from t tauri stars to the brown dwarf limit - IOPscience

The Astrophysical Journal, 749:190 (15pp), 2012 April 20 C 2012.

doi:10.1088/0004-637X/749/2/190

The American Astronomical Society. All rights reserved. Printed in the U.S.A.

EXTENDED MAGNETOSPHERES IN PRE-MAIN-SEQUENCE EVOLUTION: FROM T TAURI STARS TO THE BROWN DWARF LIMIT 1

´ Ana I. Gomez de Castro1,2 and Pablo Marcos-Arenal1

Grupo de Investigación Complutense AEGORA, Universidad Complutense de Madrid, 28040 Madrid, Spain 2 S.D. Astronom´ıa y Geodesia, Fac. de CC. Matem´ aticas, Universidad Complutense de Madrid, 28040 Madrid, Spain Received 2011 November 18; accepted 2012 February 15; published 2012 April 5

ABSTRACT Low-mass pre-main-sequence stars, i.e., T Tauri stars (TTSs), strongly radiate at high energies, from X-rays to the ultraviolet (UV). This excess radiation with respect to main-sequence cool stars (MSCSs) is associated with the accretion process, i.e., it is produced in the extended magnetospheres, in the accretion shocks on the stellar surface, and in the outflows. Although evidence of accretion shocks and outflow contribution to the high-energy excess have been recently addressed, there is not an updated revision of the magnetospheric contribution. This article addresses this issue. The UV observations of the TTSs in the well-known Taurus region have been analyzed together with the XMM-Newton observations compiled in the XEST survey. For the first time the high sensitivity of the Hubble Space Telescope UV instrumentation has allowed measurement of the UV line fluxes of TTSs to M8 type. UV- and X-ray-normalized fluxes have been determined to study the extent and properties of the TTS magnetospheres as a class. They have been compared with the atmospheres of the MSCSs. The main results from this analysis are (1) the normalized fluxes of all the tracers are correlated; this correlation is independent of the broad mass range and the hardness of the X-ray radiation field; (2) the TTS correlations are different than the MSCS correlations; (3) there is a very significant excess emission in O i in the TTSs compared with MSCSs that seems to be caused by recombination radiation from the disk atmosphere after photoionization by extreme UV radiation; the Fe ii/Mg ii recombination continuum has also been detected in several TTSs and most prominently in AA Tau; and (4) the normalized flux of the UV tracers anticorrelates with the strength of the X-ray flux, i.e., the stronger the X-ray surface flux is, the weaker the observed UV flux. This last behavior is counterintuitive within the framework of stellar dynamo theory and suggests that UV emission can be produced in the extended and dense stellar magnetosphere directly driven by local collisional processes. The brown dwarf 2MASS J12073346-3332539 has been found to follow the same flux–flux relations of the TTSs. Thus, TTS-normalized flux scaling laws seem to be extendable to the brown dwarf limit and can be used for identification/diagnosis purposes. We report the discovery of an inverse correlation between the C iv-normalized flux and the magnetospheric radius derived for stars with known magnetic fields. The normalized C iv flux is found to be ∝ exp(−αrmag ), with α = 0.5–0.7. Key words: stars: chromospheres – stars: coronae – stars: pre-main sequence – ultraviolet: stars Online-only material: color figures The first high-resolution UV spectrum of a classical T Tauri star (TTS) in this range was obtained by Herczeg et al. (2002); they observed TW Hya, a nearby TTS located just at 54 pc (van Leeuwen 2007), roughly three times closer to the Sun than the Gould Belt ring of star-forming regions (SFRs) at 140 pc where most of the TTSs have been detected. The profiles of the main resonance lines are very complex and display blueshifted or redshifted absorptions over very broad and strong emission cores. Unfortunately, the large oscillator strengths of the resonance lines make it difficult to fully determine the characteristics of the emission region since absorption components may reduce the emitted flux in an uncertain amount (the TTS environment is very complex). Observations of the Si iii] and C iii] UV semiforbidden lines confirmed the existence of a broad emission component with FWHM ranging from 100 km s−1 to 300 km s−1 ; these lines are optically thin; thus, the large broadening pointed out that the kinematics of the emission region is unresolved. As an example, an extended ring-like ionized structure has been detected around RW Aur (Gómez de Castro & Verdugo 2003). In all cases, the line fluxes pointed out to the presence of an extended and dense (ne = 109 –1010 cm−3 ) atmosphere. The TTSs have surface magnetic fields of ∼200–1 kG (Guenther et al. 1999; Johns-Krull et al. 1999), and the magnetic diffusivity is η ∼ 0.005 cm2 s−1 for the inferred temperature and density of the atmospheric

1. INTRODUCTION Low-mass pre-main-sequence (PMS) stars display a strong ultraviolet (UV) excess compared with main-sequence cool stars (MSCSs) of the same spectral type (see Gómez de Castro 2009a for a recent review); this excess is assumed to be mainly accounted for the accretion flow. UV radiation is expected to be produced at the accretion shocks where material falling from the inner disk deposits its kinetic energy on the stellar surface (Gómez de Castro & Lamzin 1999; Gullbring et al. 2000; JohnsKrull 2009). In fact, rotational modulation of the UV line fluxes has been detected in BP Tau (Simon et al. 1990; Gómez de Castro & Franqueira 1997a) and in DI Cep (Gómez de Castro & Fernández 1996). These observations show, however, that there is a UV excess even at minimum during the rotational modulation. This excess is partially caused by the unresolved outflow and jet (Gómez de Castro & Verdugo 2001), but it is dominated by the stellar magnetosphere. The UV excess is observed both in the continuum, dominated by the Balmer continuum and the H2 photodissociation bands, and in the lines. Lines range from neutral or singly ionized species, such as O i, C ii, Si ii, or Mg ii, to highly ionized species, such as C iv or Si iv, and even to tracers of very hot plasmas such as the He ii resonance transition at 1640 Å (most of these tracers are observed in the 1200–1800 Å spectral range). 1

´ Gomez de Castro & Marcos Arenal

The Astrophysical Journal, 749:190 (15pp), 2012 April 20

tracers. Henceforth, magnetic fields play a fundamental role in the atmospheric and magnetospheric evolution. Typical atmospheric/magnetospheric fluxes are about 50–100 times larger in the TTSs than in the MSCSs. This has driven to the simple rule of thumb of setting the inner limit to the accretion disk3 to ∼3–5 stellar radii. Understanding the structure of the TTS magnetosphere and the boundary with the accretion disk is a main problem in low-mass star formation, and it is a key problem in understanding the physics of jet formation since jet launching is driven from this interface (see, e.g., von Rekowski & Brandenburg 2004 or Gómez de Castro & von Rekowski 2011 for the UV output of the outflow). Crucial measurements yet to be done are as follows:

Table 1 Log of Observations Object RY Tau SU Aur HD 283572 T Tau GM Aur RW Aur HN Tau LkCa 15 UX Taud DG Taud CI Tau AA Tau DL Tau DR Tau

1. The properties of plasma at the co-rotation point between the stellar magnetosphere and the disk. 2. The rotation shear profile within the magnetosphere. 3. The magnetic and gravitational energy release in the magnetosphere.

