The Astrophysical Journal Letters, 720:L190–L194, 2010 September 10 C 2010.
doi:10.1088/2041-8205/720/2/L190
The American Astronomical Society. All rights reserved. Printed in the U.S.A.
COSMIC ORIGINS SPECTROGRAPH OBSERVATIONS OF TRANSLUCENT CLOUDS: Cyg OB2 8A∗ Theodore P. Snow1 , Joshua D. Destree1 , Eric B. Burgh1 , Ryan M. Ferguson1 , Charles W. Danforth1 , and Martin Cordiner2 1
Department of Astrophysical and Planetary Sciences, Center for Astrophysics and Space Astronomy, University of Colorado at Boulder, Campus Box 389, Boulder, CO 80309-0389, USA;
[email protected],
[email protected],
[email protected],
[email protected],
[email protected] 2 The Goddard Center for Astrobiology, NASA Goddard Space Flight Center, Code 691, 8800 Greenbelt Road, Greenbelt, MD 20771, USA;
[email protected] Received 2010 June 22; accepted 2010 July 27; published 2010 August 20
ABSTRACT Data from the Cosmic Origins Spectrograph (COS) are presented for the first highly reddened target (Cyg OB2 8A) under the COS Science Team’s guaranteed time allocation. Column densities of ionic, atomic, and molecular species are reported and implications are discussed. Data from Cyg OB2 8A demonstrate the ability to analyze highly reddened interstellar sight lines with the COS that were unavailable to previous UV instruments. Measured column densities indicate that the Cyg OB2 8A line of sight contains multiple diffuse clouds rather than a dominant translucent cloud. Key words: dust, extinction – ISM: abundances – ISM: atoms – ISM: molecules – ultraviolet: ISM
with the radio waves presumed to be coming from the stellar wind (Abbott et al. 1981; Bieging et al. 1989). Photometric data show the following magnitudes: V = 9.06, B = 10.64, EB−V = 1.60, and RV = 3.02. To date, this is the most heavily reddened star ever observed for moderate resolution UV spectroscopy. The following sections describe the data, analysis, and results for our study of Cyg OB2 8A.
1. INTRODUCTION The Cosmic Origins Spectrograph (COS) installed on board the Hubble Space Telescope represents a breakthrough in UV spectroscopy of astronomical sources (for more details on COS see J. C. Green et al. 2010 in preparation and S. Osterman et al. 2010 in preparation). One of the COS Science Team’s research programs involves observing interstellar absorption lines in clouds denser than the well-studied diffuse clouds, i.e., translucent clouds (Black & Van Dishoeck 1988; Snow & McCall 2006). With COS, we can probe deeper into clouds where neutral species and molecules rise at the expense of ionized species and H i. One aim of the science team’s guaranteed time allocation is to investigate the physics and chemistry of translucent clouds. These clouds are thought to represent the transition from diffuse to dense clouds, thereby informing us where and under what conditions species such as C i and CO begin to dominate. Figure 1 shows the progression in physical and chemical conditions from the diffuse interstellar medium (ISM) to molecular clouds, showing the expected relative abundances of H i, H2 , and carbon, going from C ii to C i to CO. We can observe all of these phases of carbon in the UV as well as H i. For the brighter stars, even H2 can be observed, given the COS’s higher sensitivity than expected in the far UV (McCandliss et al. 2010) The first star observed for this program is Cyg OB2 8A (BD +40 4227). This star, an eclipsing binary (Herbig 1967) with a period of 21.907 days (De Becker et al. 2004), is a member of an OB association about 1600 pc away, containing some of the most luminous and massive stars in the galaxy. The spectral type of component A, which we assume is the primary source of the UV radiation observed by the COS, is O5.5I(f) (Herrero et al. 2002). Component B is classified as an O5.5III(f) star about 2–3 mag fainter in V (Herbig 1967). This binary is a bright X-ray source (Harnden et al. 1979); the X-rays are thought to be from colliding stellar winds (De Becker et al. 2006, and references cited therein). Cyg OB2 8A is also a non-thermal radio source,
