Cool Stars Research Articles

Metallicities in M dwarfs: Investigating different determination techniques

Deriving metallicities for solar-like stars follows well-established methods, but for cooler stars such as M dwarfs, the determination is much more complicated due to forests of molecular lines that are present. Several methods have been developed in recent years to determine accurate stellar parameters for these cool stars (Teff≲ 4000 K). However, significant differences can be found at times when comparing metallicities for the same star derived using different methods. In this work, we determine the effective temperatures, surface gravities, and metallicities of 18 well-studied M dwarfs observed with the CARMENES high-resolution spectrograph following different approaches, including synthetic spectral fitting, analysis of pseudo-equivalent widths, and machine learning. We analyzed the discrepancies in the derived stellar parameters, including metallicity, in several analysis runs. Our goal is to minimize these discrepancies and find stellar parameters that are more consistent with the literature values. We attempted to achieve this consistency by standardizing the most commonly used components, such as wavelength ranges, synthetic model spectra, continuum normalization methods, and stellar parameters. We conclude that although such modifications work quite well for hotter main-sequence stars, they do not improve the consistency in stellar parameters for M dwarfs, leading to mean deviations of around 50–200 K in temperature and 0.1–0.3 dex in metallicity. In particular, M dwarfs are much more complex and a standardization of the aforementioned components cannot be considered as a straightforward recipe for bringing consistency to the derived parameters. Further in-depth investigations of the employed methods would be necessary in order to identify and correct for the discrepancies that remain.

A New Method for Estimating Starspot Lifetimes Based on Autocorrelation Functions

Abstract We present a method that utilizes autocorrelation functions from long-term precision broadband differential light curves to estimate the average lifetimes of starspot groups for two large samples of Kepler stars: stars with and without previously known rotation periods. Our method is calibrated by comparing the strengths of the first few normalized autocorrelation peaks using ensembles of models that have various starspot lifetimes. We find that we must mix models of short and long lifetimes together (in heuristically determined ratios) to align the models with the Kepler data. Our fundamental result is that short starspot-group lifetimes (one to four rotations) are implied when the first normalized peak is weaker than about 0.4, long lifetimes (15 or greater) are implied when it is greater than about 0.7, and in between are the intermediate cases. Rotational lifetimes can be converted to physical lifetimes if the rotation period is known. Stars with shorter rotation periods tend to have longer rotational (but not physical) spot lifetimes, and cooler stars tend to have longer physical spot lifetimes than warmer stars with the same rotation period. The distributions of the physical lifetimes are log-normal for both samples and generally longer in the first sample. The shorter lifetimes in the stars without known periods probably explain why their periods are difficult to measure. Some stars exhibit longer than average physical starspot lifetimes; their percentage drops with increasing temperature from nearly half at 3000 K to nearly zero for hotter than 6000 K.

Mechanical Energy Deposition in Cool Star Atmospheres: Resonant Coupling to Coronal Loops in M Dwarfs

Abstract The efficiency of coronal heating ε(cor) in a star can be quantified by L X/L bol, i.e., the ratio of X-ray luminosity to bolometric luminosity. The efficiency of chromospheric heating in the same star ε(chr) is typically assumed to be proportional to L(Hα)/L bol or L(Ca K)/L bol where the lines Hα and Ca K are often the two strongest emission lines in the visible spectrum: the constant of proportionality (η = ε(chr)/[L(Hα)/L bol] > 1) includes contributions from many other lines emitted by the chromosphere. In the case of the quiet Sun, it has been known for decades that, in the Sun, the efficiency of chromospheric heating is larger by a factor of ε(chr)/ε(cor) > 10 than the efficiency of coronal heating. Over the intervening years, data pertaining to ε(cor) and ε(chr) have been estimated for an increasingly large sample of main-sequence stars with spectral types later than the Sun. These data suggest that among M dwarfs, the efficiency ratio ε(chr)/ε(cor) may in some stars become smaller than in the solar case. The effect of this is such that the value of ε(cor) may become comparable to the value of ε(chr). Here, we seek to understand why coronal heating may be >10 times more efficient (relative to chromospheric heating) in certain M dwarfs than in the Sun. Using data on coronal loop properties in flaring stars, we examine the hypothesis that in M dwarfs, the enhanced efficiency of coronal heating may be related to resonant coupling between coronal loops and the source of mechanical energy in the convection zone.