BP Tau DK Tau DO Tau FM Tau CX Tau DE Tau DN Tau DP Tau IP Tau GK Tau DF Tau DM Tau UZ Tau FP Tau

For any magnetospheric study, it is fundamental to have access to UV spectroscopy in the 1200–2000 Å spectral range until the low-mass end of the TTSs. This has only been achievable with the Hubble Space Telescope (HST). For the first time the Solar Blind Channel (SBC) in the Advanced Camera for Surveys (ACS) has allowed measurement of the fluxes of the main magnetospheric resonance lines down to the M5 spectral type. Also for the first time, the high sensitivity of the Cosmic Origins Spectrograph (COS) has allowed measurement of the magnetospheric tracers in an M8 brown dwarf, 2MASS J12073346-3332539 (France et al. 2010). Thus, there is a wealth of data that permits us to revisit the flux–flux relations of the various magnetospheric tracers in the TTSs and compare them with MSCSs. Previous studies (Lemmens et al. 1992; Huélamo et al. 1998; Johns-Krull et al. 2000) only considered the sample of TTSs observed with the International Ultraviolet Explorer (IUE) and have no coverage of spectral types later than K0 (Gómez de Castro & Franqueira 1997b, hereafter GdCF97b). In Section 2, the observations used (both IUE and HST observations) as well as the data processing are described. In Section 3, the flux–flux relations are studied and the regression lines are derived. The detection of a discontinuity in the continuum of some late-type TTSs at 1650 Å is reported in Section 4; this discontinuity traces the Mg ii and Fe ii bound-free continua. In Section 5, the interpretation of the results in terms of the properties of the PMS magnetospheres is discussed. Finally, a brief summary of the main results is provided in Section 6.

Telescope

Instrumenta

Number ofb Observations

IUE IUE IUE IUE IUE IUE IUE HST IUE IUE HST HST HST IUE HST IUE HST HST HST HST HST HST HST HST HST IUE HST HST HST

SWP/LDM SWP/LDM SWP/LDM SWP/LDM SWP/LDM SWP/LDM ACS/SBC STIS/G140L SWP/LDM SWP/LDM ACS/SBC ACS/SBC ACS/SBC SWP/LDM ACS/SBC SWP/LDM ACS/SBC ACS/SBC ACS/SBC ACS/SBC ACS/SBC ACS/SBC ACS/SBC ACS/SBC ACS/SBC SWP/SBC STIS/G140L ACS/SBC ACS/SBC

14 8 3 11c 3 9c 1 1 1 3 1 1 1 11c 1 16c 1 1 1 1 1 1 1 1 1 6 1 1 1

Notes. a SWP/LDM: Short Wavelength Prime camera in low-dispersion mode. ACS/SBC: Advanced Camera System/Solar Blind Channel. STIS/G140L: Space Telescope Imaging Spectrograph/Grating G140L. b See G´ omez de Castro & Franqueira (1997b) for a compilation of all the IUE observations used as well as their dates of observations. c Variable far-UV flux; variations are dominated by the continuum. d Data are too noisy and have not been used for this study.

observations of TTSs in low-dispersion mode. The sensitivity of IUE was too low to detect the emission lines from weakline TTSs. The typical exposure time for an M0-type TTS with V = 11.5 was ∼400 minutes to reach a signal-to-noise ratio of ∼6 in the C iv line. Thus, most of the observations of late-type TTSs come from the HST Archive. The IUE data have been processed with the IUE Newly Extraction Software (INES) package (Rodr´ıguez-Pascual et al. 1999); see Huélamo et al. (2000) for an analysis of the quality of the TTS data processed with INES. The ACS on board HST obtains low-dispersion spectra of faint sources in the far-UV (1200–1700 Å) in the SBC. The spectra were obtained with the prism PR130L, which has a variable resolving power from 353 at λ ∼ 1250 Å to 40 at λ ∼ 1850 Å (see the ACS manual); for comparison, the IUE data resolving power is ∼300 over the same range. HST/ACS data have been processed by us with IRAF and the aXe reduction tool. The observations consist of a slitless spectroscopic image containing the first dispersion order and a shorter exposure, direct image (with filter F165LP), to identify the location of the source. Standard reduction (flat fielding and basic cosmetic effect removal) has been carried out with the package IRAF/CALACS. The IRAF/MULTIDRIZZLE has been used

2. OBSERVATIONS AND DATA REDUCTION We have collected for this study all the low-dispersion spectroscopic observations of TTSs in the far-UV (1200–1900 Å). The databases of the IUE and the HST have been searched for TTS spectra. Only data from stars in the Taurus SFR have been retrieved. The log of the observations sorted by spectral type is shown in Table 1. The IUE low-dispersion (∼6 Å) data were obtained with the Short Wavelength Prime (SWP) camera and cover the entire 1200–2000 Å spectral range. The characteristics of the observations (identifier, exposure time, date of observation) can be found in the study by GdCF97b, which includes all the 3

This inner limit of the accretion disk is not the limit derived from infrared observations that only trace the inner border of the dust disk.

2


The Astrophysical Journal, 749:190 (15pp), 2012 April 20 Table 2 UV Fluxes of TTSsa Star RY Tau SU Aur HD 283572 T Tau GM Aur RW Aur HN Tau LkCa 15 CI Tau AA Tau DL Tau DR Tauc BP Tau DO Tau FM Tau CX Tau DE Tauc DN Tau DP Tau IP Tau GK Tau DF Tau DM Tau UZ Tau FP Tau

O ib (10−14 erg s−1 cm−2 )

C ii (10−14 erg s−1 cm−2 )

C iv (10−14 erg s−1 cm−2 )

He ii (10−14 erg s−1 cm−2 )

9.4 ± 3.1 13.9 ± 9 ... 18.4 ± 0.7 4.5 ± 1.6 34.9 ± 23.5 0.25 1.5 ± 0.4 0.016 0.089 ± 0.016 0.085 ± 0.009 4.8 ± 1.9 22.1 ± 1.7 0.17 ... 0.061 ± 0.008 6.2 ± 2.2 2.2 ± 0.2 0.71 ± 0.06 0.21 0.25 ± 0.02 68.4 ± 28.1 2.3e ± 0.3 0.88 ± 0.06 0.044 ± 0.003

4.7 ± 1.1 4.6 ± 0.7 3.8 ± 0.9 5.4 ± 0.7 4.1 ± 1.9 11. ± 6.5 0.77 ± 0.07 1.2 ± 0.3 0.01 0.50 ± 0.01 0.039 ± 0.004 4.3 ± 1.4 7.3 ± 0.5 0.21 ± 0.02 ... 0.084 ± 0.005 2.3 ± 0.9 0.60 ± 0.07 0.84 0.24 ± 0.03 0.19 ± 0.02 11.1 ± 4.5 2.5 ± 0.4 0.87 ± 0.08 0.147 ± 0.007