2. OBSERVATION 2.1. Far UV Observations and Data Reduction Cyg OB2 8A was observed using the COS and the G130M grating (R ≈ 18,000) as part of guaranteed time proposal 11516. Four exposures were taken using four different central wavelength settings (1291 Å, 1300 Å, 1309 Å, and 1318 Å) both to fill in the 14 Å gap between detector segments and to minimize the impact of instrumental features. A total of 5159 s of exposure time were obtained over two orbits. An average pixel to pixel signal-to-noise ratio (S/N) of 4–18 was obtained covering wavelengths 1136 Å to 1459 Å (see Figure 2). Flat-fielding, alignment, and co-addition of the processed exposures were carried out using IDL routines developed by the COS Science Team specifically for COS FUV data.3 First, the data are corrected for the most egregious instrumental features. While attempts at a true “flat-fielding” of COS data show promise, the technique is not yet robust enough to improve data of moderate S/N. However, we are able to correct the narrow, ∼15% opaque features arising from ion repellor grid wires in the detector. A one-dimensional map of grid wire opacity for each detector was shifted from detector coordinates into wavelength space and divided from the flux and error vectors. Exposure time in the location of grid wires was decreased by ∼0.7, thus giving these pixels less weight in the final co-addition. We also modified the error and local exposure time at the edges of the detector segments to de-weighted flux contributions from these regions. With four different central wavelength settings per grating, any residual instrumental artifacts from
∗ Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the Data Archive at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.
3 See http://casa.colorado.edu/∼danforth/costools.html for our co-addition and flat-fielding algorithm and additional discussion.
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Figure 1. Figure from Snow & McCall (2006) showing the progression in physical and chemical conditions from the diffuse ISM to molecular clouds and the expected relative abundances of H i, H2 , and carbon. The orange region overlaid represents the region now observable with the COS.
grid wire shadows and detector segment boundaries should have negligible effect. One of the four exposures was picked as a wavelength reference, and the remaining exposures cross correlated with it. The wavelength region of cross-correlation for each case was picked to include strong ISM absorption features, and shifts were typically on the order of a resolution element or less (∼17 km s−1 ). Next, the aligned exposures were interpolated onto a uniform wavelength grid and co-added. The flux at each position was taken to be the exposure-weighted mean of flux in each exposure. Since exposure time was reduced in certain wavelength locations as noted above, pixels near detector edges and where grid wire shadows were removed received less weight than those in less suspect locations. 2.2. Near-UV Observations and Data Reduction Near-ultraviolet (NUV) spectra of Cyg OB2 8A were obtained by G. Herbig (2004, private communication) using the Keck HIRES echelle spectrograph over the wavelength range ∼3100–4100 Å at a resolving power of 52,000 (see Cordiner & Sarre 2007 for additional details). Reductions were carried out using standard iraf echelle routines, including cosmic ray removal, flat-fielding, scattered light subtraction, wavelength calibration (including shifting of the spectra to the LSR frame), and continuum rectification. The signal-to-noise of the final coadded spectra ranges from 100 to 300 per pixel. The wavelength drift between successive arc exposures bracketing the observations was 0.005 Å, so absolute wavelength calibration of the spectra should be accurate to within ±0.4 km s−1 . 3. DATA ANALYSIS All atomic, ionic, and molecular column densities were determined through profile fitting using the minimum number of cloud components necessary to satisfactorily match the observed profile. Individual cloud components were modeled assuming a Voigt profile with column density N, line width b, and central velocity v. These parameters were fit independently in each species. The resulting profiles were convolved with a line spread function appropriate to the given instrument. For the COS, we
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Figure 2. Plots of COS UV spectral data for Cyg OB2 8A.
