15 research outputs found
Classifying Single Stars and Spectroscopic Binaries Using Optical Stellar Templates
Stellar spectral classification is a fundamental tool of modern astronomy,
providing insight into physical characteristics such as effective temperature,
surface gravity, and metallicity. Accurate and fast spectral typing is an
integral need for large all-sky spectroscopic surveys like the SDSS and LAMOST.
Here, we present the next version of PyHammer, stellar spectral classification
software that uses optical spectral templates and spectral line index
measurements. PyHammer v2.0 extends the classification power to include carbon
(C) stars, DA white dwarf (WD) stars, and also double-lined spectroscopic
binaries (SB2). This release also includes a new empirical library of
luminosity-normalized spectra that can be used to flux calibrate observed
spectra, or to create synthetic SB2 spectra. We have generated physically
reasonable SB2 combinations as templates, adding to PyHammer the ability to
spectrally type SB2s. We test classification success rates on SB2 spectra,
generated from the SDSS, across a wide range of spectral types and
signal-to-noise ratios. Within the defined range of pairings described, more
than of SB2s are correctly classified.Comment: 16 pages, 7 figures, 4 tables; accepted to ApJ
Contemporaneous Observations of Luminosities and Photometric Amplitudes for M Dwarfs
While many M dwarfs are known to have strong magnetic fields and high levels
of magnetic activity, we are still unsure about the properties of their
starspots and the origin of their magnetic dynamos. Both starspots and
chromospheric heating are generated by the surface magnetic field; they produce
photometric variability and Halpha emission, respectively. Connecting
brightness variations to magnetic activity therefore provides a means to
examine M dwarf magnetism. We survey 30 M dwarfs previously identified as fast
rotating stars (Prot < 10 days). We present time-series optical photometry from
the Transiting Exoplanet Survey Satellite (TESS) and contemporaneous optical
spectra obtained using the Ohio State Multi-Object Spectrograph (OSMOS) on the
2.4m Hiltner telescope at MDM Observatory in Arizona. We measure rotation
periods and photometric amplitudes from TESS light curves using Gaussian
Processes. From the OSMOS spectra, we calculate the equivalent width of Halpha,
and LHalpha/Lbol. We find a weak positive correlation between Halpha luminosity
and the semi-amplitude, Rvar p=0.005_{-0.005}^{+0.075}. We also observe
short-term variability (between 20-45 minutes) in Halpha equivalent widths and
possible enhancement from flares consistent to recent literature.Comment: 19 pages, 9 Figures, 2 Tables, Poster Presented at Cool Stars 21,
Publication post-copy editin
Radii of 88 M subdwarfs and updated radius relations for low-metallicity M-dwarf stars
M subdwarfs are low-metallicity M dwarfs that typically inhabit the halo population of the Galaxy. Metallicity controls the opacity of stellar atmospheres; in metal-poor stars, hydrostatic equilibrium is reached at a smaller radius, leading to smaller radii for a given effective temperature. We compile a sample of 88 stars that span spectral classes K7 to M6 and include stars with metallicity classes from solar-metallicity dwarf stars to the lowest metallicity ultra subdwarfs to test how metallicity changes the stellar radius. We fit models to Palomar Double Spectrograph (DBSP) optical spectra to derive effective temperatures (T_ eff) and we measure bolometric luminosities (L_ bol) by combining broad wavelength-coverage photometry with Gaia parallaxes. Radii are then computed by combining the T_ eff and L_ bol using the StefanâBoltzman law. We find that for a given temperature, ultra subdwarfs can be as much as five times smaller than their solar-metallicity counterparts. We present color-radius and color-surface brightness relations that extend down to [Fe/H] of â2.0 dex, in order to aid the radius determination of M subdwarfs, which will be especially important for the WFIRST exoplanetary microlensing survey.Published versio
Effective Temperatures of Low-Mass Stars from High-Resolution H-band Spectroscopy
High-resolution, near-infrared spectra will be the primary tool for finding
and characterizing Earth-like planets around low-mass stars. Yet, the
properties of exoplanets can not be precisely determined without accurate and
precise measurements of the host star. Spectra obtained with the Immersion
GRating INfrared Spectrometer (IGRINS) simultaneously provide diagnostics for
most stellar parameters, but the first step in any analysis is the
determination of the effective temperature. Here we report the calibration of
high-resolution H-band spectra to accurately determine effective temperature
for stars between 4000-3000 K (K8--M5) using absorption line depths of Fe
I, OH, and Al I. The field star sample used here contains 254 K and M stars
with temperatures derived using BT-Settl synthetic spectra. We use 106 stars
