Late-type Dwarfs Research Articles

SPECULOOS: Ultracool dwarf transit survey

Context. One of the most promising avenues for the detailed study of temperate Earth-sized exoplanets is the detection of such planets in transit in front of stars that are small and near enough to make it possible to carry out a thorough atmospheric characterisation with next-generation telescopes, such as the James Webb Space telescope (JWST) or Extremely Large Telescope (ELT). In this context, the TRAPPIST-1 planets form a unique benchmark system that has garnered the interest of a large scientific community. Aims. The SPECULOOS survey is an exoplanet transit survey targeting a volume-limited (40 pc) sample of ultracool dwarf stars (of spectral type M7 and later) that is based on a network of robotic 1 m telescopes especially designed for this survey. The strategy for brighter and earlier targets leverages on the synergy with the ongoing TESS space-based exoplanet transit survey. Methods. We define the SPECULOOS target list as the sum of three non-overlapping sub-programmes incorporating the latest type objects (Teff ≲ 3000 K). Programme 1 features 365 dwarfs that are small and near enough to make it possible to detail atmospheric characterisation of an ‘Earth-like’ planet with the upcoming JWST. Programme 2 features 171 dwarfs of M5-type and later for which a significant detection of a planet similar to TRAPPIST-1b should be within reach of TESS. Programme 3 features 1121 dwarfs that are later than M6-type. These programmes form the basis of our statistical census of short-period planets around ultracool dwarf stars. Results. Our compound target list includes 1657 photometrically classified late-type dwarfs, with 260 of these targets classified, for the first time, as possible nearby ultracool dwarf stars. Our general observational strategy was to monitor each target between 100 and 200 h with our telescope network, making efficient use of the synergy with TESS for our Programme 2 targets and a proportion of targets in our Programme 1. Conclusions. Based on Monte Carlo simulations, we expect to detect up to a few dozen temperate, rocky planets. We also expect a number of them to prove amenable for atmospheric characterisation with JWST and other future giant telescopes, which will substantially improve our understanding of the planetary population of the latest-type stars.

An Increase in Small-planet Occurrence with Metallicity for Late-type Dwarf Stars in the Kepler Field and Its Implications for Planet Formation

Abstract While it is well-established that giant-planet occurrence rises rapidly with host star metallicity, it is not yet clear if small-planet occurrence around late-type dwarf stars depends on host star metallicity. Using the Kepler Data Release 25 planet candidate list and its completeness data products, we explore planet occurrence as a function of metallicity in the Kepler field’s late-type dwarf stellar population. We find that planet occurrence increases with metallicity for all planet radii R p down to at least R p ≈ 2 R ⊕, and that in the range 2 R ⊕ ≲ R p ≲ 5 R ⊕, planet occurrence scales linearly with metallicity Z. Extrapolating our results, we predict that short-period planets with R p ≲ 2 R ⊕ should be rare around early-M dwarf stars with [M/H] ≲ −0.5 or late-M dwarf stars with [M/H] ≲ +0.0. This dependence of planet occurrence on metallicity observed in the Kepler field emphasizes the need to control for metallicity in estimates of planet occurrence for late-type dwarf stars like those targeted by Kepler’s K2 extension and the Transiting Exoplanet Survey Satellite. We confirm the theoretical expectation that the small-planet occurrence–host star metallicity relation is stronger for low-mass stars than for solar-type stars. We establish that the expected solid mass in planets around late-type dwarfs in the Kepler field is comparable to the total amount of planet-making solids in their protoplanetary disks. We argue that this high efficiency of planet formation favors planetesimal accretion over pebble accretion as the origin of the small planets observed by Kepler around late-type dwarf stars.

A search for star clusters in the outskirts of the Large Magellanic Cloud: indication of clusters in the age gap

ABSTRACT The YMCA (Yes, Magellanic Clouds Again) and STEP (The SMC in Time: Evolution of a Prototype interacting late-type dwarf galaxy) projects are deep g, i photometric surveys carried out with the VLT Survey Telescope (VST) and devoted to study the outskirts of the Magellanic System. A main goal of YMCA and STEP is to identify candidate stellar clusters and complete their census out to the outermost regions of the Magellanic Clouds. We adopted a specific overdensity search technique coupled with a visual inspection of the colour–magnitude diagrams to select the best candidates and estimate their ages. To date, we analysed a region of 23 square degrees in the outskirts of the Large Magellanic Cloud, detecting 85 candidate cluster candidates, 16 of which have estimated ages falling in the so-called age gap. We use these objects together with literature data to gain insight into the formation and interaction history of the Magellanic Clouds.

