Time-resolved Ultraviolet Spectroscopy of the M-dwarf GJ 876 Exoplanetary System

Extrasolar planets orbiting M-stars may represent our best chance to discover habitable worlds in the coming decade. The ultraviolet spectrum incident upon both Earth-like and Jovian planets is critically important for proper modeling of their atmospheric heating and chemistry. In order to provide more realistic inputs for atmospheric models of planets orbiting low-mass stars, we present new near- and far-ultraviolet (NUV and FUV) spectroscopy of the M-dwarf exoplanet host GJ 876 (M4V). Using the COS and STIS spectrographs aboard the Hubble Space Telescope, we have measured the 1150-3140A spectrum of GJ 876. We have reconstructed the stellar HI LyA emission line profile, and find that the integrated LyA flux is roughly equal to the rest of the integrated flux (1150-1210A + 1220-3140A) in the entire ultraviolet bandpass (F(LyA)/F(FUV+NUV) ~0.7). This ratio is ~ 2500x greater than the solar value. We describe the ultraviolet line spectrum and report surprisingly strong fluorescent emission from hot H2 (T(H2) > 2000 K). We show the light-curve of a chromospheric + transition region flare observed in several far-UV emission lines, with flare/quiescent flux ratios >= 10. The strong FUV radiation field of an M-star (and specifically LyA) is important for determining the abundance of O2 -- and the formation of biomarkers -- in the lower atmospheres of Earth-like planets in the habitable zones of low-mass stars.Comment: 6 pages, 4 figures. ApJL accepte

The Mass of the Black Hole in XTE J1118+480

We present contemporaneous, broadband, near-infrared spectroscopy (0.9-2.45 μm) and H-band photometry of the black hole X-ray binary, XTE J1118+480. We determined the fractional dilution of the NIR ellipsoidal light curves of the donor star from other emission sources in the system by comparing the absorption features in the spectrum with field stars of known spectral type. We constrained the donor star spectral type to K7 V-M1 V and determined that the donor star contributed 54% ± 27% of the H-band flux at the epoch of our observations. This result underscores the conclusion that the donor star cannot be assumed to be the only NIR emission source in quiescent X-ray binaries. The H-band light curve shows a double-humped asymmetric modulation with extra flux at orbital phase 0.75. The light curve was fitted with a donor star model light curve, taking into account a constant second flux component based on the dilution analysis. We also fitted models that included emission from the donor star, a constant component from the accretion disk, and a phase-variable component from the bright spot where the mass accretion stream impacts the disk. These simple models with reasonable estimates for the component physical parameters can fully account for the observed light curve, including the extra emission at phase 0.75. From our fits, we constrained the binary inclination to 68° ≤ i ≤ 79°. This leads to a black hole mass of 6.9 M_☉ ≤ M_(BH) ≤ 8.2 M_☉. Long-term variations in the NIR light curve shape in XTE J1118+480 are similar to those seen in other X-ray binaries and demonstrate the presence of continued activity and variability in these systems even when in full quiescence

Near-Infrared Spectroscopy of Low Mass X-ray Binaries : Accretion Disk Contamination and Compact Object Mass Determination in V404 Cyg and Cen X-4

We present near-infrared (NIR) broadband (0.80--2.42 $\mu$m) spectroscopy of two low mass X-ray binaries: V404 Cyg and Cen X-4. One important parameter required in the determination of the mass of the compact objects in these systems is the binary inclination. We can determine the inclination by modeling the ellipsoidal modulations of the Roche-lobe filling donor star, but the contamination of the donor star light from other components of the binary, particularly the accretion disk, must be taken into account. To this end, we determined the donor star contribution to the infrared flux by comparing the spectra of V404 Cyg and Cen X-4 to those of various field K-stars of known spectral type. For V404 Cyg, we determined that the donor star has a spectral type of K3 III. We determined the fractional donor contribution to the NIR flux in the H- and K-bands as $0.98 \pm .05$ and $0.97 \pm .09$, respectively. We remodeled the H-band light curve from \citet{sanwal1996} after correcting for the donor star contribution to obtain a new value for the binary inclination. From this, we determined the mass of the black hole in V404 Cyg to be $M_{BH}= 9.0^{+.2}_{-.6}M_{\odot}$. We performed the same spectral analysis for Cen X-4 and found the spectral type of the donor star to be in the range K5 -- M1V. The donor star contribution in Cen X-4 is $0.94\pm.14$ in the H-band while in the K-band, the accretion disk can contribute up to 10% of the infrared flux. We remodeled the H-band light curve from \citet{shahbaz1993}, again correcting for the fractional contribution of the donor star to obtain the inclination. From this, we determined the mass of the neutron star as $M_{NS}= 1.5^{+.1}_{-.4}M_{\odot}$. However, the masses obtained for both systems should be viewed with some caution since contemporaneous light curve and spectral data are required to obtain definitive masses