10.1 ± 2.8 19.3 ± 5.6 20.1 ± 3.0 48.8 ± 1.1 12.2 ± 2.6 29 ± 17 ... 8.5 ± 0.4 ... ... 5.6 25.6 ± 8.9 32.0 ± 5.8 ... 1.4 ± 0.1 0.18 ± 0.02 9.1 ± 2.6 3.8 ± 0.04 ... 2.4 ± 0.02 1.55 ± 0.05 24.0 ± 3.8 6.9 ± 0.3 1.6 ± 0.1 0.27 ± 0.03

4.9 ± 1.5 6.8 ± 0.8 8.7 ± 2.0 14.9 ± 1.0 6.2 ± 2.8 ... ... 2.1 ± 0.4 ... ... ... 6.3 ± 3.0 15.6 ± 2.9 ... 1.8 ± 0.2 ... 3.6 ± 2.3 2.3 ± 0.4 ... 1.3 ± 0.2 0.8 ± 0.1 9.2 ± 3.7 6.4 ± 0.2 ... ...

Notes. a Fluxes are not extinction corrected. b O i may be affected by geocoronal emission in the SBC/ACS data. c Measurements in the table come from the IUE observations.

for cosmic ray removal, geometric distortion correction, and the combination of drizzle exposures. The position of the star in world coordinates is obtained using the SExtractor tool on the direct image of the target. Finally, the aXe reduction tool is used to calibrate the spectrum. The aXe tool requires the instrument calibration files and the configuration files obtained from the direct image to perform the one-dimensional (1D) spectral extraction. Those files contain information about errors and data quality, positioning of the source, description of the spectral local trace on the frame, and the dispersion solution of the spectral order and calibration files. The local trace is required to obtain the extraction direction, and the dispersion solution is determined to associate each of the pixels on the image with a wavelength. A set of spectral bins is created and the amount of counts corresponding to each bin produces the final 1D spectrum. Some of the ACS/SBC observations were split into several subexposures. We have reduced each one of them independently, but, since no significant variations have been detected, the spectra have been co-added. Finally, DM Tau and LkCa 15 data were retrieved from the HST archive. These stars were part of a large observing program with the Space Telescope Imaging Spectrograph (STIS) and the grism G140L, which also included several targets already in the IUE low-dispersion sample. These two targets were the only ones that had not been previously observed with IUE. The FUV spectra of all the TTSs studied in this work are shown in Figure 1. The FUV continuum is clearly underexposed in the IUE observations except for DR Tau; however, the main emission lines can be cleanly identified. The most prominent

features are O i (1305 Å), C ii (1335 Å), Si iv (1400 Å), C iv (1550 Å), He ii (1640 Å), C i (1657 Å), O iii] (1666 Å), Si ii (1808, 1817 Å), Si iii] (1892 Å), and C iii] (1908 Å). The Lymanband H2 lines are also observed in the spectra; they are especially conspicuous in the T Tau spectrum (Brown et al. 1981). The shape of the FUV continuum is cleanly detected in the HST/ACS observations, but the lines are barely detectable above the C iv line wavelength: only very strong He ii features are susceptible to being measured. We have determined the O i, C ii, C iv, and He ii fluxes for the stars in the sample (see Table 2). The continuum contribution to the line flux is negligible in the IUE spectra, but, in the HST/ACS spectra, a proper fit to the continuum was needed. For this purpose, “clean” spectral windows have been selected to avoid the strongest H2 features that are at 1431 Å, 1446 Å, 1490 Å, 1505 Å, 1547 Å, and 1562 Å (see the Brown et al. 1984 analysis of T Tau spectrum and the high-resolution spectrum of TW Hya studied by Herczeg et al. 2002). The fit for DE Tau is shown in Figure 2 to compare with the fits of Ingleby et al. (2009). The measured fluxes have been corrected from extinction according to Valencic et al. (2004). The main properties of the TTSs in this sample are summarized in Table 3 (see the Appendix for a detailed description of the bibliographic sources and the uncertainties in the main stellar parameters). To assist in the characterization of the TTSs, they have been plotted in a J − H versus H − K diagram in Figure 3; infrared magnitudes have been extracted from the “Two Micron All Sky Survey” (2MASS; Skrutskie et al. 2006) for all sources. The MSCS location, for spectral types GO to M5, 3



Figure 1. Top: FUV spectrum of the TTSs observed with the IUE. Middle: FUV spectrum of TTSs observed with HST/STIS. Bottom: FUV spectrum of TTSs observed with HST/ACS. IUE spectrum of DR Tau has been overplotted on its ACS spectra for comparison. (A color version of this figure is available in the online journal.)

3. SCALING OF MAGNETOSPHERIC PROPERTIES: FROM TTSs TO BROWN DWARFS

is traced by the XMM-Newton-selected sample of nearby bright X-ray sources (López-Santiago et al. 2007); note that there is a turnover point at M0 types. There is a tight (J − H), (H − K) correlation in our sample that agrees with the already reported results from Meyer et al. (1997) for the locus of the TTSs in the J − H, H − K diagram.

To analyze the properties of the atmospheres/magnetospheres of the TTSs, we have compared them with a test population of main-sequence stars of similar spectral types. The test sample 4



Figure 2. Fit of the continuum in the spectrum of DE Tau used to derive the line fluxes. (A color version of this figure is available in the online journal.)

Table 3 Sample Properties Object RY Tau SU Aur HD 283572 T Tau GM Aur RW Aur HN Tau LkCa 15 CI Tau AA Tau DL Tau DR Tau BP Tau DO Tau FM Tau CX Tau DE Tau DN Tau DP Tau IP Tau GK Tau DF Tau DM Tau UZ Tau FP Tau

Spectral Type

L∗

AV

H

K

Lacc (L )

SED

SiO

V sin(i) (km s−1 )

Age log τ (yr)

Period (days)

G1 G1 G5a K0 K3 K3 K5 K5 K7 K7 K7 K7 K7 M0 M0 M0 M0 M0 M0 M0 M0 M1, M3.5 M1 M1 M4

9.6 7.8 5.78b 7.8 1.2 1.72b 0.2 1.0 1.3 1.1 1.0 1.7 1.3 1.4 0.5 0.56 1.2 1.2 0.2 0.7 1.4 0.47, 0.53 d 0.3 0.6 0.4

2.20 0.90 0.3c 1.8 1.1 0.5d 1.2 1.0 2.1 1.4 1.6 1.0 1.0 2.4 1.9 1.3d 1.1 0.8 0.5 0.9 1.1 1.6e 0.6 1.0 0.1

6.13 ± 0.06 6.56 ± 0.02 7.01 ± 0.03 6.24 ± 0.02 8.60 ± 0.02 7.62 ± 0.04 9.47 ± 0.03 ... 8.43 ± 0.04 8.55 ± 0.02 8.68 ± 0.03 7.80 ± 0.05 8.22 ± 0.02 8.24 ± 0.03 9.39 ± 0.02 9.05 ± 0.02 8.27 ± 0.02 8.34 ± 0.03 9.69 ± 0.02 8.89 ± 0.02 8.11 ± 0.03 7.26 ± 0.02 ... 8.01 ± 0.02 9.18 ± 0.02

