l153 a subarcsecond binary radio source associated with ... - IOPscience

The Astrophysical Journal, 544:L153–L156, 2000 December 1 q 2000. The American Astronomical Society. All rights reserved. Printed in U.S.A.

A SUBARCSECOND BINARY RADIO SOURCE ASSOCIATED WITH THE X-RAY–EMITTING YOUNG STELLAR OBJECT YLW 15 Jose´ M. Girart,1 Luis F. Rodrı´guez,2 and Salvador Curiel3 Received 2000 April 11; accepted 2000 September 26; published 2000 November 17

ABSTRACT YLW 15 is a remarkable object since it exhibits phenomena, such as strong millimeter emission and association with a bipolar outflow, that characterize extremely young stars (class 0 or I objects) while at the same time presenting strong, time-variable X-ray emission that is ubiquitous and detected characteristically in more evolved objects. Our Very Large Array observations reveal that YLW 15 is a subarcsecond (00. 6) radio binary, with one of the components spatially extended and the other unresolved. We discuss the possibility that the components of the binary system may have different characteristics also at other wavelengths, possibly as a result of different evolutionary status, and discuss future observations that may test this hypothesis. Subject headings: ISM: individual (YLW 15) — ISM: jets and outflows — stars: formation — X-rays: ISM 1996; Shu et al. 1997). At the present time, the detection of X-rays from class I sources is rare (as compared with its ubiquity in the more evolved YSOs), and this could be due to their intrinsic average lower L X or, alternatively, because of the significant absorption of the soft X-rays by the dense envelope surrounding these objects (Carkner, Kozak, & Feigelson 1998). Indeed, CO observations toward some of the X-ray–emitting sources show that the molecular outflows are compact and the red and blue lobes are largely overlapped, which indicates a nearly pole-on configuration, i.e., the geometrical configuration that minimizes the strong absorption by the dense envelope (Sekimoto et al. 1997). On the other hand, by analyzing the properties of four of these molecular outflows, Sekimoto et al. (1997) found that the X-ray–emitting protostars are in the late class I stage. YLW 15 (IRS 43) is an infrared protostar with a bolometric luminosity of ∼10 L, (Young, Lada, & Wilking 1986; Wilking, Lada, & Young 1989) embedded in the L1681B molecular cloud within the nearby (160 pc) r Oph molecular cloud complex. It has been classified as a very young, class I object (Andre´, WardThompson, & Barsony 1993) on the basis of its infrared spectral energy distribution and its association with a compact bipolar outflow (Bontemps et al. 1996). As expected for this kind of object, it is surrounded by a dense, compact dusty envelope with a mass of 0.05–0.3 M, (Andre´ & Montmerle 1994; Motte, Andre´, & Neri 1998). Its radio continuum emission has been well studied but at a moderate angular resolution (∼100) showing no variability (Andre´, Montmerle, & Feigelson 1987; Leous et al. 1991). VLBI milliarcsecond angular resolution observations of YLW 15 show that the emission is resolved out at “peristellar” scales (i.e., its size is larger than 1.2 # 10 13 cm), suggesting that the emission is thermal in origin and probably comes from circumstellar ionized winds (Andre´ et al. 1992). Most interesting is its X-ray emission, first detected by ROSAT (Casanova et al. 1995), which shows quasi-periodic X-ray flares, with an X-ray luminosity of 1032 ergs s21 (Tsuboi et al. 2000). A “superflare” with an X-ray peak luminosity of *1034–1036 ergs s21, or L X ∼ 10–1000 L ,, was observed during a few hours (Grosso et al. 1997). Thus, the X-ray luminosity during the “superflare” was larger than the stellar luminosity of YLW 15. Grosso et al. (1997) suggested that the flare arises from a magnetically confined, low-density plasma bubble with a diameter of ∼0.05–0.3 AU around the young star. Because of its classification as a very young star, with an age of roughly 105 yr, YLW 15 has been