used the on-orbit empirical instrumental line spread function characterized by Ghavamian et al. (2009). In the analysis of COS data, the continuum was fit simultaneously with the absorption profiles using a low order polynomial (between 1 and 3) whenever possible. For all chi-square minimization fitting to COS data, we used the non-linear leastsquares curve fitting algorithm MPFIT, by C. Markwardt,4 to find the best-fit parameter values. MPFIT is a set of routines that uses the Levenberg–Marquardt technique to minimize the square of deviations between data and a user-defined model (Markwardt 2009). These routines are based on the MINPACK1 Fortran package by Mor`e and collaborators.5 A thorough analysis and discussion of the reliability of profile fits to COS data is beyond the scope of this short letter. Such an analysis of the reliability of best fit column densities and errors will be undertaken in a future work (J. D. Destree et al. 2010, in preparation). Atomic and ionic column density errors were estimated from the errors returned from the MPFIT routines and also by observing how sensitive the best fit column density was to changes in initial fit parameters. NUV Spectra were fitted with Voigt-profile interstellar line models using the vapid code (Howarth 2002). Error estimates on measured column densities are the ±1σ ranges of models refitted to 100 replicated spectra of each line. For each replication, random Poisson noise was added to the spectrum, with rms equal to that of the local continuum noise. In all cases, where more than one spectral line was available for a given species, the same cloud component model was fitted simultaneously to all available lines to maximally constrain the profile fit. The transitions used in the present study were taken from several previous studies and compilations (Black & Van Dishoeck 1988; Carrington & Ramsay 1982; Destree et al. 2010; Jenkins & Tripp 2001; Larsson & Siegbahn 1983; Morton 1991; Morton & Noreau 1994; Morton 2003) Observations of neutral carbon species, specifically C i and CO, can be used as indicators of the presence of translucent material along a given line of sight (Burgh et al. 2010). Both of these species are observed in the COS data. Five bands of the CO Fourth Positive (A–X) band system, from the (3–0) to the (7–0) are detected in the data with good signal-to-noise. Lower-lying bands were not covered by the G130M grating and higher-lying bands were too weak to detect. 4 5
http://purl.com/net/mpfit http://www.netlib.org/minpack
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Figure 3. Plots of spectral data with best-fit model overlaid (bold line) for CH+ , Ti ii, N i and Mn ii, and CO. Velocities for N i are based on the feature at 1200.22 Å. CO velocities are relative to the R(0) line in each band.
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Table 1 Best-fit Total Column Densities and Calculated Depletions Species Hi Ni Oi Na i Mg ii Si Ki Ca ii Ti ii Mn ii Fe i Ni ii CH CH+ CN CO
Instrument
log (N)
COS COS COS HIRES COS COS HIRES HIRES HIRES COS HIRES COS
21.95: 17.5 ± 0.5 18.6 ± 0.3 13.42 ± 0.01 16.8 ± 0.4 14.5 ± 0.3 12.70 ± 0.03 13.46+0.07 −0.05 12.60 ± 0.01 14.3 ± 0.4 12.34+0.01 −0.02 14.0 ± 0.2
HIRES HIRES HIRES COS
13.51+0.03 −0.04
Depletion −0.3 −0.1 −0.8 −2.9 −2.3 −1.2 −2.2
14.07 ± 0.01 12.36+0.03 −0.02 14.9 ± 0.5
Although the resolving power of COS does not allow for the resolution of the individual rovibrational lines in each band, we used a line-profile fitting technique to determine the best-fit physical parameters. Assuming a single velocity component, a grid of models of various total column density, N(CO), Doppler broadening parameter, b, and rotational excitation temperature, Trot , were generated. Each model was convolved with the COS line spread function (Ghavamian et al. 2009), compared to the data and a chi-square value calculated. 4. RESULTS Table 1 shows the best-fit column densities and associated standard errors for all atomic, ionic, and molecular species measured. Figure 3 shows a few example profile fits to atomic, ionic, and molecular lines. One goal of the COS cold ISM guaranteed time program is to analyze the chemical and physical conditions of clouds and sight lines that could not be analyzed realistically with previous UV instruments. One way that we can make inferences as to the physical conditions of the clouds in this line of sight is by calculating how greatly various elements have been depleted out of the gas phase. To do this, we need both measurements of standard abundances, and an estimate of the total hydrogen column