5.40 ± 0.02 5.99 ± 0.02 6.87 ± 0.02 5.32 ± 0.02 8.28 ± 0.02 7.02 ± 0.02 8.38 ± 0.02 ... 7.79 ± 0.02 8.05 ± 0.02 7.96 ± 0.02 6.87 ± 0.02 7.74 ± 0.02 7.30 ± 0.02 8.76 ± 0.02 8.81 ± 0.02 7.80 ± 0.02 8.02 ± 0.02 8.76 ± 0.02 8.39 ± 0.02 7.47 ± 0.02 6.74 ± 0.02 ... 7.47 ± 0.03 8.87 ± 0.02

1.6 0.1 ... 0.9 0.18 ... 0.07 0.03 0.47 0.13 0.32 1.03 0.23 0.29 0.3 ... 0.16 0.04 0.01 0.02 0.06 ... 0.08 0.02 0.001

0.05 0.37 ... −0.04 0.46 −0.50 ... ... ... ... 0.32 −0.61 −0.48 ... ... −0.14 −0.44 0.45 ... −0.45 0.20 −1.13 ... −0.66 −0.39

1.36 0.98 ... −0.1 1.19 0.32 ... ... ... ... 1 0.27 0.55 ... ... 0.48 0.43 0.22 ... 0.99 1.03 0.17 ... 0.45 0.22

52.2 66.2 75.0 20.1 12.4 17.2 ... 12.5 ... ... ... 10.0 7.8 ... ... 18.2 10.0 8.1 ... 11.0 18.7 16.1 ... 15.9 26.6

6.38 ± 0.09 6.80 ± 0.08 6.94 ± 0.08 ... 6.9 ± 0.2 7.20 ± 0.11 ... 6.70 ± 0.16 ... ... ... 5.9 ± 0.2 6.51 ± 0.12 ... ... 6.12 ± 0.09 ... 6.15 ± 0.11 ... 6.6 ± 0.2 ... 6.28 ± 0.17 ... ... 6.04 ± 0.14

9.53 3.5 ... 2.8 12 5.4 ... 5.9 ... ... ... 7.3 7.6 ... ... 0 7.6 6.6 ... 3.3 4.7 8.5

Notes. a Herbig & Bell (1988) Catalog. b Bertout et al. (2007). c Walter et al. (1987). d The luminosities of DF Tau A and DF Tau B are provided from Bertout et al. (2007). e Furlan et al. (2006).

5

0 0



Figure 3. Location in the J − H, H − K diagram of the TTSs in the Taurus SFR observed in the FUV. The MSCS location (López-Santiago et al. 2007) is marked in green. The Meyer et al. (1997) fit to the TTS infrared excess is also indicated. (A color version of this figure is available in the online journal.)

In the following, the correlations between the normalized fluxes of several spectral tracers are analyzed.

has been extracted from two studies: Ayres et al. (1995) for G0 to K4 types and Linsky et al. (1982) for K5 to M5.6 types. The observed line fluxes depend on the total emitting volume and the line emissivity, (ne , Te ), as 1 l (ne , Te )ne nH dV , Fl = 4π d 2 where ne and nH are the electron and hydrogen density, respectively, d is the distance to the star, and Te is the electron temperature. The emitting volume is difficult to constrain in the TTS environment. First, UV emission is not expected to occur in a narrow layer above the stellar surface as in cool stars. Second, significant line emission may be produced in the large stellar magnetospheres that are expected to extend to the inner disk border. Ionized filaments are expected to radiate in this large and sheared area. Note that when the spectral lines are resolved, the line broadening is highly suprathermal (see, e.g., Gómez de Castro 2009b). Thus, it seems reasonable to model the emitting volume as Vl = χl Hl 4π R∗2 , where the factor 4π R∗2 takes into account the size of the stellar surface and χl Hl provides an estimate of the thickness of the emitting volume and its clumpiness. Since the stellar bolometric flux, Fbol , is Fbol (σ T 4 4π R∗2 )/4π d 2 , the rate Fl /Fbol provides a measure of the line emissivity weighted over an unknown thickness but corrected from stellar radii and surface temperature. In this manner, the normalized fluxes are corrected from scaling effects associated with the broad range of mass, luminosity, and stellar radius covered by the TTS sample studied in this work. Note that the emission measure formalism has been consciously avoided given its limitations for the study of TTSs.

3.1. F (C iv)/Fbol versus F (C ii)/Fbol In Figure 4, F (C iv)/Fbol is represented versus F (C ii)/Fbol for TTSs and cool main-sequence stars in the logarithmic scale. TTS-normalized fluxes are on average about two orders of magnitude larger than those of their main-sequence analogs. However, F (C iv)/Fbol scales with F (C ii)/Fbol in a similar manner; more precisely, 1. For TTSs: log(F (C iv)/Fbol ) = (0.90 ± 0.07) log(F (C ii)/Fbol ) + (0.1 ± 0.3) with rms = 0.236. 2. For main-sequence cool stars (hereafter MSCSs): log(F (C iv)/Fbol ) = (1.14 ± 0.05) log(F (C ii)/Fbol ) + (1.0 ± 0.3) with rms = 0.163. Note that the regression lines are nearly parallel to the extinction direction; this may mimic some environmental effects in the TTSs. At the bottom corner of Figure 4, the histogram of the distances to the MSCS regression line of both TTS and MSCS populations is plotted. The distance from every point to the MSCS regression line has been computed and then a histogram of distances has been derived for both populations. As expected, the MSCS distribution strongly peaks at distance 0, but notice that the dispersion of the TTS distribution is not broad (0.4 dex) and it is well centered. There is only one brown dwarf detected, 2MASS J120733463332539, that has been observed with high enough spectral 6



-2

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-8 -8

-6

-4

-2

Figure 4. C iv vs. C ii plot for TTSs compared with MSCSs. TTSs are plotted with triangles and the error bars are marked. MSCSs are plotted with filled circles. The large circle marks the location of the brown dwarf 2MASS J12073346-3332539, which was detected for the first time in the UV with HST/COS (France et al. 2010). The MSCS and TTS regression lines are plotted with dashed and continuous lines, respectively. The bottom right inset displays the distribution of the distances of the stars in both samples to the MSCS regression line. (A color version of this figure is available in the online journal.)

resolution in the UV to measure the fluxes of the main UV resonance lines (France et al. 2010). This brown dwarf is a young M8 type and, as shown in Figure 4, its UV excess is similar to that of the TTSs, being stronger than what is detected in some M0–M5 TTSs. Thus, while in MSCSs the normalized flux increases as the stellar mass decreases, in TTSs the normalized flux is not related to inherent stellar properties such as mass or effective temperature.