1. INTRODUCTION

The evolutionary stages of young stellar objects (YSOs) can be estimated from the spectral energy distribution in the IR wavelengths (e.g., Lada 1991). Thus, class 0, the youngest YSOs, and class I sources are believed to be “true” protostars, which have a circumstellar disk of ∼100 AU and a dense envelope of *103 AU associated with them and often power molecular outflows and Herbig-Haro outflows. Class II sources are the classical T Tauri stars (CTTSs), which also have a well-defined circumstellar disk but without a massive envelope. Finally, class III sources are weak-lined T Tauri stars (WTTSs) that have little or no disk material. CTTSs and WTTSs have nearly ubiquitous X-ray emission associated with them, with X-ray luminosities of L X ∼ 1024 to 1023 L , (e.g., Feigelson & Montmerle 1999; Glassgold, Feigelson, & Montmerle 2000). It has been suggested that in general CTTSs emit fewer X-rays than WTTSs do (Neuha¨user 1997). However, Stelzer, Neuha¨user, & Hambaryan (2000) have found some contradicting evidence. The mechanism that produces the X-rays in these objects is believed to be coronal solar-like magnetic activity but much more luminous than that of the Sun because of the objects’ faster rotation (e.g., Neuha¨user 1997). Because of the anchoring effect of the stellar magnetic field with the disk magnetic field, CTTSs have slower rotation than WTTSs, which implies a weaker coronal X-ray intensity of the CTTSs with respect to the WTTSs (e.g., Ko¨nigl 1991). Nevertheless, even though in a long-term average the spin rate of the star equals that of the inner edge of the disk, the complex magnetic field distribution of the disk-star system oscillates, which triggers enhanced X-ray flaring; i.e., this mechanism also contributes to the X-ray emission, particularly in hard X-rays (Shu et al. 1997). On the other hand, the dozen class I sources that emit X-ray emission tend to have higher LX and harder spectra than the T Tauri stars (e.g., Casanova et al. 1995; Koyama et al. 1996). This has been proposed to be due to the larger accretion rate of class I sources and that for these sources the reconnection events caused by the star-disk differential rotation dominate the X-ray luminosity (Hayashi, Shibata, & Matsumoto 1 Department of Astronomy, University of Illinois at Champaign-Urbana, 1002 West Green Street, Urbana, IL 61801. 2 Instituto de Astronomı´a, Universidad Nacional Auto´noma de Me´xico, Campus Morelia, Apdo. Postal 3-72 (Xangari), Morelia, Michoaca´n, 58089, Mexico. 3 Instituto de Astronomı´a, Universidad Nacional Auto´noma de Me´xico, Apdo. Postal 70-264, Me´xico, DF, 04510, Mexico.

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Radio Astronomy Observatory4 at 3.6 cm on 1989 May 5. The phase center was located at the position of the nearby source s YLW 16, a(J2000) p 16 h 27 m 28.0 , d(J2000) p 224739 0 330. 0. The phase calibrator was 16262298, and the absolute amplitude calibrator was 3C 286. The data were edited and calibrated following the standard VLA procedures with AIPS. Maps were done using robust weighting of 0.5, which yielded a synthesized beam of 00. 51 # 00. 21, 1237. The achieved rms noise of the map was 17 mJy beam21. 3. RESULTS AND DISCUSSION