Chromospheric activity of nearby Sun-like stars

Context.The chromospheric emission in the cores of the Ca IIH & K lines of late-type dwarfs is a well known indicator of magnetic activity that decreases with increasing stellar age.Aims.I use this indicator to investigate the formation history of nearby G- and early K-type stars with origins at galactocentric distances similar to that of the region where the Sun was born.Methods.A parent sample of single main-sequence stars with near-solar metallicity and known magnetic activity levels is built from catalogues of stellar atmospheric parameters and chromospheric activity indices. A kinematical approach usesGaiaastrometric data to differentiate thin disc stars from thick disc stars. Measured distributions ofR′HKchromospheric activity indices are compared with Monte Carlo simulations based on an empirical model of chromospheric activity evolution.Results.The thin disc includes a significant fraction of Sun-like stars with intermediate activity levels (2 × 10−5≤R′HK≤ 6 × 10−5), while most early K- and G-type stars from the thick disc are inactive (R′HK< 2 × 10−5). The chromospheric activity distribution among nearby Sun-like dwarfs from the thin disc can be explained by a combination of an old (>6–7 Gyr) star formation event (or events) and a more recent (<3 Gyr) burst of star formation. Such an event is not required to account for theR′HKindex distributions of nearby thick disc stars.Conclusions.The distribution of magnetic activity among local G- and early K-type stars with a near-solar metallicity bears the imprint of an important star formation event that occurred ~1.9–2.6 Gyr ago in the thin disc of the Milky Way.

Fundamental stellar parameters of benchmark stars from CHARA interferometry

Context.Benchmark stars are crucial as validating standards for current as well as future large stellar surveys of the Milky Way. However, the number of suitable metal-poor benchmark stars is currently limited, owing to the difficulty in determining reliable effective temperatures (Teff) in this regime.Aims.We aim to construct a new set of metal-poor benchmark stars based on reliable interferometric effective temperature determinations and a homogeneous analysis. The aim is to reach a precision of 1% inTeff, as is crucial for sufficiently accurate determinations of the full set of fundamental parameters and abundances for the survey sources.Methods.We observed ten late-type metal-poor dwarfs and giants: HD 2665, HD 6755, HD 6833, HD 103095, HD 122563, HD 127243, HD 140283, HD 175305, HD 221170, and HD 224930. Only three of them (HD 103095, HD 122563, and HD 140283) have previously been used as benchmark stars. For the observations, we used the high-angular-resolution optical interferometric instrument PAVO at the CHARA array. We modelled angular diameters using 3D limb-darkening models and determined effective temperatures directly from the Stefan-Boltzmann relation, with an iterative procedure to interpolate over tables of bolometric corrections. Surface gravities (log(g)) were estimated from comparisons to Dartmouth stellar evolution model tracks. We collected spectroscopic observations from the ELODIE and FIES spectrographs and estimated metallicities ([Fe/H]) from a 1D non-local thermodynamic equilibrium (NLTE) abundance analysis of unblended lines of neutral and singly ionised iron.Results.We inferredTeffto better than 1% for five of the stars (HD 103095, HD 122563, HD 127243, HD 140283, and HD 224930). The effective temperatures of the other five stars are reliable to between 2 and 3%; the higher uncertainty on theTefffor those stars is mainly due to their having a larger uncertainty in the bolometric fluxes. We also determined log(g) and [Fe/H] with median uncertainties of 0.03 dex and 0.09 dex, respectively.Conclusions.This study presents reliable and homogeneous fundamental stellar parameters for ten metal-poor stars that can be adopted as a new set of benchmarks. The parameters are based on our consistent approach of combining interferometric observations, 3D limb-darkening-modelling and spectroscopic observations. The next paper in this series will extend this approach to dwarfs and giants in the metal-rich regime.

Statistics and properties of emission-line regions in the local volume dwarf galaxies

ABSTRACT We used the H α images from a large sample of nearby late-type dwarf galaxies to investigate properties of their emission structure. The sample consists of 300 galaxies of the irregular (Irr), Magellanic irregular (Im), blue compact dwarf (BCD), and transition (Tr) types situated within a distance of 11 Mpc. In each galaxy, we indicated the number of compact H ii regions, the presence of bubble-like or filament-like structures, the presence of a faint diffuse emission, and a sign of the global burst. The larger the luminosity of a galaxy, the greater number of compact H ii sources in it. The integral and specific star formation rates (SFRs) of the dwarf increase steeply with the increase of the number of H ii regions showing the evidence of the epidemic character of the star formation process. The dwarf galaxies with emission-line bubbles, or filaments, or signs of the global star formation burst have approximately the same hydrogen-mass-to-luminosity ratio as that of the whole sample objects. However, their mean SFR is significantly higher than that of other galaxies in the sample. Emission bubble-like structures are found in the nearby dwarfs with a frequency of one case per four to five galaxies. Their linear diameters are close to those expected for supernova remnants. The mean specific SFR for the nearby late-type dwarfs is close to the Hubble parameter, H0 = −10.14 dex (yr)−1, consistent with the sluggish cosmic star formation history of galaxies of this kind.