Near-Infrared Light Curves of the Black Hole Binary A0620-00

We measured the near-infrared orbital light curve of the black hole binary A0620-00 in 1995 and 1996. The light curves show an asymmetric, double-humped modulation with extra emission in the peak at orbital phase 0.75. There were no significant changes in the shape of the light curve over the one-year observation period. There were no sharp dips in the light curves nor reversals of the asymmetry between the two peaks as seen in earlier observations. The light curves are well fit by models incorporating ellipsoidal variations from the mass-losing K-type star plus a beamed bright spot on the accretion disk around the compact star. The long-term stability of the light curve shape rules out superhumps and star spots as sources of asymmetry when we observed A0620-00. The ellipsoidal variations yield a lower limit i >= 38 deg on the orbital inclination. The light curves show no eclipse features, which places an upper limit i <= 75 deg. This range of inclinations constrains the mass of the compact object to 3.3 < M_1 < 13.6 Msun. The light curves do not further constrain the orbital inclination because the contribution of the accretion disk to the observed flux is unknown. We argue that a previous attempt to measure the near-infrared flux from the accretion disk using the dilution of the 12CO(2,0) bandhead in the spectrum of the K star is not reliable because the band strength depends strongly on surface gravity.Comment: Accepted for publication in the Astronomical Journal. 17 pages, 4 figures. Prepared using AASTEX V. 5.

A Survey of Far Ultraviolet Spectroscopic Explorer Observations of Cataclysmic Variables

During its lifetime, the Far Ultraviolet Spectroscopic Explorer (FUSE) was used to observe 99 cataclysmic variables in 211 separate observations. Here, we present a survey of the moderate resolution (R\simeq10,000), far-ultraviolet (905 - 1188 {\deg}A), time-averaged FUSE spectra of cataclysmic variables (CVs). The FUSE spectra are morphologically diverse. They show contributions from the accretion disk, the disk chromosphere, disk outflows, and the white dwarf, but the relative contribution of each component varies widely as a function of CV subtype, orbital period and evolutionary state, inclination, mass accretion rate, and magnetic field strength of the white dwarf. The data reveal information about the structure, temperature, density and mass flow rates of the disk and disk winds, the temperature of the white dwarf and the effects of ongoing accretion on its structure, and the long-term response of the systems to disk outbursts. The complete atlas of time-averaged FUSE spectra of CVs are available at the Multimission Archive at Space Telescope Science Institute as a High Level Science Product.Comment: ApJS, in press. The extra panels in the figure sets for Figures 1 and 2 are included at the end of the manuscrip

The Ultraviolet Radiation Environment Around M dwarf Exoplanet Host Stars

The spectral and temporal behavior of exoplanet host stars is a critical input to models of the chemistry and evolution of planetary atmospheres. At present, little observational or theoretical basis exists for understanding the ultraviolet spectra of M dwarfs, despite their critical importance to predicting and interpreting the spectra of potentially habitable planets as they are obtained in the coming decades. Using observations from the Hubble Space Telescope, we present a study of the UV radiation fields around nearby M dwarf planet hosts that covers both FUV and NUV wavelengths. The combined FUV+NUV spectra are publically available in machine-readable format. We find that all six exoplanet host stars in our sample (GJ 581, GJ 876, GJ 436, GJ 832, GJ 667C, and GJ 1214) exhibit some level of chromospheric and transition region UV emission. No "UV quiet" M dwarfs are observed. The bright stellar Ly-alpha emission lines are reconstructed, and we find that the Ly-alpha line fluxes comprise ~37-75% of the total 1150-3100A flux from most M dwarfs; > 10^{3} times the solar value. The F(FUV)/F(NUV) flux ratio, a driver for abiotic production of the suggested biomarkers O2 and O3, is shown to be ~0.5-3 for all M dwarfs in our sample, > 10^{3} times the solar ratio. For the four stars with moderate signal-to-noise COS time-resolved spectra, we find UV emission line variability with amplitudes of 50-500% on 10^{2} - 10^{3} s timescales. Finally, we observe relatively bright H2 fluorescent emission from four of the M dwarf exoplanetary systems (GJ 581, GJ 876, GJ 436, and GJ 832). Additional modeling work is needed to differentiate between a stellar photospheric or possible exoplanetary origin for the hot (T(H2) \approx 2000-4000 K) molecular gas observed in these objects.Comment: ApJ, accepted. 16 pages, 10 figures. On-line data at: http://cos.colorado.edu/~kevinf/muscles.htm