Stellar Chromospheric Variability

Cool stars with convective envelopes of spectral types F and later tend to exhibit magnetic activity throughout their atmospheres. The presence of strong and variable magnetic fields is evidenced by photospheric starspots, chromospheric plages and coronal flares, as well as by strong Ca ii H+K and Hα emission, combined with the presence of ultraviolet resonance lines. We review the drivers of stellar chromospheric activity and the resulting physical parameters implied by the observational diagnostics. At a basic level, we explore the importance of stellar dynamos and their activity cycles for a range of stellar types across the Hertzsprung–Russell diagram. We focus, in particular, on recent developments pertaining to stellar rotation properties, including the putative Vaughan–Preston gap. We also pay specific attention to magnetic variability associated with close binary systems, including RS Canum Venaticorum, BY Draconis, W Ursae Majoris and Algol binaries. At the present time, large-scale photometric and spectroscopic surveys are becoming generally available, thus leading to a resurgence of research into chromospheric activity. This opens up promising prospects to gain a much improved understanding of chromospheric physics and its wide-ranging impact.

EMISSA (Exploring Millimeter Indicators of Solar-Stellar Activity)

Context. Due to their wide wavelength coverage across the millimeter to centimeter (mm–cm) range and their increased sensitivity, modern interferometric arrays facilitate observations of the thermal and non-thermal radiation that is emitted from different layers in the outer atmospheres of stars. Aims. We study the spectral energy distribution (Sobs(ν)) of main-sequence stars based on archival observations in the mm–cm range with the aim to study their atmospheric stratification as a function of stellar type. Methods. The main-sequence stars with significant detection in mm bands were identified in the ALMA Science Archive. These data were then complemented with spectral flux data in the extreme ultraviolet to cm range as compiled from various catalogues and observatory archives. We compared the resultant Sobs(ν) of each star with a photospheric emission model (Smod(ν)) calculated with the PHOENIX code. The departures of Sobs(ν) from Smod(ν) were quantified in terms of a spectral flux excess parameter (ΔS∕Smod) and studied as a function of stellar type. Results. The initial sample consists of 12 main-sequence stars across a broad range of spectral types from A1 to M3.5 and the Sun-as-a-star as reference. The stars with Teff = 3000–7000 K (F–M type) showed a systematically higher Sobs(ν) than Smod(ν) in the mm–cm range. Their ΔS∕Smod exhibits a monotonic rise with decreasing frequency. The steepness of this rise is higher for cooler stars in the Teff = 3000–7000 K range, although the single fully convective star (Teff ~ 3000 K) in the sample deviates from this trend. Meanwhile, Sobs(ν) of the A-type stars agrees with Smod(ν) within errors. Conclusions. The systematically high ΔS∕Smod in F–M stars indicates hotter upper atmospheric layers, that is, a chromosphere and corona in these stars, like for the Sun. The mm–cm ΔS∕Smod spectrum offers a way to estimate the efficiency of the heating mechanisms across various outer atmospheric layers in main-sequence stars, and thereby to understand their structure and activity. We emphasise the need for dedicated surveys of main-sequence stars in the mm–cm range.

EvryFlare. iv. Detection of Periodicity in Flare Occurrence from Cool Stars with TESS

Abstract Phased flaring, or the periodic occurrence of stellar flares, may probe electromagnetic star−planet interaction (SPI), binary interaction, or magnetic conditions in spots. For the first time, we explore flare periodograms for a large sample of flare stars to identify periodicity due to magnetic interactions with orbiting companions, magnetic reservoirs, or rotational phase. Previous large surveys have explored periodicity at the stellar rotation period, but we do not assume periods must correspond with rotation in this work. Two-minute TESS light curves of 284 cool stars are searched for periods of 1–10 days using two newly developed periodograms. Because flares are discrete events in noisy and incomplete data, typical periodograms are not well suited to detect phased flaring. We construct and test a new Bayesian likelihood periodogram and a modified Lomb–Scargle periodogram. We find six candidates with a false-alarm probability below 1%. Three targets are ≥3σ detections of flare periodicity; the others are plausible candidates that cannot be individually confirmed. Periods range from 1.35 to 6.7 days and some, but not all, correlate with the stellar rotation period or its 1/2 alias. Periodicity from two targets may persist from TESS Cycle 1 into Cycle 3. The periodicity does not appear to persist for the others. Long-term changes in periodicity may result from the spot evolution observed from each candidate, which suggests magnetic conditions play an important role in sustaining periodicity.