density. For all standard abundances, we used the solar system abundances as compiled by Lodders (2003). Since no data exist for Cyg OB2 8A that cover the molecular hydrogen bands in the Far UV, we must estimate the H2 column density in another manner. The most reliable proxy for H2 available in the present study is CH (the column densities of CH and H2 are known to be well correlated). Using the correlation with CH from Sheffer et al. (2008), we estimate that log NH2 ≈ 20.96. From this, we estimate that the total hydrogen column density is ∼1022 . Elemental depletions were calculated for all species where the column density of the dominant ionization state of the atom was measured (see Table 1). Comparing these values to standard diffuse ISM values (Jenkins 2009), we find that the elemental depletion pattern for Cyg OB2 8A is fairly consistent with previously studied diffuse clouds. All five bands of CO were fit simultaneously, and the minimum chi-square produced the following best-fit parameters: log N(CO) = 14.9 ± 0.5, b = 3.4 ± 1.4, and Trot < 4.6 (1σ upper limit). A determination of the column density for C i was attempted; however, due to the lack of knowledge of any unre-
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solved velocity structure, we were unable to obtain a reliable C i column density. The previously mentioned estimate of the H2 column density results in a molecular fraction of 0.17 and CO/H2 = 8.6 × 10−7 . We conclude from this that the gas along this line of sight is consistent with the diffuse ISM and does not contain any significant amount of translucent material (see Figure 2 of Burgh et al. 2010). 5. DISCUSSION AND FUTURE WORK This is the first Letter reporting results of the COS Science Team’s translucent interstellar cloud program. The target star, Cyg OB2 8A, is the most heavily reddened star yet observed with a moderate resolution UV spectrograph. Perhaps the most important result of this first observation is the clear demonstration that heavily reddened hot stars can realistically be observed in the UV using the COS, a feat that was almost impossible with previous UV instruments. As expected from the target’s large reddening, the total column densities of most neutral species, as well as molecules, are higher than those seen in previously studied diffuse-cloud sight lines. In particular, the total hydrogen column density is over 1022 cm−2 . The CO/H2 ratio is normal for diffuse clouds, within the uncertainties. The depletions are compatible with diffuse clouds, but not as compatible with the expected values for translucent clouds. There are some hints of complex structure in the COS absorption lines, and the HIRES NUV spectra show evidence for multiple components in the atomic and molecular absorption lines, indicating the presence of multiple clouds along the line of sight. We conclude that this star lies behind several diffuse cloud components, not a single translucent cloud. In a later paper, we will join our COS data with observations from telescopes in several other spectral regions from visible to infrared to radio. This Letter has been the first in a series on heavily reddened clouds observed with the COS and ground-based telescopes. The data described here will make a useful standard for comparisons with true translucent cloud sight lines. This research was supported by NASA grant NNX08AC14G. We thank the other members of the COS Science Team for developing data co-addition techniques. We thank Kevin France for his helpful comments, George Herbig for providing the raw Keck HIRES data, and Rachel Destree for her constant editing help. Facilities: HST (COS) REFERENCES Abbott, D. C., Bieging, J. H., & Churchwell, E. 1981, ApJ, 250, 645 Bieging, J. H., Abbott, D. C., & Churchwell, E. 1989, ApJ, 340, 518 Black, J. H., & Van Dishoeck, E. F. 1988, ApJ, 331, 986 Burgh, E. B., France, K., & Jenkins, E. B. 2010, ApJ, 708, 334 Carrington, A., & Ramsay, D. A. 1982, Phys. Scr., 25, 272 Cordiner, M. A., & Sarre, P. J. 2007, A&A, 472, 537 De Becker, M., Rauw, G., Sana, H., Pollock, A. M. T., Pittard, J. M., Blomme, R., Stevens, I. R., & van Loo, S. 2006, MNRAS, 371, 1280 De Becker, M., et al. 2004, Ap&SS, 297, 291 Destree, J. D., Williamson, K. E., & Snow, T. P. 2010, ApJ, 712, L48 Ghavamian, P., et al. 2009, COS Instr. Sci. Report 2009-01 (Baltimore, MD: STScI), http://www.stsci.edu/hst/cos/documents/isrs/ISR2009_ 01(v1).pdf Harnden, F. R., et al. 1979, ApJ, 234, 51 Herbig, G. H. 1967, Cont. Lick Obs. No., 245 Herrero, A., Puls, J., & Najaro, F. 2002, A&A, 396, 949
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