Main-sequence, fully convective M dwarfs in eclipsing binaries are observed to be larger than stellar evolutionary models predict by as much as 10%-15%. 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 , but that at higher masses (0.18 < M < 0.4 MSun), the discrepancy is only about 6% and comparable to results from interferometry and eclipsing binaries. At the lowest masses (0.08 < M < 0.18 MSun), we find that the discrepancy between observations and theory is 13%-18%, 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.
We present a library of empirical stellar spectra created using spectra from the Sloan Digital Sky Survey's Baryon Oscillation Spectroscopic Survey. The templates cover spectral types O5 through L3, ...are binned by metallicity from −2.0 dex through +1.0 dex, and are separated into main-sequence (dwarf) stars and giant stars. With recently developed M dwarf metallicity indicators, we are able to extend the metallicity bins down through the spectral subtype M8, making this the first empirical library with this degree of temperature and metallicity coverage. The wavelength coverage for the templates is from 3650 to 10200 at a resolution of better than R ∼ 2000. Using the templates, we identify trends in color space with metallicity and surface gravity, which will be useful for analyzing large data sets from upcoming missions like the Large Synoptic Survey Telescope. Along with the templates, we are releasing a code for automatically (and/or visually) identifying the spectral type and metallicity of a star.
Abstract
Ultrahot Jupiters are gas giants that orbit so close to their host star that they are tidally locked, causing a permanent hot dayside and a cooler nightside. Signatures of their nonuniform ...atmospheres can be observed with high-resolution transit transmission spectroscopy by resolving time-dependent velocity shifts as the planet rotates and varying areas of the evening and morning terminator are probed. These asymmetric shifts were seen for the first time in iron absorption in WASP-76b. Here, we search for other atoms/ions in the planets transmission spectrum and study the asymmetries in their signals. We detect Li
i
, Na
i
, Mg
i
, Ca
ii
, V
i
, Cr
i
, Mn
i
, Fe
i
, Ni
i
, and Sr
ii
, and tentatively detect H
i
, K
i
, and Co
i
, of which V, Cr, Ni, Sr
ii
, and Co have not been reported before. We notably do not detect Ti or Al, even though these species should be readily observable, and hypothesize this could be due to condensation or cold trapping. We find that the observed signal asymmetries in the detected species can be explained in different ways. We find a relation between the expected condensation or ionization temperatures and the strength of the observed asymmetry, which could indicate rain-out or recombination on the nightside. However, we also find a dependence on the signal broadening, which could imply a two-zoned atmospheric model, in which the lower atmosphere is dominated by a day-to-night wind, while the upper atmosphere is dominated by a vertical wind or outflow. These observations provide a new level of modeling constraint and will aid our understanding of atmospheric dynamics in highly irradiated planets.
The abundance of refractory elements in giant planets can provide key insights into their formation histories
. Owing to the low temperatures of the Solar System giants, refractory elements condense ...below the cloud deck, limiting sensing capabilities to only highly volatile elements
. Recently, ultra-hot giant exoplanets have allowed for some refractory elements to be measured, showing abundances broadly consistent with the solar nebula with titanium probably condensed out of the photosphere
. Here we report precise abundance constraints of 14 major refractory elements on the ultra-hot giant planet WASP-76b that show distinct deviations from proto-solar and a sharp onset in condensation temperature. In particular, we find nickel to be enriched, a possible sign of the accretion of the core of a differentiated object during the evolution of the planet. Elements with condensation temperatures below 1,550 K otherwise closely match those of the Sun
before sharply transitioning to being strongly depleted above 1,550 K, which is well explained by nightside cold-trapping. We further unambiguously detect vanadium oxide on WASP-76b, a molecule long suggested to drive atmospheric thermal inversions
, and also observe a global east-west asymmetry
in its absorption signals. Overall, our findings indicate that giant planets have a mostly stellar-like refractory elemental content and suggest that temperature sequences of hot Jupiter spectra can show abrupt transitions wherein a mineral species is either present or completely absent if a cold trap exists below its condensation temperature
.
Abstract
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.
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 Sloan Digital Sky Survey (SDSS) and the Large Sky Area Multi-Object Fiber Spectroscopic Telescope. Here, we present the next version of PyHammer, a stellar spectral classification software that uses optical spectral templates and spectral line index measurements. PyHammer v2.0 extends the classification power to include dwarf carbon stars, DA white dwarf 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 the ability to spectrally type SB2s to PyHammer. 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 95% of SB2s are correctly classified.
Abstract
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 H
α
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 (
P
rot
< 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 2.4 m 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 H
α
, and
L
H
α
/
L
bol
. We find a weak positive correlation between H
α
luminosity and the semiamplitude,
R
var
(
p
=
0.005
−
0.005
+
0.075
). We also observe short-term variability (between 20 and 45 minutes) in H
α
equivalent widths and possible enhancement from flares consistent to recent literature.
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 (Teff) and we measure bolometric luminosities (Lbol) by combining broad wavelength-coverage photometry with Gaia parallaxes. Radii are then computed by combining the Teff and Lbol using the Stefan-Boltzman 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 −2.0 dex, in order to aid the radius determination of M subdwarfs, which will be especially important for the WFIRST exoplanetary microlensing survey.
Most of the molecules detected thus far in exoplanet atmospheres, such as water and CO, are present for a large range of pressures and temperatures. In contrast, metal hydrides exist in much more ...specific regimes of parameter space, and so can be used as probes of atmospheric conditions. Iron hydride (FeH) is a dominant source of opacity in low-mass stars and brown dwarfs, and evidence for its existence in exoplanets has recently been observed at low resolution. We performed a systematic search of archival CARMENES near-infrared data for signatures of FeH during transits of 12 exoplanets. These planets span a large range of equilibrium temperatures (600 4000 K) and surface gravities (2.5 3.5). We did not find a statistically significant FeH signal in any of the atmospheres, but obtained potential low-confidence signals (signal-to-noise ratio ∼ 3) in two planets, WASP-33b and MASCARA-2b. Previous modeling of exoplanet atmospheres indicate that the highest volume mixing ratios (VMRs) of 10−7 to 10−9 are expected for temperatures between 1800 and 3000 K and log . The two planets for which we find low-confidence signals are in the regime where strong FeH absorption is expected. We performed injection and recovery tests for each planet and determined that FeH would be detected in every planet for VMRs , and could be detected in some planets for VMRs as low as . Additional observations are necessary to conclusively detect FeH and assess its role in the temperature structures of hot-Jupiter atmospheres.
Abstract
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
μ
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
α
emission. The
H
−
M
′ 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
L
′, NB4.05, and
M
′ bands. The excess flux is well described by a blackbody component with
T
disk
≈ 460 K and
R
disk
≈ 65
R
J
and is expected to trace continuum emission from small grains in a protolunar disk. We derive an extinction of
A
V
≈ 2.3 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 84 ± 9 deg, indicating a tumultuous dynamical evolution or a stellar-like formation pathway. From the measured H
α
flux and the estimated companion mass,
M
p
≈ 30
M
J
, we derive an accretion rate of
M
̇
≈
10
−
6.5
M
J
yr
−
1
. 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.
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