Figure 1 shows the subarcsecond resolution map of the 3.6 cm continuum emission of YLW 15. The emission clearly shows two distinct objects separated by ∼00. 6, which at the distance of r Oph (160 pc) implies a projected distance of only ∼100 AU. Table 1 shows the fluxes and deconvolved sizes of the two sources obtained by Gaussian fitting. The deconvolution takes into account the synthesized beam dimensions and approximately corrects for the effects of bandwidth smearing. The expected bandwidth smearing (resulting from YLW 15 not being at the phase center of the observations) at the position of YLW 15 is ∼00. 5, with a position angle (P.A.) p177. VLA 1 is clearly resolved, although with a somewhat curved shape, in the southeast-northwest direction, with a deconvolved major axis of ∼130 AU. The molecular outflow associated with YLW 15 is compact, with the blueshifted and redshifted lobes almost overlapping (Bontemps et al. 1996), so it is not possible to derive its axis of symmetry and compare it with that of the radio emission. As noted by Sekimoto et al. (1997), if this were a pole-on outflow, the detection of X-ray emission would be easier since less gas and dust would be expected toward the source. On the other hand, VLA 2 is consistent with an unresolved (less than 00. 4) source. In order to compare the total flux density of the binary radio sources with previous observations, a lower angular resolution map was made by applying a Gaussian taper of 400 kl to the visibility data. The total flux density measured was 3.2 5 0.1 mJy, which is fully consistent with previous measurements at this wavelength (Andre´ et al. 1992). No variability is seen at 6 cm either (Andre´ et al. 1987; Stine et al. 1988; Leous et al. 1991), which strongly suggests that neither of the two radio sources we detect is a strongly variable source. Using all the existing data at centimeter wavelengths (3.6, 6, and 20 cm), we derive a spectral index of 0.1 5 0.1 for VLA 1 1 VLA 2. Because of the morphological resemblance, VLA 1 is probably a thermal radio jet, which suggests that it could be the powering source of the molecular outflow (all known thermal radio jets are associated with molecular outflows and/or Herbig-Haro outflows). The thermal radio jets have spectral indices typically in the 0.2–1.5 range

Fig. 1.—Subarcsecond angular resolution map at 3.6 cm of YLW 15. Contours are 23, 3, 5, 7, 9, …, and 25 times the rms noise of the map, 17 mJy beam21. The half-power contour of the synthesized beam is shown in the bottom left corner. The image has not been deconvolved for beam pattern.

proposed to be representative of the youngest low-mass stars detectable in X-rays (Grosso et al. 1997). The quasi-periodic X-ray flares of YLW 15 were explained by Montmerle et al. (2000) as due to “fast rotation of the central star with respect to the accretion disk, which results in star-disk shearing of the magnetic lines, producing magnetic reconnection and mass loss and eventually extremely high X-ray luminosities.” This situation is somewhat new and different from the “classical” quasi–steady state situation expected for the protostars and T Tauri stars. Montmerle et al. (2000) suggests that the situation observed in YLW 15 precedes that from the protostars and T Tauri stars, and we refer the reader to this paper for a detailed discussion of rotation and X-ray production. In this Letter, we present deep subarcsecond angular resolution Very Large Array (VLA) observations at 3.6 cm toward this very young class I object that shows that it actually is a radio binary and suggest new observations that may provide a deeper understanding of its nature. 2. OBSERVATIONS

Sensitive radio continuum interferometric observations were carried out with the 27 antennas of the VLA of the National

4 NRAO is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.

TABLE 1 YLW 15: Parameters of the Radio Emission Fluxa Source

a(J2000)

d(J2000)

Ipeak (mJy beam21)

Stotal (mJy)

Deconvolved Sizeb (arcsec)

P.A. (deg)

VLA 1 . . . . . . VLA 2 . . . . . .

16 27 26.913 16 27 26.936

224 40 50.14 224 40 50.63

0.62 5 0.02 0.30 5 0.02

1.78 5 0.07 0.79 5 0.07

(0.82 5 0.05) # (0.18 5 0.02) &0.4

25 5 2 …

Note.—Units of right ascension are hours, minutes, and seconds, and units of declination are degrees, arcminutes, and arcseconds. a Fluxes corrected for primary beam response. b Corrected for bandwidth smearing.