C iv[uv1] emission). If the lines are excited by X-ray radiation, both would be naturally formed in the recombination cascade. X-ray radiation as soft as 50 eV, corresponding to blackbody temperatures of 0.56 MK, would be enough for this purpose. The intriguing issue is that this correlation only applies to a very small subsample of TTSs: those TTSs producing significant He ii emission. Note that the C iv resonance multiplet is detected in all the TTSs while the He ii is only detected in a subset. This is not a selection effect associated with the TTS brightness; UV bright TTSs, such as RW Aur, have very weak, if present at all, He ii emission.

3.2. F (C iv)/Fbol versus F (He ii)/Fbol The correlation between C iv and He ii fluxes is very strong both in TTSs and MSCSs and, as shown in Figure 5, the scaling law is very similar.

3.3. F (LX )/Fbol versus F (He ii)/Fbol

1. For TTSs: log(F (C iv)/Fbol ) = (1.01 ± 0.08) log(F (He ii)/Fbol ) + (0.4 ± 0.3) with rms = 0.207. 2. For MSCSs: log(F (C iv)/Fbol ) = (1.01 ± 0.05) log(F (He ii)/Fbol ) + (0.3 ± 0.2) with rms = 0.205.

In the left panel of Figure 6, the X-ray luminosity is compared with the He ii luminosity. The He ii line at 1640 Å is the “Hα” line of He ii, and it is considered to be a coronal indicator in late-type stars where the line is partly formed by recombination following photoionization by extreme coronal UV radiation (Hartmann et al. 1979; Ayres et al. 1995). However, the detailed analysis of the He ii flux of T Tau by Brown et al. (1984) suggested that, at least for T Tau, the emission is most easily accounted for by collisional excitation plus photon trapping in the He ii Ly-β line at 256 Å. The main rationale behind this claim was that: (1) the observed X-ray flux from T Tau is two orders of magnitude smaller than the expected if the late-type star scaling is applied and (2) there is not a high enough photoionization flux to produce He iii in T Tau as it is in solar-type atmospheres. Figure 6 confirms that the He ii

Thus, in TTSs, as in MSCSs, the He ii and C iv lines seem to be produced in the same physical region. This is not surprising. In a collisional plasma, the excitation energy of the upper level of the He ii line at 1640 Å is 48.4 eV comparable with the ionization potential of C iii (47.9 eV). Such a high electron temperature would naturally excite the upper levels of C iv that will produce the C iv line emission in the recombination cascade (provided that the electron density is low enough to avoid collisionally depopulating the low-energy resonance level producing the 7



-2

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-6

-8 -8

-6

-4

-2

Figure 5. C iv flux vs. He ii flux plot for TTSs compared with MSCSs. The bottom right inset displays the distribution of the distances of the stars in both samples to the MSCS regression line. Plotting symbols are indicated in Figure 4. (A color version of this figure is available in the online journal.)

Figure 6. X-ray flux vs. He ii plot (top) and C iv (bottom) for TTSs compared with MSCSs. Plotting symbols are indicated in Figure 4.

Thus, He ii is not a coronal proxy in TTSs and, in fact, the X-ray flux decreases when the He ii radiation grows. Note that He ii measurements and X-ray fluxes are only available for a small fraction of the TTSs: RY Tau, SU Aur, HD 283572, T Tau, BP Tau, and FM Tau. All but FM Tau are early-type TTSs, with spectral types similar to those in the Coronathon sample (Ayres et al. 1995). All these spectra were obtained either with IUE or HST/STIS, whose spectral resolution is high enough to resolve the He ii line from the C i (1656 Å) or the O iii] (1666 Å)

line excitation proceeds through different channels in MSCS and TTS populations. The regression lines are as follows. 1. For TTSs: log(F (LX )/Fbol ) = (−0.11 ± 0.08) log(F (He ii)/Fbol ) + (4.0 ± 0.3) with rms = 0.145. 2. For MSCSs: log(F (LX )/Fbol ) = (1.2±0.2) log(F (He ii)/Fbol )+(3.±1) with rms = 0.392. 8



-2

-2

-4

-4

-6

-6

-6

-4

-6

-2

-4

-2

Figure 7. C iv (top) and C ii (bottom) fluxes vs. O i flux plot for TTSs compared with MSCSs. Plotting symbols are indicated in Figure 4. The bottom right inset displays the distribution of the distances of the stars in both samples to the MSCS regression line. (A color version of this figure is available in the online journal.)

log(F (C ii)/Fbol ) = (0.75 ± 0.05) log(F (O i)/Fbol ) − (1.1 ± 0.2) with rms = 0.187. 2. For MSCSs: log(F (C iv)/Fbol ) = (1.4 ± 0.1) log(F (O i)/Fbol ) + (2.7 ± 0.6) rms = 0.254. log(F (C ii)/Fbol ) = (1.2 ± 0.1) log(F (O i)/Fbol ) + (1.2 ± 0.5) with rms = 0.214. Hence, there is a prominent O i emission excess not only when compared with high-energy tracers, such as the C iv line, but also when compared with low-excitation tracers such as the C ii line. At the bottom corner of Figure 7, the histogram of the distances to the MSCS regression line is plotted for both TTS and MSCS populations. As expected, the MSCS distribution strongly peaks at distance 0; however, the TTS distribution is fully shifted to the left. Although, the O i line can be collisionally excited in the jet or pumped in the recombination of O ii, it may also be excited by the Bowen mechanism from H Ly-β (Bowen 1947). Photoionization of the disk surface by Lyman continuum photons is known to create a photodissociation region in the disk atmosphere that pumps the H2 Lyman band (Herczeg et al. 2004; Ingleby et al. 2009). Thus, the extreme UV (EUV) power radiated by the stellar magnetosphere could be pumping O i emission from the atmosphere of the inner disk. To further test this possibility, we have plotted the O i emission versus the U-band excess. This has been often used as a tracer of the EUV power radiated in the accretion process (see, e.g., Ingleby et al. 2009, who use it to evaluate whether the radiation from the H2 molecule is pumped by the accretion luminosity). As shown in Figure 8, there is only a mild correlation; though, in general, stars with accretion rates >0.1L∗ clearly have the largest O i fluxes. Note that inclination effects could be associated with this scatter as well (unfortunately, information about the disk inclination with respect to the line of sight is scarce). The O i flux does not correlate with other disk parameters such as the infrared spectral energy distribution (SED), the strength of the SiO band, or the H − K infrared color. Neither does the O i flux correlate with the stellar age or rotation period (see Figure 9 for the plot of the age versus the normalized flux of O i).