The Smallest Scale of Hierarchy Survey (SSH). I. Survey Description.

Abstract The Smallest Scale of Hierarchy (SSH) survey is an ongoing strategic large program at the Large Binocular Telescope, aimed at the detection of faint stellar streams and satellites around 45 late-type dwarf galaxies located in the Local Universe within ≃10 Mpc. SSH exploits the wide-field, deep photometry provided by the Large Binocular Cameras in the two wide filters g and r. This paper describes the survey, its goals, and the observational and data reduction strategies. We present preliminary scientific results for five representative cases (UGC 12613, NGC 2366, UGC 685, NGC 5477 and UGC 4426) covering the whole distance range spanned by the SSH targets. We reach a surface brightness limit as faint as μ(r) ∼ 31 mag arcsec−2 both for targets closer than 4−5 Mpc, which are resolved into individual stars, and for more distant targets through the diffuse light. Our analysis reveals the presence of extended low surface brightness stellar envelopes around the dwarfs, reaching farther out than what traced by the integrated light, and as far out as, or even beyond, the observed H I disk. Stellar streams, arcs, and peculiar features are detected in some cases, indicating possible perturbation, accretion, or merging events. We also report on the discovery of an extreme case of Ultra Diffuse Galaxy (μg(0) = 27.9 mag/arcsec2) in the background of one of our targets, to illustrate the power of the survey in revealing extremely low surface brightness systems.

Spectroscopic Follow-up of Discoveries from the NEOWISE Proper Motion Survey

Abstract We present low-resolution near-infrared spectra of discoveries from an all-sky proper motion search conducted using multi-epoch data from the Wide-field Infrared Survey Explorer. Using the data from NEOWISE, along with the AllWISE catalog, Schneider et al. conducted an all-sky proper motion survey to search for nearby objects with high proper motions. Here, we present a follow-up spectroscopic survey of 65 of their discoveries, which focused primarily on potentially nearby objects (d < 25 pc), candidate late-type brown dwarfs (≥L7), and subdwarf candidates. We found 31 new M dwarfs, 18 new L dwarfs, and 11 new T dwarfs. Of these, 13 are subdwarfs, including one new sdL1 and two new sdL7s. Eleven of these discoveries, with spectral types ranging from M7 to T7 (including one subdwarf) are predicted to be within 25 pc, adding to the number of known objects in the solar neighborhood. We also discovered three new early-type T subdwarf candidates, one sdT1, one sdT2, and one sdT3, which would increase the number of known early-type T subdwarfs from two to five.

Evidence of New Magnetic Transitions in Late-type Dwarfs from Gaia DR2

Abstract The second Gaia data release contains the identification of 147,535 low-mass ( ) rotational modulation variable candidates on (or close to) the main sequence, together with their rotation period and modulation amplitude. The richness, the period and amplitude range, and the photometric precision of this sample make it possible to unveil, for the first time, signatures of different surface inhomogeneity regimes in the amplitude–period density diagram. The modulation amplitude distribution shows a clear bimodality, with an evident gap at periods P ≲ 2 days. The low-amplitude branch, in turn, shows a period bimodality with a main clustering at periods P ≈ 5–10 days and a secondary clustering of ultra-fast rotators at P ≲ 0.5 day. The amplitude–period multimodality is correlated with the position in the period–absolute magnitude (or period–color) diagram, with the low- and high-amplitude stars occupying different preferential locations. Here we argue that such a multimodality provides further evidence of the existence of different regimes of surface inhomogeneities in young and middle-age low-mass stars and we lay out possible scenarios for their evolution, which manifestly include rapid transitions from one regime to another. In particular, the data indicate that stars spinning up close to breakup velocity undergo a very rapid change in their surface inhomogeneities configuration, which is revealed here for the first time. The multimodality can be exploited to identify field stars of age ∼100–600 Myr belonging to the slow-rotator low-amplitude sequence, for which age can be estimated from the rotation period via gyrochronology relationships.