A large sub-Neptune transiting the thick-disk M4 V TOI-2406

Context. Large sub-Neptunes are uncommon around the coolest stars in the Galaxy and are rarer still around those that are metal-poor. However, owing to the large planet-to-star radius ratio, these planets are highly suitable for atmospheric study via transmission spectroscopy in the infrared, such as with JWST. Aims. Here we report the discovery and validation of a sub-Neptune orbiting the thick-disk, mid-M dwarf star TOI-2406. The star’s low metallicity and the relatively large size and short period of the planet make TOI-2406 b an unusual outcome of planet formation, and its characterisation provides an important observational constraint for formation models. Methods. We first infer properties of the host star by analysing the star’s near-infrared spectrum, spectral energy distribution, and Gaia parallax. We use multi-band photometry to confirm that the transit event is on-target and achromatic, and we statistically validate the TESS signal as a transiting exoplanet. We then determine physical properties of the planet through global transit modelling of the TESS and ground-based time-series data. Results. We determine the host to be a metal-poor M4 V star, located at a distance of 56 pc, with properties Teff = 3100 ± 75 K, M* = 0.162 ± 0.008M⊙, R* = 0.202 ± 0.011R⊙, and [Fe∕H] = −0.38 ± 0.07, and a member of the thick disk. The planet is a relatively large sub-Neptune for the M-dwarf planet population, with Rp = 2.94 ± 0.17R⊕ and P= 3.077 d, producing transits of 2% depth. We note the orbit has a non-zero eccentricity to 3σ, prompting questions about the dynamical history of the system. Conclusions. This system is an interesting outcome of planet formation and presents a benchmark for large-planet formation around metal-poor, low-mass stars. The system warrants further study, in particular radial velocity follow-up to determine the planet mass and constrain possible bound companions. Furthermore, TOI-2406 b is a good target for future atmospheric study through transmission spectroscopy. Although the planet’s mass remains to be constrained, we estimate the S/N using amass-radius relationship, ranking the system fifth in the population of large sub-Neptunes, with TOI-2406 b having a much lower equilibrium temperature than other spectroscopically accessible members of this population.

A Near-infrared Survey of UV-excited Molecular Hydrogen in Photodissociation Regions

Abstract We present a comparative study of the near-infrared (NIR) H2 line emission from five regions near hot young stars: Sharpless 140, NGC 2023, IC 63, the Horsehead Nebula, and the Orion Bar. This emission originates in photodissociation or photon-dominated regions (PDRs), interfaces between photoionized and molecular gas near hot (O) stars or reflection nebulae illuminated by somewhat cooler (B) stars. In these environments, the dominant excitation mechanism for NIR emission lines originating from excited rotational-vibrational (rovibrational) levels of the ground electronic state is radiative or UV excitation (fluorescence), wherein absorption of far-UV photons pumps H2 molecules into excited electronic states from which they decay into the upper levels of the NIR lines. Our sources span a range of UV radiation fields (G 0 = 102−105) and gas densities (n H = 104−106 cm−3), enabling examination of how these properties affect the emergent spectrum. We obtained high-resolution (R ≈ 45,000) spectra spanning 1.45–2.45 μm on the 2.7 m Harlan J. Smith Telescope at McDonald Observatory with the Immersion Grating INfrared Spectrometer (IGRINS), detecting up to over 170 transitions per source from excited vibrational states (v = 1–14). The populations of individual rovibrational levels derived from these data clearly confirm UV excitation. Among the five PDRs in our survey, the Orion Bar shows the greatest deviation of the populations and spectrum from pure UV excitation, while Sharpless 140 shows the least deviation. However, we find that all five PDRs exhibit at least some modification of the level populations relative to their values under pure UV excitation, a result we attribute to collisional effects.