with precise temperatures in the literature to calibrate our method with
typical errors of about 140 K, and systematic uncertainties less than 120
K. For the broadest applicability, we present T--line-depth-ratio
relationships, which we test on 12 members of the TW Hydrae Association and at
spectral resolving powers between 10,000--120,000. These ratios offer a
simple but accurate measure of effective temperature in cool stars that is
distance and reddening independent.Comment: 19 pages, 11 figures and 3 tables. Accepted in Ap
Transmission spectroscopy of the ultra-hot Jupiter MASCARA-4 b: Disentangling the hydrostatic and exospheric regimes of ultra-hot Jupiters
Ultra-hot Jupiters (UHJs), rendering the hottest planetary atmospheres, offer
great opportunities of detailed characterisation with high-resolution
spectroscopy. MASCARA-4 b is a recently discovered close-in gas giant belonging
to this category. In order to refine system and planet parameters, we carried
out radial velocity measurements and transit photometry with the CORALIE
spectrograph and EulerCam at the Swiss 1.2m Euler telescope. We observed two
transits of MASCARA-4 b with the high-resolution spectrograph ESPRESSO at ESO's
Very Large Telescope. We searched for atomic, ionic, and molecular species via
individual absorption lines and cross-correlation techniques. These results are
compared to literature studies on UHJs characterised to date. With CORALIE and
EulerCam observations, we updated the mass of MASCARA-4 b (1.675 +/- 0.241
Jupiter masses) as well as other system and planet parameters. In the
transmission spectrum derived from ESPRESSO observations, we resolve excess
absorption by H, H, Na D1 & D2, Ca+ H & K, and a few strong
individual lines of Mg, Fe and Fe+. We also present the cross-correlation
detection of Mg, Ca, Cr, Fe and Fe+. The absorption strength of Fe+
significantly exceeds the prediction from a hydrostatic atmospheric model, as
commonly observed in other UHJs. We attribute this to the presence of Fe+ in
the exosphere due to hydrodynamic outflows. This is further supported by the
positive correlation of absorption strengths of Fe+ with the H line.
Comparing transmission signatures of various species in the UHJ population
allows us to disentangle the hydrostatic regime (as traced via the absorption
by Mg and Fe) from the exospheres (as probed by H and Fe+) of the
strongly irradiated atmospheres.Comment: 13 pages, 9 figures, accepted to A&
Magnetic inflation and Stellar Mass. II. On the radii of wingle, rapidly rotating, fully convective M-dwarf stars
Main-sequence, fully convective M dwarfs in eclipsing binaries are observed to be larger than stellar evolutionary models predict by as much as 10%â15%. A proposed explanation for this discrepancy involves effects from strong magnetic fields, induced by rapid rotation via the dynamo process. Although, a handful of single, slowly rotating M dwarfs with radius measurements from interferometry also appear to be larger than models predict, suggesting that rotation or binarity specifically may not be the sole cause of the discrepancy. We test whether single, rapidly rotating, fully convective stars are also larger than expected by measuring their distribution. We combine photometric rotation periods from the literature with rotational broadening () measurements reported in this work for a sample of 88 rapidly rotating M dwarf stars. Using a Bayesian framework, we find that stellar evolutionary models underestimate the radii by 10 \% \mbox{--}15{ \% }_{-2.5}^{+3}, but that at higher masses (0.18 < M < 0.4 M Sun), the discrepancy is only about 6% and comparable to results from interferometry and eclipsing binaries. At the lowest masses (0.08 < M < 0.18 M Sun), we find that the discrepancy between observations and theory is 13%â18%, and we argue that the discrepancy is unlikely to be due to effects from age. Furthermore, we find no statistically significant radius discrepancy between our sample and the handful of M dwarfs with interferometric radii. We conclude that neither rotation nor binarity are responsible for the inflated radii of fully convective M dwarfs, and that all fully convective M dwarfs are larger than models predict.The authors would like to thank the referee for the thoughtful report, which greatly improved the manuscript. The authors would also like to thank Lisa Prato and Larissa Nofi for IGRINS training, and Heidi Larson, Jason Sanborn, and Andrew Hayslip for operating the DCT during our observations. We would also like to thank Jen Winters, Jonathan Irwin, Paul Dalba, Mark Veyette, Eunkyu Han, and Andrew Vanderburg for useful discussions and helpful comments on this work. Some of this work was supported by the NASA Exoplanet Research Program (XRP) under grant No. NNX15AG08G issued through the Science Mission Directorate.These results made use of the Lowell Observatory's Discovery Channel Telescope, supported by Discovery Communications, Inc., Boston University, the University of Maryland, the University of Toledo and Northern