No. 2, 2000

GIRART, RODRI´GUEZ, & CURIEL

(e.g., Anglada 1996), suggesting that VLA 2 likely has a flat or negative spectrum. A relationship between the radio and X-ray fluxes is well established for a large variety of active coronal stars (e.g., Gu¨del & Benz 1993 and references therein). Gu¨del & Benz (1993) found a correlation between the X-ray and the quiescent non-thermal radio luminosities: log (L X/ergs s21) . log (L R / ergs s21 Hz21) 1 15.5. These results hold over many orders of magnitude and for many classes of stars (note, however, that Smith, Gu¨del, & Benz 1999 found that the more luminous sources are systematically less X-ray bright compared to their radio luminosity). Some of these stars exhibit a nearly flat or negative spectrum at 8 GHz (e.g., Fox et al. 1994; Gu¨del & Benz 1996), as it is expected for VLA 2. This correlation is purely observational, and although it suggests that the heating mechanism that generates the X-ray emission is also responsible for accelerating the nonthermal electrons that emit the synchrotron radio continuum (Gu¨del & Benz 1996), the physical mechanism behind this correlation is not very well understood. Nevertheless, it can still be used to roughly check if the observed parameters are consistent with VLA 2 being associated with the X-ray source. The VLA 2 radio luminosity at 3.6 cm is 2 # 10 16 ergs s21 Hz21. Thus, if it has a peristellar origin, then the expected X-ray luminosity will be ∼8 # 10 31 ergs s21. This value is comparable with the X-ray luminosity range measured by Tsuboi et al. (2000): (3–20) # 10 31 ergs s21. In addition, our results on VLA 2 do not exclude that its radio emission has a peristellar origin; Andre´ et al. (1992) did not detect the radio emission from milliarcsecond angular resolution VLBI observations, which led them to suggest that the emission is not arising at peristellar scales (*1.2 # 10 13 cm), where strong, organized magnetic structures generate gyrosynchrotron radiation from mildly relativistic electrons, as observed in other strong radio sources in r Oph, and instead the emission should be the result of thermal freefree emission. However, our detection of a double radio source suggests an alternative interpretation. First, VLA 1 appears resolved at scales of 130 AU, suggesting that the emission is thermal free-free. On the other hand, VLA 2 appears unresolved, but the VLBI sensitivity, with an upper limit of the correlated flux of &1.0 mJy beam21 (Andre´ et al. 1992), was not enough to detect this source (its flux density is ∼0.8 mJy). In addition, we note that the lack of strong variability is not incompatible with a nonthermal nature: the radio continuum emission of SVS4/EC95, another very strong X-ray YSO, shows marginal variability by only ∼20% (Smith et al. 1999). The available VLA observations of YLW 15 were not sensitive enough to detect variability at this level for VLA 2. In summary, for YLW 15 we propose a scenario consisting of a binary system, with VLA 1, a protostar (type class 0 or I), powering the molecular outflow, being less evolved than VLA 2, the energy source of the X-ray emission. The observational evidence for such a scenario, although inconclusive, is the following: the morphological resemblance of VLA 1 to the thermal radio jets suggests that this source is possibly tracing the powering source of the molecular outflow, i.e., a protostar (class 0 or I). If so, then it is quite plausible that the VLA 2 source is nonthermal and therefore is tracing a more evolved YSO. Although the system was most probably formed coevally, differences in mass will lead to different evolutionary stages. This possible scenario gives a new explanation for the characteristics of YLW 15 and in particular accounts for the “hybrid” nature (in the sense that it simultaneously exhibits phenomena that are characteristic of different stellar evolutionary