Wavelength Dependence of Activity-induced Photometric Variations for Young Cool Stars in Hyades

Abstract We investigate photometric variations due to stellar activity that induce systematic radial-velocity errors (so-called “jitter”) for the four targets in the Hyades open cluster observed by the K2 mission (EPIC 210721261, EPIC 210923016, EPIC 247122957, and EPIC 247783757). Applying Gaussian process regressions to the K2 light curves and the near-infrared (NIR) light curves observed with the IRSF 1.4 m telescope, we derive the wavelength dependences of the photometric signals due to stellar activity. To estimate the temporal variations in the photometric variability amplitudes between the two observation periods of K2 and IRSF, separated by more than 2 yr, we analyze a number of K2 targets in Hyades that have also been observed in Campaigns 4 and 13 and find a representative variation rate over 2 yr of 38% ± 71%. Taking this temporal variation into account, we constrain projected sizes and temperature contrast properties of the starspots in the stellar photosphere to be approximately 10% and 0.95%, respectively. These starspot properties can induce relatively large differences in the variability amplitude over different observational passbands, and we find that radial-velocity jitter may be more suppressed in the NIR than previously expected. Our result supports profits of ongoing exoplanet search projects that are attempting to detect or confirm young planets in open clusters via radial-velocity measurements in the NIR.

Slingshot Prominences, Formation, Ejection and Cycle Frequency in Cool Stars

AbstractStars lose mass and angular momentum during their lifetimes. Observations of H-alpha absorption of a number of low mass stars, show prominences transiting the stellar disc and being ejected into the extended stellar wind. Analytic modelling have shown these M-dwarf coronal structures growing to be orders of magnitude larger than their solar counterparts. This makes prominences responsible for mass and angular momentum loss comparable to that due to the stellar wind. We present results from a numerical study which used magnetohydrodynamic simulations to model the balance between gravity, magnetic confinement, and rotational acceleration. This allows us to study the time dependent nature of prominence formation. We demonstrate that a prominence, formed beyond the co-rotation radius, is ejected into the extended stellar wind in the slingshot prominence paradigm. Mass, angular momentum flux and ejection frequency have been calculated for a representative cool star, in the so-called Thermal Non-Equilibrium (TNE) regime.

A scaling relationship for non-thermal radio emission from ordered magnetospheres: from the top of the main sequence to planets

ABSTRACT In this paper, we present the analysis of incoherent non-thermal radio emission from a sample of hot magnetic stars, ranging from early-B to early-A spectral type. Spanning a wide range of stellar parameters and wind properties, these stars display a commonality in their radio emission which presents new challenges to the wind scenario as originally conceived. It was thought that relativistic electrons, responsible for the radio emission, originate in current sheets formed, where the wind opens the magnetic field lines. However, the true mass-loss rates from the cooler stars are too small to explain the observed non-thermal broad-band radio spectra. Instead, we suggest the existence of a radiation belt located inside the inner magnetosphere, similar to that of Jupiter. Such a structure explains the overall indifference of the broad-band radio emissions on wind mass-loss rates. Further, correlating the radio luminosities from a larger sample of magnetic stars with their stellar parameters, the combined roles of rotation and magnetic properties have been empirically determined. Finally, our sample of early-type magnetic stars suggests a scaling relationship between the non-thermal radio luminosity and the electric voltage induced by the magnetosphere’s co-rotation, which appears to hold for a broader range of stellar types with dipole-dominated magnetospheres (like the cases of the planet Jupiter and the ultracool dwarf stars and brown dwarfs). We conclude that well-ordered and stable rotating magnetospheres share a common physical mechanism for supporting the generation of non-thermal electrons.