Denuded dwarfs demystified: gas loss from dSph progenitors and implications for the minimum mass of galaxies

The placement of early-type dwarf galaxies (dSphs and dEs) with respect to the Potential Plane defined by pressure-supported late-type dwarf galaxies (dIs and BCDs) has been determined from surface brightness profiles. dEs and the most luminous dSphs lie on the Plane, suggesting that they emerged from late-type dwarfs that converted most of their gas into stars. However, there is a critical value of the potential at which dSphs start to fall systematically below the Plane, with the deviation growing as the potential becomes shallower. The displacements are attributed to depletion of baryons through gas loss, smaller galaxies having lost proportionately more gas. The critical potential corresponds to an escape velocity of 50 $\pm$ 8 km/s, which is what is expected for gas with a temperature of 13,000 $\pm$ 4,000 K, typical of a low-metallicity HII region. This suggests that photoionization was responsible for instigating the loss of gas by galaxies with potentials shallower than the critical value, with evacuation occurring over a few tens of millions of years. Extreme ratios of dynamical to luminous masses observed for the smallest dSphs are an artifact of mass loss. Because the efficiency with which gas was converted into stars was lower for dSphs with shallower potentials, there should be a minimum baryonic mass for a galaxy below which the stellar mass is negligible. Gross extrapolation of the trend of inferred gas masses with stellar masses suggests a value between 500 and 10,000 M$_\odot$. The corresponding dynamical mass is below 10$^6$ M$_\odot$.

The evolution of ultra-diffuse galaxies in nearby galaxy clusters from the Kapteyn IAC WEAVE INT Clusters Survey

We study the population of ultra-diffuse galaxies (UDGs) in a set of eight nearby ($z <$ 0.035) galaxy clusters, from the Kapteyn IAC WEAVE INT Clusters Survey ($\texttt{KIWICS}$). We report the discovery of 442 UDG candidates in our eight field of views, with 247 of these galaxies lying at projected distances < 1 R$_{200}$ from their host cluster. With the aim of testing theories about their formation, we study the scaling relations of UDGs comparing with different types of galaxies, finding that in the full parameter space they behave as dwarf galaxies and their colors do not seem to correlate with their effective radii. To investigate the influence of the environment on the evolution of UDGs we analyze their structural properties as functions of the projected clustercentric distance and the mass of their host cluster. We find no systematic trends for the stellar mass nor effective radius as function of the projected distance. However, the fraction of blue UDGs seems to be lower towards the center of clusters, and UDGs in the inner and outer regions of clusters have different S\'ersic index and axis ratio distributions. Specifically, the axis ratio distributions of the outer and inner UDGs resemble the axis ratio distributions of, respectively, late-type dwarfs and dwarf ellipticals in the Fornax Cluster suggesting an environmentally-driven evolution and another link between UDGs and dwarf galaxies. In general our results suggest strong similarities between UDGs and smaller dwarf galaxies in their structural parameters and their transformation within clusters.

Impact of metallicity and star formation rate on the time-dependent, galaxy-wide stellar initial mass function

The stellar initial mass function (IMF) is commonly assumed to be an invariant probability density distribution function of initial stellar masses. These initial stellar masses are generally represented by the canonical IMF, which is defined as the result of one star formation event in an embedded cluster. As a consequence, the galaxy-wide IMF (gwIMF) should also be invariant and of the same form as the canonical IMF; gwIMF is defined as the sum of the IMFs of all star-forming regions in which embedded clusters form and spawn the galactic field population of the galaxy. Recent observational and theoretical results challenge the hypothesis that the gwIMF is invariant. In order to study the possible reasons for this variation, it is useful to relate the observed IMF to the gwIMF. Starting with the IMF determined in resolved star clusters, we apply the IGIMF-theory to calculate a comprehensive grid of gwIMF models for metallicities, [Fe/H] ∈ (−3, 1), and galaxy-wide star formation rates (SFRs), SFR ∈ (10−5, 105) M⊙ yr−1. For a galaxy with metallicity [Fe/H] &lt; 0 and SFR &gt; 1 M⊙ yr−1, which is a common condition in the early Universe, we find that the gwIMF is both bottom light (relatively fewer low-mass stars) and top heavy (more massive stars), when compared to the canonical IMF. For a SFR &lt; 1 M⊙ yr−1 the gwIMF becomes top light regardless of the metallicity. For metallicities [Fe/H] &gt; 0 the gwIMF can become bottom heavy regardless of the SFR. The IGIMF models predict that massive elliptical galaxies should have formed with a gwIMF that is top heavy within the first few hundred Myr of the life of the galaxy and that it evolves into a bottom heavy gwIMF in the metal-enriched galactic centre. Using the gwIMF grids, we study the SFR−Hα relation and its dependency on metallicity and the SFR. We also study the correction factors to the Kennicutt SFRK − Hα relation and provide new fitting functions. Late-type dwarf galaxies show significantly higher SFRs with respect to Kennicutt SFRs, while star-forming massive galaxies have significantly lower SFRs than hitherto thought. This has implications for gas-consumption timescales and for the main sequence of galaxies. We explicitly discuss Leo P and ultra-faint dwarf galaxies.