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stages) of its observational parameters. Several close binaries with components in apparently different evolutionary stages are known, constituting the class of binaries with infrared companions (e.g., Koresko, Herbst, & Leinert 1997). Recently, Anglada, Rodrı´guez, & Torrelles (2000) have found that SVS 13 is also a subarcsecond radio binary and suggested that the components could be in different evolutionary stages. If VLA 1 is a protostar powering the molecular outflow, what type of object is VLA 2? Other radio sources in r Oph with a well-known peristellar origin (Andre´ et al. 1991, 1992; Phillips, Lonsdale, & Feigelson 1991) are class III sources, and all of them have X-ray emission (Casanova et al. 1995). However, their radio emission is highly variable (e.g., Andre´ 1996) and they are diskless. Therefore, it is unlikely that VLA 2 belongs to this class: to explain the YLW 15 X-ray properties the presence of a disk is required (Montmerle et al. 2000). On the other hand, class II sources also have ubiquitous X-ray emission, but their luminosity is significantly lower than YLW 15, ∼1030 ergs s21 (e.g., Preibisch, Neuha¨user, & Stanke 1998 and references therein). Montmerle et al. (2000) show that the YLW 15 X-ray properties are well fitted by a model with a central star rotating faster than the accretion disk, a situation that should precede the class II or even in some cases the class I stage. But on the other hand, our results suggest that VLA 2 is possibly in a stage that follows the strong outflowing phase, when the protostars power molecular outflows. Only a dozen protostars have associated X-ray emission with luminosities in the 1030–1031 ergs s21 range, except YLW 15 and SVS 63 (Ozawa et al. 1999), which have X-ray luminosities an order of magnitude higher. This implies, as suggested by Montmerle et al. (2000), that the time range for the luminous X-ray phase observed in YLW 15 and SVS 63 should be small—less than a few times 105 yr. It is important to remark that we cannot rule out the possibility that the VLA 2 emission is thermal free-free radiation from a partially ionized wind, as in VLA 1. If this is the case, then the peristellar gyrosynchrotron expected from the X-ray–emitting YSO may be absorbed by the free-free opacity (e.g., Martin 1996). For this scenario, further observations will be required to elucidate which of the two sources, VLA 1 or 2, is associated with the X-ray emission. On the other hand, Montmerle et al. (2000) proposed that the collimated hot coronal winds, a consequence of the magnetic shearing and reconnection due to the star-disk differential rotation, may contribute or even be responsible for the radio continuum emission associated with the protostars. This would imply that VLA 1 may equally be the X-ray source. However, if so, it will be difficult to explain the correlation found between the radio luminosity and the momentum rate of the outflows (e.g., Anglada 1996) and, in addition, one should expect the X-ray emission to be associated with the centimeter emission (which is almost ubiquitous in protostars: Anglada et al. 1998), which so far is not observed (although the high column densities of gas and dust could be absorbing the X-rays). Besides, in some sources it has been possible to observe spectroscopically in the optical the origin of the jets (e.g., Bacciotti, Eislo¨ffel, & Ray 1999), and the observed low-ionization species ([O i], [S ii], [N ii]) and derived electron temperature (104 K) are not suggestive of the presence of a significant amount of coronal gas in the outflow. In summary, our detection of a binary system in YLW 15 does not allow us to strongly constrain the physical conditions of this interesting X-ray source but rather suggests a range of possible scenarios that could be explored in the future. Clearly,

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additional observations are required to determine more firmly the nature of this source. Multifrequency subarcsecond angular resolution observations will allow us to establish the nature of both VLA 1 and VLA 2. A detailed VLA study will establish the centimeter characteristics of the sources. Sensitive VLBI observations will elucidate whether or not the radio emission from VLA 2 arises from peristellar scales, in which case it would likely be associated with the observed X-ray emission. Future subarcsecond observations of the dust emission at millimeter and infrared wavelengths could establish whether one or both objects have dust associated with them and if additional

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sources are present in the zone. Finally, high angular resolution X-ray observations will address if the high-energy emission is coming from one or both sources. We thank the referee, Thierry Montmerle, for valuable comments. J. M. G. is grateful for the hospitality at the Instituto de Astronomı´a-UNAM, where most of this work was carried out. J. M. G. is supported by NSF grant AST 96-13999. L. F. R. and S. C. acknowledge support from DGAPA, UNAM, and CONACyT, Me´xico.

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