NEREA (Near Earths and high-Res Exoplanet Atmospheres), a red/near-IR spectrograph concept for the Gran Telescopio Canarias

NEREA (Near Earths and high-Res Exoplanet Atmospheres) is a stable, compact, high-resolution spectrometer concept for the Gran Telescopio Canarias (GTC). The spectrograph is designed to be a common user instrument; however, its specifications are governed by two main science goals. (1) The discovery and characterization of planets around late-M type stars. Using the GTC’s large 10 m collecting area, NEREA would be capable of reaching out for planets around cooler, smaller, and dimmer stars than current facilities, contributing to completing the census of small planets in our stellar neighborhood (<30 pc); (2) The characterization of planetary atmospheres from Hot-Jupiters to Super-Earths. NEREA would be capable of detecting species including H2O, CH4, CO2, and ionized metals in planetary atmospheres; as well as detecting and resolving individual Na I, Hα, and He I line profiles, improving our understanding of the physical and chemical evolution of planetary atmospheres. Owing to the GTC’s collecting area, the current planetary sample size can be expected to increase by ∼40 times. The spectrometer preliminary design follows a single-pass echelle layout and is fiber-fed with both a rectangular 22 μm × 198 μm science fiber and a 29 μm calibration fiber. The system is designed to achieve a resolving power of 100,000 in the red-NIR regime (0.7 to 1.7 μm). The device footprint is ∼0.25 m × 1 m × 0.75 m, allowing the system to be temperature and pressure stabilized (<50 cm / s) in a small, compact container. The concept applies a 320 mm × 90 mm, R4 blazed, diffraction grating as the disperser and a 60.6-deg P-SF67 prism as the cross disperser. A 325 mm focal length, off-axis, parabolic mirror is employed as the collimator and the camera consists of a custom designed 325 mm refocusing lens. The instrument is designed to efficiently couple to the GTC AO system, for which we assume moderate performance of 0.3″ in our wavelength range. The spectrometer would also be able to perform efficiently without adaptive optics at a resolving power of 40,000 or, with losses, at a resolving power of 70,000 to 100,000.

Electron-Impact Excitation of Ge iii and Photoionisation of Ge ii

Abstract Energy levels, Einstein A-values and oscillator strengths are calculated for doubly-ionised germanium (Ge iii). We have undertaken a study of the electron-impact excitation of Ge iii and have also completed a study of the photoionisation of Ge ii. Ge iii target structures were generated using the relativistic GRASP0 package. Several scattering models were investigated to assess the uncertainty quantification of our results. Initially, the lowest lying 64 jj fine-structure levels were included in the close-coupling expansion resulting from 14 non-relativistic configurations. A second larger model was subsequently generated using the GASP package and incorporated the lowest 589 jj fine-structure levels from 17 non-relativistic configurations. Both the DARC and Breit-Pauli parallel suite of R-matrix collision codes were utilised in the scattering calculations to generate the collision strengths for incident electron-energies between 0 and 5 Rydbergs and subsequently the Maxwellian averaged effective collision strengths for temperatures in the range 1000 to 200000 Kelvin. The photoionisation calculations comprised of two studies, a DARC and a Breit-Pauli calculation incorporating the 250 lowest lying levels. Photoionisation cross sections for Ge ii are presented in the range 0-5 Ryds. Due to the lack of data available in the literature, the present radiative data and electron-impact/photoionisation cross sections are compared between the models presented in this paper. To assess their significance for astrophysics, the data are incorporated into the stellar atmosphere package sterne. Test models for the heavy-metal subdwarf LSIV−14○116 are not affected by the new data, but models for cooler stars may be sensitive to them.

The Aligned Orbit of the Eccentric Warm Jupiter K2-232b

Abstract Measuring the obliquity distribution of stars hosting warm Jupiters may help us to understand the formation of close-orbiting gas giants. Few such measurements have been performed due to practical difficulties in scheduling observations of the relatively infrequent and long-duration transits of warm Jupiters. Here, we report a measurement of the Rossiter–McLaughlin effect for K2-232 b, a warm Jupiter on an 11.17 day orbit with an eccentricity of 0.26. The data were obtained with the Automated Planet Finder during two separate transits. The planet’s orbit appears to be well aligned with the spin axis of the host star, with a projected spin–orbit angle of λ = −11.°1 ± 6.°6. Combined with the other available data, we find that high obliquities are almost exclusively associated with planets that either have an orbital separation greater than 10 stellar radii or orbit stars with effective temperatures hotter than 6000 K. This pattern suggests that the obliquities of the closest-orbiting giant planets around cooler stars have been damped by tidal effects.