lines; thus, pollution of the He ii flux by other features can be neglected. The presence of two groups in Figure 6, at low and high UV line flux excesses, is not associated with stellar mass or temperature segregation effects. For instance, the cluster with the higher FC iv and FHe ii fluxes is constituted by RY Tau, T Tau, BP Tau, and FM Tau with spectral types G1, K0, K7, and M0, respectively. As an additional test, the F (LX )/Fbol versus F (C iv)/Fbol has been plotted in the right panel of Figure 6; there are few more sources in this plot. The trend is clearly confirmed. The strength of the UV tracers grows as the X-ray radiation decreases, suggesting that magnetic energy dissipation is preferentially channeled to the UV in the dense magnetospheric environment of the TTSs. The X-ray radiation is dominated by the bremsstrahlung radiation of hot electrons in the stellar coronae (see, e.g., Guedel et al. 2007 analysis of the TTS spectra). In dense environments, the X-ray radiation can be rapidly absorbed and reprocessed in the UV channel (see, e.g., the simulations in Gómez de Castro & Verdugo 2003). 3.4. F (C iv)/Fbol versus F (O i)/Fbol and F (C ii)/Fbol versus F (O i)/Fbol Finally, the O i flux is compared with the C ii and C iv fluxes. Note that the O i line is one of the strongest lines produced in the Earth geocorona; thus, HST observations may be significantly polluted by this contribution given the low Earth orbit of HST. Fortunately, the contribution from the Earth geocorona to the O i flux seems to be negligible in the ACS/SBC spectra since O i fluxes from the ACS/SBC observations compare well with the IUE fluxes obtained at a much higher geosynchronous orbit. As shown in Figure 7, a very large O i excess is found in the TTSs when compared with the MSCSs. The brown dwarf 2MASS J12073346-3332539 shares the same effect; in fact, it is on the TTS regression line. 1. For TTSs: log(F (C iv)/Fbol ) = (0.70 ± 0.06) log(F (O i)/Fbol ) − (0.9 ± 0.2) with rms = 0.236. 9


The Astrophysical Journal, 749:190 (15pp), 2012 April 20 8

6

4

2

0 20

Figure 8. Accretion luminosity from Table 3 is represented vs. the O i flux for the TTSs. The locations of stars with spectral types earlier than M0 are plotted with red triangles. Stars of spectral types M0 or later are plotted with blue squares. (A color version of this figure is available in the online journal.)

40

60

vsin(i) (km/s)

Figure 10. Histogram of the projected stellar rotation velocity (v sin(i)) for the entire TTS sample (thick black line). The histograms corresponding to the TTSs with and without Fe ii/Mg ii recombination continuum are overdrawn in red dashed and blue pointed lines, respectively. Note that stars with large projected rotation velocities, suggesting observations close to edge-on, display the Fe ii/Mg ii recombination continuum. (A color version of this figure is available in the online journal.)

expected to be produced on the disk surface as a result of the photoionization of matter in the disk atmosphere. The bump is observed in HN Tau, AA Tau, and in about half of the latest spectral types (K7 to M5). There is no correlation between the presence of the bump and the strength of other physical (accretion luminosity, rotation period) or spectral (UV line fluxes, infrared excess) tracers. There is only minor evidence of the sources displaying the jump having larger values of v sin(i), suggesting that the strength may be related to the inclination angle (see Figure 10). This slight indication is further supported by AA Tau observations; AA Tau displays the largest and cleanest jump, and it is known to have a prominent warp (O’Sullivan et al. 2005). Within the subsample of sources displaying the bump, there is a correlation between the strength of the bump and the UV line flux (see Figure 11), suggesting the relevance of the recombination processes in UV line excitation. Figure 9. TTS age (from Table 3) is represented vs. the O i flux. The locations of stars with spectral types earlier than M0 are plotted with red triangles. Stars of spectral types M0 or later are plotted with blue squares. Both populations are cleanly separated, with more massive stars being older than the less massive stars. However, there is no clear correlation between the age and O i flux for either of the two populations. (A color version of this figure is available in the online journal.)

5. DISCUSSION In Section 3, normalized fluxes of some prominent atmospheric/magnetospheric tracers are derived. It is shown that (1) the normalized flux is not related to the stellar mass or effective temperature and (2) the well-defined scaling laws applied to the TTSs are significantly different from the observed in the MSCSs (see Table 4 for a summary). The strongest deviation is found for the X-ray versus He ii- or C iv-normalized flux correlation since the slope of the regression line is negative (the stronger the He ii, the weaker the X-ray flux). In Figure 12, C iv-, O i-, and C ii-normalized fluxes are plotted versus the stellar luminosity for MSCSs and TTSs. MSCSs have normalized fluxes typically between 10−7 and 10−6 unless for the M-flare stars whose normalized fluxes are enhanced by one to two orders of magnitude with respect to G-K stars. Normalized fluxes of the TTSs are, at least, comparable with those of the M-flare stars and well above their main-sequence

4. THE FAR-UV CONTINUUM For the first time, the ACS/SBC observations have allowed us to obtain a well-exposed far-UV continuum spectrum of cool TTSs. As shown in Figure 1, some late-type stars display a sharp discontinuity at ∼1650 Å, which corresponds to an energy of ∼7.5 eV. Thus, although H2 dissociation and 2γ continuum contribute to the far-UV continuum, there is a clear, nonnegligible contribution from the recombination (bound-free) continuum of Fe ii (χi = 7.87 eV) and Mg ii (χi = 7.65 eV) in some TTSs. These recombination continua are naturally 10


The Astrophysical Journal, 749:190 (15pp), 2012 April 20 Table 4 Summary of Fittingsa Tracers C iv vs. C ii C iv vs. He ii X-ray vs. He ii C iv vs. O i C ii vs. O i

TTS Fit

MSCS Fit

(0.9±0.1) fs (C iv) = 1.2+1.3 −0.6 fs (C ii) +2.5 fs (C iv) = 2.5−1.3 fs (He ii)(1.0±0.1) 4 (−0.11±0.08) fs (X-ray) = 1.0+1 −0.5 × 10 fs (He ii) +0.07 (0.70±0.06) fs (C iv) = 0.13−0.05 fs (O i) (0.75±0.05) fs (C ii) = 0.08+0.05 −0.03 fs (O i)

(1.1±0.1) fs (C iv) = 10+10 −5 fs (C ii) +1.2 fs (C iv) = 2.0−0.8 fs (He ii)(1.0±0.1) 3 (1.2±0.2) fs (X-ray) = 1.0+0.3 −0.2 × 10 fs (He ii) +1.5 (1.4±0.1) fs (C iv) = 0.5−0.4 fs (O i) (1.2±0.1) fs (C ii) = 15+35 −10 fs (O i)

Note. a fs (l) is the normalized flux of a given spectral tracer “l,” i.e., F (l)/Fbol .

