21 research outputs found
The effects of metallicity and magnetism on the radii of M dwarf stars
M dwarfs are ubiquitous in the galaxy, yet their fundamental properties are not precisely known. Radii are particularly difficult to determine because M dwarfs are intrinsically small and faint, leading to only a few radius determinations using either long-baseline optical interferometry or eclipses of binary stars. Observations rarely agree with models, and the scatter in M dwarf radius relations is significantly larger and less understood than it is for higher mass stars. I explored the two main hypotheses evoked to explain discrepancies between model radii and observed radii, namely effects from metallicity and strong magnetic fields. I conducted a spectroscopic survey of M dwarfs with a wide range of metallicities and derived radii using the Stefan-Boltzmann law in order to constrain radius relations for the lowest mass and lowest metallicity stars. I found that solar metallicity stars can be up to five times larger than their low-metallicity counterparts for a given effective temperature, but that metallicity has a relatively small effect on mass- or luminosity-to-radius relations. To test the effect of magnetism on radii, I determined a statistical distribution of radii for magnetically active M dwarfs by combining measured rotational broadening values with literature rotation periods. I found that the magnetically active stars were on average 10-15% larger than model predictions and that models and observations were most discrepant for the lowest-mass stars. To deduce whether the 10-15% radius discrepancy could be due entirely to the spotted nature of these stars, I determined the spot temperature and spot filling fraction of one of the most magnetically active stars in my sample. I measured a high spot filling fraction, spot temperatures several hundred Kelvin lower than the photosphere temperature, and I also detected evidence of faculae on the stellar surface. I concluded that spots are the primary cause for models overestimating the sizes of low-mass stars, and that stellar-evolution models should consider the effects of spots to more accurately predict the sizes and temperature of all M dwarfs
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
A Search for FeH in Hot-Jupiter Atmospheres with High-Dispersion Spectroscopy
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
4000K) 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 (SNR3) in two planets,
WASP-33b and MASCARA-2b. Previous modeling of exoplanet atmospheres indicate
that the highest volume mixing ratios (VMRs) of 10 to 10 are
expected for temperatures between 1800 and 3000K 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 10.
Additional observations are necessary to conclusively detect FeH and assess its
role in the temperature structures of hot Jupiter atmospheres.Comment: Accepted to AAS journal
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 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.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
Pictoris b through the eyes of the upgraded CRIRES+
Context: High-resolution spectrographs fed by adaptive optics (AO) provide a
unique opportunity to characterize directly imaged exoplanets. Observations
with such instruments allow us to probe the atmospheric composition, spin
rotation, and radial velocity of the planet, thereby helping to reveal
information on its formation and migration history. The recent upgrade of the
Cryogenic High-Resolution Infrared Echelle Spectrograph (CRIRES+) at the VLT
makes it a highly suitable instrument for characterizing directly imaged
exoplanets.
Aims: In this work, we report on observations of Pictoris b with
CRIRES+ and use them to constrain the planets atmospheric properties and update
the estimation of its spin rotation.
Methods: The data were reduced using the open-source \textit{pycrires}
package. We subsequently forward-modeled the stellar, planetary, and systematic
contribution to the data to detect molecules in the planet's atmosphere. We
also used atmospheric retrievals to provide new constraints on its atmosphere.
Results: We confidently detected water and carbon monoxide in the atmosphere
of Pictoris b and retrieved a slightly sub-solar carbon-to-oxygen
ratio, which is in agreement with previous results. The interpretation is
hampered by our limited knowledge of the C/O ratio of the host star. We also
obtained a much improved constraint on its spin rotation of
km/s, which gives a rotation period of hours, assuming no
obliquity. We find that there is a degeneracy between the metallicity and
clouds, but this has minimal impact on the retrieved C/O, , and
radial velocity. Our results show that CRIRES+ is performing well and stands as
a highly useful instrument for characterizing directly imaged planets.Comment: Accepted for publication in A&
Retrieval survey of metals in six ultra-hot Jupiters: Trends in chemistry, rain-out, ionisation and atmospheric dynamics
Ground-based high-resolution spectroscopy (HRS) has detected numerous
chemical species and atmospheric dynamics in exoplanets, most notably ultra-hot
Jupiters (UHJs). However, quantitative estimates on abundances have been
challenging but are essential for accurate comparative characterisation and to
determine formation scenarios. In this work we retrieve the atmospheres of six
UHJs (WASP-76~b, MASCARA-4~b, MASCARA-2~b, WASP-121~b, HAT-P-70~b and
WASP-189~b) with ESPRESSO and HARPS-N/HARPS observations, exploring trends in
eleven neutral species and dynamics. While Fe abundances agree well with
stellar values, Mg, Ni, Cr, Mn and V show more variation, highlighting the
difficulty in using a single species as a proxy for metallicity. We find that
Ca, Na, Ti and TiO are under-abundant, potentially due to ionisation and/or
night-side rain-out. Our retrievals also show that relative abundances between
species are more robust, consistent with previous works. We perform spatially-
and phase-resolved retrievals for WASP-76~b and WASP-121~b given their high
signal-to-noise observations, and find the chemical abundances in each of the
terminator regions are broadly consistent. We additionally constrain dynamics
for our sample through Doppler shifts and broadening of the planetary signals
during the primary eclipse, with median blue shifts between 0.9-9.0~km/s
due to day-night winds. Furthermore, we constrain spectroscopic masses for
MASCARA-2~b and HAT-P-70~b consistent with their known upper limits, but we
note that these may be biased due to degeneracies. This work highlights the
importance of future HRS studies to further probe differences and trends
between exoplanets.Comment: 26 pages, 11 figures, 5 tables, published in A
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&