Martin A. Bitner

A SPITZER MIPS STUDY OF 2.5-2.0 M ☉ STARS IN SCORPIUS-CENTAURUS

We have obtained Spitzer Space Telescope Multiband Imaging Photometer for Spitzer (MIPS) 24 μm and 70 μm observations of 215 nearby, Hipparcos B- and A-type common proper-motion single and binary systems in the nearest OB association, Scorpius-Centaurus. Combining our MIPS observations with those of other ScoCen stars in the literature, we estimate 24 μm B+A-type disk fractions of 17/67 (25+6 – 5%), 36/131 (27+4 – 4%), and 23/95 (24+5 – 4%) for Upper Scorpius (~11 Myr), Upper Centaurus Lupus (~15 Myr), and Lower Centaurus Crux (~17 Myr), respectively, somewhat smaller disk fractions than previously obtained for F- and G-type members. We confirm previous IRAS excess detections and present new discoveries of 51 protoplanetary and debris disk systems, with fractional infrared luminosities ranging from L IR/L * = 10–6 to 10–2 and grain temperatures ranging from T gr = 40 to 300 K. In addition, we confirm that the 24 μm and 70 μm excesses (or fractional infrared luminosities) around B+A-type stars are smaller than those measured toward F+G-type stars and hypothesize that the observed disk property dependence on stellar mass may be the result of a higher stellar companion fraction around B- and A-type stars at 10-200 AU. Finally, we note that the majority of the ScoCen 24 μm excess sources also possess 12 μm excess, indicating that Earth-like planets may be forming via collisions in the terrestrial planet zone at ~10-100 Myr.

The Masses and Evolutionary State of the Stars in the Dwarf Nova SS Cygni

The dwarf nova SS Cygni is a close binary star consisting of a K star transferring mass to a white dwarf by way of an accretion disk. We have obtained new spectroscopic observations of SS Cyg. Fits of synthetic spectra for Roche lobe-filling stars to the absorption-line spectrum of the K star yield the amplitude of the K stars radial velocity curve and the mass ratio, KK = 162.5 ± 1.0 km s-1 and q = MK/MWD = 0.685 ± 0.015. The fits also show that the accretion disk and white dwarf contribute a fraction f = 0.535 ± 0.075 of the total flux at 5500 A. Taking the weighted average of our results with previously published results obtained using similar techniques, we find KK = 163.7 ± 0.7 km s-1 and q = 0.683 ± 0.012. The orbital light curve of SS Cyg shows an ellipsoidal variation diluted by light from the disk and white dwarf. From an analysis of the ellipsoidal variations, we limit the orbital inclination to the range 45° ≤ i ≤ 56°. The derived masses of the K star and white dwarf are MK = 0.55 ± 0.13 M☉ and MWD = 0.81 ± 0.19 M☉, where the uncertainties are dominated by systematic errors in the orbital inclination. The K star in SS Cyg is 10%-50% larger than an unevolved star with the same mass and thus does not follow the mass-radius relation for zero-age main-sequence stars, nor does it follow the ZAMS mass-spectral type relation. Its mass and spectral type are, however, consistent with models in which the core hydrogen has been significantly depleted.

OBSERVATIONAL CONSTRAINTS ON COOL DISK MATERIAL IN QUIESCENT BLACK HOLE BINARIES

We consider current observational constraints on the presence of cool, optically thick disk material in quiescent black hole binaries, specifically focusing on a case study of the prototypical system A0620� 00. Such material might be expected to be present theoretically but is usually claimed to make a negligible contribution at optical and IR wavelengths. The primary argument is based on measurements of the veiling of stellar photospheric absorption lines,inwhichitisassumedthatthediskspectrumisfeatureless.Weusesimulatedspectratoexplorethesensitivity of veiling measurements to uncertainties in companion temperature, gravity, and metallicity. We find that the derived veiling is extremely sensitive to a mismatch between the temperature and metallicity of the companion and template but that the effect of a plausible gravity mismatch is much smaller. In general, the resulting uncertainty in the amount of veiling is likely to be much larger than the usually quoted statistical uncertainty. We also simulate spectra in which the disk has an emergent spectrum similar to the star and find that in this case, optical veiling constraints are moderately robust. This is because the rotational broadening of the disk is so large that the two line profiles effectively decouple, and the measurement of the depth of stellar lines is largely unbiased by the disk component. We note, however, that this is only true at intermediate resolutions or higher and that significant bias might still affect low-resolution IR observations. Assuming that the optical veiling is reliable, we then examine the constraints on the temperature and covering factor of any optically thick disk component. These are stringent if the disk is warm (TeA k3500 K), but very temperature sensitive, and cooler disks are largely unconstrained by optical measurements. Current IR veiling estimates do not help much, representing rather high upper limits. Probably the best constraint comes from the relative amplitudes of ellipsoidal variations in different bands, as these are sensitive to differences in veiling that are expected for disks cooler than the companion star. A significant disk contribution in the IR, up to � 25%, is not ruled out in this or any other way considered, however. Subject headingg accretion, accretion disks — binaries: close — black hole physics

TEXES Observations of Pure Rotational H2 Emission from AB Aurigae

We present observations of pure rotational molecular hydrogen emission from the Herbig Ae star, AB Aur. Our observations were made using the Texas Echelon Cross Echelle Spectrograph (TEXES) at the NASA Infrared Telescope Facility and the Gemini North Observatory. We searched for H_2 emission in the S(1), S(2), and S(4) lines at high spectral resolution and detected all three. By fitting a simple model for the emission in the three transitions, we derive T = 670 ± 40 K and M = 0.52 ± 0.15 M_⊙ for the emitting gas. On the basis of the 8.5 km s^(-1) FWHM of the S(2) line, assuming the emission comes from the circumstellar disk, and with an inclination estimate of the AB Aur system taken from the literature, we place the location for the emission near 18 AU. Comparison of our derived temperature to a disk structure model suggests that UV and X-ray heating are important in heating the disk atmosphere.