where genuine magnetospheric emission seems to be mixed with recombination radiation from the disk surface. The accretion luminosities have been extracted from Ingleby et al. (2009); they are derived from the U-band excess following Gullbring et al. (1998) and the median U from photometry in Herbst et al. (1994). Figure 13 displays a soft trend to have larger surface fluxes for larger accretion luminosities. Note that, although DR Tau is a peculiar star that could escape from the trend, GK Tau also significantly deviates from it. To quantify it, a least-squares fit has been applied to the three line excesses (see Table 5). There seems to be a rough power-law dependence of the excess on the accretion luminosity: ˆfl ∝ L0.6 ac with an rms = 0.29 for the best fit, the C iv line. For comparison, the line fluxes are plot versus the accretion luminosity in Figure 14. There is a linear scaling between the UV line flux and the accretion luminosity as otherwise expected, since the accretion luminosity is derived from the strength of the Balmer continuum. Thus, the regression lines in Table 5 can be used to derive the accretion luminosity directly from the UV line fluxes. In summary, the accretion luminosities derived from the U-band excess rather seem to be a measure of the volume of warm plasma in the TTSs. The atmospheric/magnetospheric sources of this excess during PMS evolution can be associated with three main heating sources. 1. The classic inside-out atmospheric heating is associated with the magnetic activity; during PMS evolution, the large thickness of the convective layer results in an enhancement of the magnetic activity before stabilization in the zero-age main sequence. 2. The outside–in atmospheric heating is driven by (1) accretion shocks and (2) the shearing of the stellar magnetic field from the inner border of the disk that rotates slower than the stellar surface. Note that the rotation periods of TTSs are about 7–8 days (Ω∗ = 0.8–0.9 day−1 ), while the Keplerian frequency is Ωk = 0.6 day−1 (M/M )1/2 (r/3R )−3/2 . The corotation radius, where the shear is inverted (the Keplerian frequency is smaller than the stellar frequency), is typically at some 17.5 R for these parameters. 3. Dissipative magnetospheric processes are expected to occur on the extended stellar magnetosphere such as plasma waves, magnetospheric pulsation, collision less shocks, or magnetic reconnection events that are poorly analyzed in the TTS environment.

Figure 11. C iv (top) and O i (bottom) fluxes vs. the strength of the Fe ii/Mg ii recombination continuum jump. The strength of the jump is represented by the ratio between the continuum flux at the short-wavelength edge of the bump to the flux at the long-wavelength edge. The rightmost point corresponds to AA Tau.

analogs. The normalized flux of DF Tau seems to be further enhanced by its binary nature. DF Tau binary is composed of two TTSs of types M1 and M3.5 (Tamazian et al. 2002). Since the components are not resolved in the UV spectra, the total UV flux has been divided by the total bolometric flux of the two stars (0.53 and 0.47 solar luminosities, respectively), as shown in Table 3. However, even with these provisions, the normalized flux is enhanced by about an order of magnitude when compared with the rest of the TTSs. In all cases, FP Tau seems to be the closest to the MSCS behavior and especially to the M-flare stars. The UV line flux dependence on the accretion luminosity is analyzed in Figure 13. A normalized flux excess (ˆf) has been derived for late-type TTSs (spectral types K3 to M3) using FP Tau as a reference to compare the level of the excess for different spectral tracers. The normalized flux for a given spectral line l is defined as ˆfl = log(Fl /Fbol )∗ − log(Fl /Fbol )FPTau , where l refers to the O i, C ii, and C iv lines. As expected, the variations are correlated and the excess is the highest in the O i line,

The first two mechanisms excite UV emission close to the surface of the star producing an extended stellar atmosphere, while magnetospheric radiation is most likely to be produced in a narrow shear layer, at the magnetospheric boundary with the inner disk border, where the plasma density is high (see Figure 15 for an illustration). The magnetospheric cavity resides within a warm disk of molecular and atomic gas that it is also 11



Figure 12. Normalized fluxes are represented vs. the stellar luminosities for C ii, C iv, and O i (bottom) in the left, center, and right panels, respectively. TTSs and MSCSs are plotted with triangles and circles, respectively. Error bars for the UV normalized fluxes of the TTSs are indicated.

Table 5 Summary of Fittingsa Variables C iv excess vs. Lac C ii excess vs. Lac O i excess vs. Lac F(C iv) vs. Lac F(C ii) vs. Lac F(O i) vs. Lac

Fit

rms

ˆfl /ˆfl FPTau

log ˆfs (C iv) = (0.6 ± 0.2) log Lac + 2.5 ± 0.2 log ˆfs (C ii) = (0.6 ± 0.3) log Lac + 2.2 ± 0.3 log ˆfs (O i) = (0.7 ± 0.3) log Lac + 3.0 ± 0.5 log F(C iv) = (1.11 ± 0.15) log Lac − 11.0 ± 0.2 log F(C ii) = (1.11 ± 0.13) log Lac − 11.4 ± 0.2 log F(O i) = (1.3 ± 0.2) log Lac − 11.1 ± 0.3

0.29 0.41 0.47 0.41 0.38 0.57

316 158 1000

Note. a fs (l) is the normalized flux excess of a given spectral tracer “l,” i.e., log((F (l)/Fbol )∗ /(F (l)/Fbol )FPTau ).

12



Figure 13. fˆl = log(Fl /Fbol )∗ − log(Fl /Fbol )FPTau is represented vs. the accretion luminosity for the TTSs in the sample for the lines C ii, C iv, and O i represented with circles, squares, and triangles, respectively. The regression line of the best fits is shown.

represented in Figure 15. This gas disk is well traced by the CO fundamental (Δν = 1) ro-vibrational transition at 2.3 μm and it has been detected in nearly all CTTS (Najita et al. 2003). In many stars, the CO lines show the classic double-peaked profile produced in rotating rings. The location of the emission region can be derived assuming Keplerian rotation in a thin disk. Moreover, the profile also contains information on the radial variation of the gas emission. The emission peaks at about 0.04 AU (8.6 R ); this radius is generally smaller than the corotation radius, while the inner rim of the dusty disk traced by the infrared excess is further outwards and it is not represented in Figure 15. The size of the magnetosphere is set by the balance between the toroidal component of the stellar magnetic flux and the angular momentum of the infalling matter (Ghosh & Lamb 1979); thus, Bp Bt 4π r 2 Δr M˙ ac rVk , 4π where Bp and Bt are the poloidal and toroidal components of the field, respectively, r is the magnetosphere radius, Δr is the thickness of the shear layer, M˙ ac is the accretion rate, and Vk is the Keplerian velocity at the magnetospheric radius. Following Lamb (1989), the radius of the magnetospheric cavity is given by γ Bp2 r 2 = M˙ ac Vk

Figure 14. Line fluxes are represented vs. the accretion luminosity for the TTSs in the sample for the lines C ii, C iv, and O i.