Surface Rotation and Photometric Activity for Kepler Targets. II. G and F Main-sequence Stars and Cool Subgiant Stars

Abstract Dark magnetic spots crossing the stellar disk lead to quasiperiodic brightness variations, which allow us to constrain stellar surface rotation and photometric activity. The current work is the second of this series, where we analyze the Kepler long-cadence data of 132,921 main-sequence F and G stars and late subgiant stars. Rotation-period candidates are obtained by combining wavelet analysis with autocorrelation function. Reliable rotation periods are then selected via a machine-learning (ML) algorithm, automatic selection, and complementary visual inspection. The ML training data set comprises 26,521 main-sequence K and M stars from Paper I. To supplement the training, we analyze in the same way as Paper I, i.e., automatic selection and visual inspection, 34,100 additional stars. We finally provide rotation periods P rot and associated photometric activity proxy S ph for 39,592 targets. Hotter stars are generally faster rotators than cooler stars. For main-sequence G stars, S ph spans a wider range of values with increasing effective temperature, while F stars tend to have smaller S ph values in comparison with cooler stars. Overall for G stars, fast rotators are photometrically more active than slow rotators, with S ph saturating at short periods. The combined outcome of the two papers accounts for average P rot and S ph values for 55,232 main-sequence and subgiant FGKM stars (out of 159,442 targets), with 24,182 new P rot detections in comparison with McQuillan et al. The upper edge of the P rot distribution is located at longer P rot than found previously.

Stellar Metallicities from SkyMapper Photometry. II. Precise Photometric Metallicities of ∼280,000 Giant Stars with [Fe/H] &lt; −0.75 in the Milky Way

Abstract The Milky Way’s metal-poor stars are nearby ancient objects that are used to study early chemical evolution and the assembly and structure of the Milky Way. Here we present reliable metallicities of ∼280,000 stars with −3.75 ≲ [Fe/H] ≲ −0.75 down to g = 17 derived using metallicity-sensitive photometry from the second data release of the SkyMapper Southern Survey. We use the dependency of the flux through the SkyMapper v filter on the strength of the Ca ii K absorption features, in tandem with SkyMapper u, g, i photometry, to derive photometric metallicities for these stars. We find that metallicities derived in this way compare well to metallicities derived in large-scale spectroscopic surveys, and we use such comparisons to calibrate and quantify systematics as a function of location, reddening, and color. We find good agreement with metallicities from the APOGEE, LAMOST, and GALAH surveys, based on a standard deviation of σ ∼ 0.25 dex of the residuals of our photometric metallicities with respect to metallicities from those surveys. We also compare our derived photometric metallicities to metallicities presented in a number of high-resolution spectroscopic studies to validate the low-metallicity end ([Fe/H] &lt; −2.5) of our photometric metallicity determinations. In such comparisons, we find the metallicities of stars with photometric [Fe/H] &lt; −2.5 in our catalog show no significant offset and a scatter of σ ∼ 0.31 dex level relative to those in high-resolution work when considering the cooler stars (g − i &gt; 0.65) in our sample. We also present an expanded catalog containing photometric metallicities of ∼720,000 stars as a data table for further exploration of the metal-poor Milky Way.