Near-Infrared Spectra of the Black Hole X-Ray Binary A0620-00

We present broadband near-IR (NIR) spectra of A0620-00 obtained with SpeX on the IRTF. The spectrum is characterized by a blue continuum on which are superimposed broad emission lines of H I and He II and a host of narrower absorption lines of neutral metals and molecules. Spectral type standard star spectra scaled to the dereddened spectrum of A0620-00 in K exceed the A0620-00 spectrum in J and H for all stars of spectral type K7 V or earlier, demonstrating that the donor star, unless later than K7 V, cannot be the sole NIR flux source in A0620-00. In addition, the atomic absorption lines in the K3 V spectrum are too weak with respect to those of A0620-00 even at 100% donor star contribution, restricting the spectral type of the donor star in A0620-00 to later than K3 V. Comparison of the A0620-00 spectrum to scaled K star spectra indicates that the CO absorption features are significantly weaker in A0620-00 than in field dwarf stars. Fits of scaled model spectra of a Roche lobe-filling donor star to the spectrum of A0620-00 show that the best match to the CO absorption lines is obtained when the C abundance is reduced to [C/H] = -1.5. The donor star contribution in the H wave band is determined to be (82 ± 2)%. Combined with previous published results from Froning & Robinson and Marsh et al., this gives a precise mass for the black hole in A0620-00 of M1 = 9.7 ± 0.6 M☉.

HD 101088, AN ACCRETING 14 AU BINARY IN LOWER CENTAURUS CRUX WITH VERY LITTLE CIRCUMSTELLAR DUST ∗

We present high-resolution (R = 55, 000) optical spectra obtained with MIKE on the 6.5 m Magellan Clay Telescope as well as Spitzer MIPS photometry and Infrared Spectrometer low-resolution (R ~ 60) spectroscopy of the close (14 AU separation) binary, HD 101088, a member of the ~12 Myr old southern region of the Lower Centaurus Crux subgroup of the Scorpius-Centaurus OB association. We find that the primary and/or secondary is accreting from a tenuous circumprimary and/or circumsecondary disk despite the apparent lack of a massive circumbinary disk. We estimate a lower limit to the accretion rate of 1\times 10^{-9} \;M_{\odot } \;{\rm yr}^{-1}

Synthetic Spectra of Cool, Roche Lobe-filling Stars in Close Binary Systems

SRC=http://ej.iop.org/images/0004-637X/714/2/1542/apj342600ieqn1.gif/>, which our multiple observation epochs show varies over a timescale of months. The upper limit on the 70 μm flux allows us to place an upper limit on the mass of dust grains smaller than several microns present in a circumbinary disk of 0.16 M moon. We conclude that the classification of disks into either protoplanetary or debris disks based on fractional infrared luminosity alone may be misleading.

Broadband Near‐Infrared Spectroscopy of X‐Ray Binaries

We present the details of LinBrod, a program that calculates synthetic spectra of cool Roche lobe-filling stars in close binary systems. The program has two modes of operation. In the primary mode it calculates the spectra by adding wavelength-dependent, velocity-shifted specific intensities from the distorted and gravity-darkened surface of the lobe-filling star. The wavelength-dependent specific intensities are calculated externally using the ATLAS9 stellar atmosphere program and a modified version of the MOOG spectrum synthesis program. In its secondary mode LinBrod calculates phase-dependent line-broadening functions that can be convolved with the spectra of nonrotating single stars to yield approximate synthetic spectra. We use the line-broadening functions to isolate and assess the effects of the physical processes that broaden the absorption lines in the spectra of lobe-filling stars. The synthetic spectra can be used to extract radial velocities, mass ratios, and chemical abundances from observed spectra of lobe-filling stars. Originally written to analyze observations of the secondary stars in X-ray binaries containing black holes, the program can also be used to analyze observations of the secondary stars in Algol systems, cataclysmic variables, and low-mass X-ray binaries containing neutron stars.

TEXES on Gemini

We present broadband NIR spectroscopy of the X‐ray binaries A0620‐00, Cen X‐4, V404 Cyg, and GX 13+1. The NIR spectra consist of emission from the late‐type donor star and the accretion disk. In A0620‐00 and Cen X‐4, the donor star is the dominant component but there are emission lines from the accretion disk as well as evidence of a continuum disk component. In V404 Cyg, there are no signatures of the accretion disk in the NIR spectrum, whereas in GX13+1, the accretion disk is the dominant emitter. Using spectra of field stars of known spectral type, we have examined the donor star absorption spectra in these systems. The apparent spectral types of the donor stars in A0620‐00 and Cen X‐4 are those of late‐type dwarf stars (K7 or later). In A0620‐00, the weakness of the 12CO bandhead features relative to the atomic lines indicates a depleted carbon abundance in the system, [C/H] = −1.5. The spectrum of Cen X‐4 does not show this abundance anomaly. The donor stars in the V404 Cyg and GX13+1 are evolved, bu...

A MAGELLAN MIKE AND SPITZER MIPS STUDY OF 1.5-1.0 M ☉ STARS IN SCORPIUS-CENTAURUS

TEXES is a versatile mid-infrared spectrograph, which has been used on the NASA IRTF since 2000. It is capable of high spectral resolution (R ≈ 100,000), which is well suited for observations of interstellar and circumstellar molecules and ions, as well as molecules in planetary and stellar atmospheres. It has been installed on Gemini North where its point source sensitivity is expected to improve by a factor of 7, and its angular resolution will improve by 8/3.