with μ = B∗ R∗3 , which is the equatorial magnetic moment of the star. For typical TTS parameters, B∗ 4/7 R∗ 1/7 2/7 rmag = 9.2 R γ 1 kG R −2/7 M∗ 1/7 Lac × , 1032 erg s−1 M where γ 2/7 is a factor about unity (γ = (Bt /Bp )(Δr/r) 0.5–0.8, see Lamb 1989). The main uncertainties in the physics, namely, the ratio between the toroidal and the poloidal components and the relative thickness of the sheared magnetosphere, are enclosed in this factor. Note that the magnetospheric radius should decrease as the accretion luminosity increases, i.e., the pressure of the matter flow onto the magnetosphere produces a shrinkage of the cavity. Therefore, the UV flux should increase as the size of the magnetospheric cavity decreases. The magnetospheric radius can be calculated for a small subset of the stars in Table 2. Surface magnetic fields have been measured for T Tau, GM Aur, BP Tau, DF Tau, DE Tau, DN Tau, and GK Tau (see Johns-Krull 2007). In Figure 16, the ˆfC iv versus rmag is represented; the values used for rmag calculation are provided in Table 6. The C iv line has been selected for this plot because it is expected to be the least affected by the contribution of unresolved jets and disks (Gómez de Castro & Ferro-Fontán 2005). As shown in the figure, typical magnetospheric radii are between 5 and 12 solar radii; thus, the TTS magnetospheres do not seem to reach the corotation radius. However, these values of rmag are comparable with the radius estimated for the ion belt detected around RW Aur: 2.7–6.1 R∗ (see Gómez de Castro & Verdugo 2003) as well as the radius of the gas disk traced by the CO observations.

with

Bt Δr Bp r in terms of the mean azimuthal pitch Bt /Bp in the region of radial width Δr at the inner edge of the Keplerian disk, where the stellar magnetosphere more strongly interacts with the accretion flow. Thus, 1/7 γ 2 μ4 rmag = 2 GM∗ Mac γ =

13



Figure 15. Illustration of the inner disk and magnetospheric structure. (A color version of this figure is available in the online journal.) Table 6 Parameters of the Stars for rmag Calculationa Star

B∗ (kG)

M∗ (M )

R∗ (R )

T Tau

2.37

2.30 (2.11)

3.31 (3.3)

0.9+1.2 −0.6

Lac (L )

GM Aur

2.22

0.70 (1.22)

1.78 (1.48)

0.18+0.21 −0.16

BP Tau

2.17

0.70 (0.77)

1.99 (1.91)

0.23+0.29 −0.20

DE Tau

1.12

0.32 (0.66)

2.45 (3.15)

0.16+0.16 −0.05

DN Tau

2.0

0.51 (0.70

2.09 (2.06)

0.04+0.07 −0.01

GK Tau

2.28

0.69 (0.76)

2.15 (2.05)

0.06+0.08 −0.02

Note. a B∗ , M∗ , and R∗ have been selected from Johns-Krull (2007). R∗ is derived from the luminosity and the effective temperature of the stars. Values of M∗ and R∗ in brackets correspond to Johns-Krull & Gafford (2002).

log(F (C iv)/Fbol ) = (−0.29±0.06)rmag +(4.67±0.53), with rms = 0.262. One might think that this correlation is biased by the normalization by Fbol given that T Tau, a K0-type TTS, is the star with the largest ˆfC iv , while DN Tau and DK Tau, with the lowest fluxes, are M0 type. However, there is also a correlation between the C iv flux (fC iv ) and rmag , as shown in the bottom panel of Figure 15. To summarize, we conclude that ˆfC iv ∝ exp(−αrmag ), with α = 0.5–0.7. The correlation coefficient between these two magnitudes is −0.829. Finally, note that if the average line emissivity is assumed to be similar in all TTSs, the normalized flux could be used as a tracer of the thickness of the UV radiating region. Let us assume that the UV radiation is dominated by an extended atmospheric envelope, then the expression above points to a shrinkage of the envelope by two orders of magnitude ∝ exp(−αrmag ) along the spectral sequence, since ˆfl = Fl /Fbol ∝ χl Hl . Otherwise, if the UV radiation were dominated by dissipative processes in the star–disk shear layer, then Fl ∝ R∗2 exp(−αrmag ).

Figure 16. Top panel: ˆfl = log(FC iv /Fbol )∗ −log(FC iv /Fbol )FPTau is represented vs. the magnetospheric radius for the TTSs with measured magnetic fields. Bottom panel: C iv flux vs. magnetospheric radius for the same sample; note that the trend is clearly apparent even for non-normalized fluxes.

A trend is clearly apparent with larger ˆfl corresponding to smaller rmag as otherwise expected. Only DE Tau with a magnetic field of 1 kG, about half of the ∼2 kG fields reported for the rest of the stars in the sample, significantly deviates from it. The regression lines represented in Figure 16 are: log(F (C iv)/Fbol ) = (−0.23 ± 0.06)rmag + (3.96 ± 0.53), including DE Tau and excluding DE Tau is

6. SUMMARY The joint analysis of the fluxes of the O i (1305 Å), C ii (1335 Å), C iv (1550 Å), and He ii (1640 Å) lines and the 14



X-ray flux of the TTSs has driven us to the following conclusions:

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1. There is a power-law scaling among the X-rays and UV-normalized fluxes that it is not associated with the mass of the star or its spectral type. 2. The precise power-law scaling depends on the tracers considered, and it is different than the observed in MSCSs. 3. High-energy tracers such as the He ii line or the X-ray flux are only observed in roughly one-third of the TTSs. 4. There are evidences of the flux of low-ionization species and, in particular, the O i flux being polluted by recombination processes on the disk atmosphere. The Fe ii/Mg ii recombination continuum has also been detected in several TTSs, most prominently in AA Tau. 5. The normalized flux of the UV tracers anticorrelates with the strength of the X-ray flux; the stronger the X-ray surface flux is, the weaker the observed UV flux. This last behavior is counterintuitive within the framework of stellar dynamo theory and suggests that UV emission can be produced in the extended and dense stellar magnetosphere driven directly by local collisional processes. UV fluxes are only available for one brown dwarf, 2MASS J12073346-3332539, which follows the same flux–flux relations of the TTSs. Finally, we report the discovery of an inverse correlation between the C iv-normalized flux and the magnetospheric radius derived for stars with known magnetic fields. The normalized C iv flux is found to be ∝ exp(−αrmag ), with α = 0.5–0.7. This result can be interpreted as evidence of shrinkage of the UV emission region as the magnetospheric radius increases. A.I.G. acknowledges the support from the Spanish Ministry of Science and Innovation through the grant AYA2008-06423C03-01. Facilities: HST APPENDIX SAMPLE PROPERTIES The main properties of the stars in this sample have been selected from various sources and compiled in Table 2 for reference. Spectral types are compiled from Furlan et al. (2006). Stellar luminosities have been taken from Ingleby et al. (2009) and compared with those obtained by Bertout et al. (2007). Bertout et al. (2007) derived the stellar luminosities from the IC flux because the contribution of accretion is the smallest; bolometric corrections were taken from Kenyon & Hartmann (1995). We have found average discrepancies between both sets of values of ∼32% ± 22%, with typically higher values in Ingleby et al. (2009). As for the analysis of the data, accretion luminosities from Ingleby et al. (2009) will be used. The main source for extinctions is also Ingleby et al. (2009) that adopted Kenyon & Hartmann (1995) values. We have cross-checked the AV values with the hydrogen column densities required to fit the X-ray spectrum of the sources in the XEST survey (Guedel et al. 2007); there is a very broad scatter with no obvious correlation (the best fit is nH (cm−2 ) = [(2.3 ± 1.6)AV + (0.1 ± 2.3)] × 1021 with rms = 0.485).

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