The Abundance of Lead in Four Metal-poor Stars

Abstract Cowan et al. review how roughly half the elements heavier than iron found in the Sun are produced by rapid neutron capture and half by slow neutron capture, the r- and s-processes. In the Sun, their relative contribution to individual elemental abundances is well understood, except for the lightest and heaviest elements beyond iron. Their contributions are especially uncertain for the heaviest nonradioactive element, lead (Pb, Z = 82). This is constrained by deriving lead abundances in metal-poor stars. For in the most metal-poor halo stars, strontium and heavier elements are found in the solar r-process proportion; s-process elements appear only at metallicities above one-thirtieth solar. In unevolved metal-poor stars of roughly solar heavy-element content, only two UV Pb lines are detectable. Four such stars have high-resolution spectra of the strongest line, Pb ii at 2203.53 Å. Roederer et al. analyzed this line in one star, deriving a lead-to-iron abundance ratio 10 times solar. This and its blueshifted profile suggested strong s-process production. This work analyzes the UV spectra of all four stars. Calculations including a predicted Fe i line blueward of the Pb ii line, and assuming the lead abundance scales with r-process abundances, match all four profiles extremely well. A scaled s-process contribution might improve the match to the much lower lead abundance found in the unevolved star analyzed previously, but its s-process excess is modest. An Fe ii line blends the other lead line, Pb i at 2833.05 Å, which constrains the lead abundance only in the coolest star.

Oxygen-Rich Cool Stars in the Cepheus Region. New Spectroscopic Observations of Selected Objects from the KP2001 Catalog

Oxygen-Rich Cool Stars in the Cepheus Region. New Spectroscopic Observations of Selected Objects from the KP2001 Catalog

An Atacama Large Millimeter/submillimeter Array Survey of Chemistry in Disks around M4–M5 Stars

Abstract M-stars are the most common hosts of planetary systems in the Galaxy. Protoplanetary disks around M-stars thus offer a prime opportunity to study the chemistry of planet-forming environments. We present an Atacama Large Millimeter/submillimeter Array survey of molecular line emission toward a sample of five protoplanetary disks around M4–M5 stars (FP Tau, J0432+1827, J1100-7619, J1545-3417, and Sz 69). These observations can resolve chemical structures down to tens of astronomical units. Molecular lines of 12CO, 13CO, C18O, C2H, and HCN are detected toward all five disks. Lines of H2CO and DCN are detected toward 2/5 and 1/5 disks, respectively. For disks with resolved C18O, C2H, HCN, and H2CO emission, we observe substructures similar to those previously found in disks around solar-type stars (e.g., rings, holes, and plateaus). C2H and HCN excitation conditions estimated interior to the pebble disk edge for the bright disk J1100-7619 are consistent with previous measurements around solar-type stars. The correlation previously found between C2H and HCN fluxes for solar-type disks extends to our M4–M5 disk sample, but the typical C2H/HCN ratio is higher for the M4–M5 disk sample. This latter finding is reminiscent of the hydrocarbon enhancements found by previous observational infrared surveys in the innermost (&lt;10 au) regions of M-star disks, which is intriguing since our disk-averaged fluxes are heavily influenced by flux levels in the outermost disk, exterior to the pebble disk edge. Overall, most of the observable chemistry at 10–100 au appears similar for solar-type and M4–M5 disks, but hydrocarbons may be more abundant around the cooler stars.

The Swan: Data-driven Inference of Stellar Surface Gravities for Cool Stars from Photometric Light Curves

Abstract Stellar light curves are well known to encode physical stellar properties. Precise, automated, and computationally inexpensive methods to derive physical parameters from light curves are needed to cope with the large influx of these data from space-based missions such as Kepler and TESS. Here we present a new methodology that we call “The Swan,” a fast, generalizable, and effective approach for deriving stellar surface gravity ( ) for main-sequence, subgiant, and red giant stars from Kepler light curves using local linear regression on the full frequency content of Kepler long-cadence power spectra. With this inexpensive data-driven approach, we recover to a precision of ∼0.02 dex for 13,822 stars with seismic values between 0.2 and 4.4 dex and ∼0.11 dex for 4646 stars with Gaia-derived values between 2.3 and 4.6 dex. We further develop a signal-to-noise metric and find that granulation is difficult to detect in many cool main-sequence stars (T eff ≲ 5500 K), in particular K dwarfs. By combining our measurements with Gaia radii, we derive empirical masses for 4646 subgiant and main-sequence stars with a median precision of ∼7%. Finally, we demonstrate that our method can be used to recover to a similar mean absolute deviation precision for a TESS baseline of 27 days. Our methodology can be readily applied to photometric time series observations to infer stellar surface gravities to high precision across evolutionary states.