Arizona University; the Immersion Grating Infrared Spectrograph (IGRINS) that was developed under a collaboration between the University of Texas at Austin and the Korea Astronomy and Space Science Institute (KASI) with the financial support of the US National Science Foundation under grant AST-1229522, of the University of Texas at Austin, and of the Korean GMT Project of KASI; data taken at The McDonald Observatory of The University of Texas at Austin; and data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by NASA and the NSF. (NNX15AG08G - NASA Exoplanet Research Program (XRP); Discovery Communications, Inc.; Boston University; University of Maryland; University of Toledo; Northern Arizona University; AST-1229522 - US National Science Foundation; University of Texas at Austin; Korean GMT Project of KASI; NASA; NSF
A Model for (Quasi-)Periodic Multiwavelength Photometric Variability in Young Stellar Objects
We present radiation transfer models of rotating young stellar objects (YSOs)
with hotspots in their atmospheres, inner disk warps and other 3-D effects in
the nearby circumstellar environment. Our models are based on the geometry
expected from the magneto-accretion theory, where material moving inward in the
disk flows along magnetic field lines to the star and creates stellar hotspots
upon impact. Due to rotation of the star and magnetosphere, the disk is
variably illuminated. We compare our model light curves to data from the
Spitzer YSOVAR project (Morales-Calderon et al. 2014, Cody et al. 2014) to
determine if these processes can explain the variability observed at optical
and mid-infrared wavelengths in young stars. We focus on those variables
exhibiting "dipper" behavior that may be periodic, quasi-periodic, or
aperiodic. We find that the stellar hotspot size and temperature affects the
optical and near-infrared light curves, while the shape and vertical extent of
the inner disk warp affects the mid-IR light curve variations. Clumpy disk
distributions with non-uniform fractal density structure produce more
stochastic light curves. We conclude that the magneto-accretion theory is
consistent with certain aspects of the multi-wavelength photometric variability
exhibited by low-mass YSOs. More detailed modeling of individual sources can be
used to better determine the stellar hotspot and inner disk geometries of
particular sources.Comment: Accepted to Ap
Confirmation of Asymmetric Iron Absorption in WASP-76b with HARPS
Hot Jupiters are predicted to have hot, clear daysides and cooler, cloudy
nightsides. Recently, an asymmetric signature of iron absorption has been
resolved in the transmission spectrum of WASP-76b using ESPRESSO on ESO's Very
large Telescope. This feature is interpreted as being due to condensation of
iron on the nightside, resulting in a different absorption signature from the
evening than from the morning limb of the planet. It represents the first time
that a chemical gradient has been observed across the surface of a single
exoplanet. In this work, we confirm the presence of the asymmetric iron feature
using archival HARPS data of four transits. The detection shows that such
features can also be resolved by observing multiple transits on smaller
telescopes. By increasing the number of planets where these condensation
features are detected, we can make chemical comparisons between exoplanets and
map condensation across a range of parameters for the first time.Comment: 7 page, 3 figures. Accepted to ApJ
Characterizing the Protolunar Disk of the Accreting Companion GQ Lupi B
GQ Lup B is a young and accreting, substellar companion that appears to drive
a spiral arm in the circumstellar disk of its host star. We report
high-contrast imaging observations of GQ Lup B with VLT/NACO at 4-5 m and
medium-resolution integral field spectroscopy with VLT/MUSE. The optical
spectrum is consistent with an M9 spectral type, shows characteristics of a
low-gravity atmosphere, and exhibits strong H emission. The
color is 1 mag redder than field dwarfs with similar spectral types
and a detailed analysis of the spectral energy distribution (SED) from optical
to mid-infrared wavelengths reveals excess emission in the , NB4.05, and
bands. The excess flux is well described by a blackbody component with
K and
and is expected to trace continuum emission from small grains in a protolunar
disk. We derive an extinction of mag from the broadband SED
with a suspected origin in the vicinity of the companion. We also combine 15 yr
of astrometric measurements and constrain the mutual inclination with the
circumstellar disk to deg, indicating a tumultuous dynamical
evolution or a stellar-like formation pathway. From the measured H flux
and the estimated companion mass, , we
derive an accretion rate of . We speculate that the disk is in a
transitional stage in which the assembly of satellites from a pebble reservoir
has opened a central cavity while GQ Lup B is in the final stages of its
formation.Comment: 28 pages, 11 figures, accepted for publication in A