Serendipitous X-Ray Sources in the Chandra HRC Field around Alpha Centauri

Abstract For more than a decade, Alpha Centauri AB (G2 V+K1 V) has been observed by Chandra, in a long-term program to follow coronal (T ∼ 106 K) activity cycles of the two sunlike stars. Over 2008.4–2017.8, 19 HRC-I exposures were taken, each about 10 ks in duration, and spaced about six months apart. Beyond monitoring the AB X-ray luminosities, the HRC-I sequence represents a unique decadal record of the dozen, or so, serendipitous X-ray sources in the α Cen field, which is at low Galactic latitude and thus dominated by nearby stars. For the present study, the 10 brightest candidates were considered. Only a handful of these were persistent; most were variable, some highly so, flaring in a few epochs, weak or absent in the others. All 10 X-ray sources have Gaia objects within about 2″; mostly late-type dwarfs, but a few giants. However, two of the proposed optical counterparts have statistically significant offsets, and possible conflicts between the X-ray and optical properties. Another of the candidates brightened by a factor of 100 in X-rays during a single exposure, briefly attaining super-flare status. The Gaia counterpart is anomalously blue for its absolute G-magnitude and likely is a WD+dM pair. To the extent that the low Galactic latitude field is representative, the Chandra time-domain view emphasizes that the high-energy stellar sky is biased toward transient sources, so any snapshot survey surely will miss many of the most interesting objects.

Discovery and characterisation of long-period eclipsing binary stars from Kepler K2 campaigns 1, 2, and 3

Context. The Kepler K2 mission now makes it possible to find and study a wider variety of eclipsing binary stars than has been possible to-date, particularly long-period systems with narrow eclipses. Aims. Our aim is to characterise eclipsing binary stars observed by the Kepler K2 mission with orbital periods longer than P ≈ 5.5 days. Methods. The ellc binary star model has been used to determine the geometry of eclipsing binary systems in Kepler K2 campaigns 1, 2 and 3. The nature of the stars in each binary is estimated by comparison to stellar evolution tracks in the effective temperature – mean stellar density plane. Results. 43 eclipsing binary systems have been identified and 40 of these are characterised in some detail. The majority of these systems are found to be late-type dwarf and sub-giant stars with masses in the range 0.6–1.4 solar masses. We identify two eclipsing binaries containing red giant stars, including one bright system with total eclipses that is ideal for detailed follow-up observations. The bright B3V-type star HD 142883 is found to be an eclipsing binary in a triple star system. We observe a series of frequencies at large multiples of the orbital frequency in BW Aqr that we tentatively identify as tidally induced pulsations in this well-studied eccentric binary system. We find that the faint eclipsing binary EPIC 201160323 shows rapid apsidal motion. Rotational modulation signals are observed in 13 eclipsing systems, the majority of which are found to rotate non-synchronously with their orbits. Conclusions. The K2 mission is a rich source of data that can be used to find long period eclipsing binary stars. These data combined with follow-up observations can be used to precisely measure the masses and radii of stars for which such fundamental data are currently lacking, e.g., sub-giant stars and slowly-rotating low-mass stars.

Two planetary systems with transiting Earth-sized and super-Earth planets orbiting late-type dwarf stars

ABSTRACTWe present two new planetary systems found around cool dwarf stars with data from the K2 mission. The first system was found in K2-239 (EPIC 248545986), characterized in this work as M3.0V and observed in the 14th campaign of K2. It consists of three Earth-sized transiting planets with radii of 1.1, 1.0, and 1.1 R⊕, showing a compact configuration with orbital periods of 5.24, 7.78, and 10.1 d, close to 2:3:4 resonances. The second was found in K2-240 (EPIC 249801827), characterized in this work as M0.5V and observed in the 15th campaign. It consists of two transiting super-Earths with radii 2.0 and 1.8 R⊕ and orbital periods of 6.03 and 20.5 d. The equilibrium temperatures of the atmospheres of these planets are estimated to be in the range of 380–600 K and the amplitudes of signals in transmission spectroscopy are estimated at ∼10 ppm.

The stellar content of the XMM-Newton slew survey

Aims. We present a detailed analysis of the stellar content of the current version of the XMM-Newton slew survey (XMMSL2). Methods. Since stars emit only a small fraction of their total luminosity in the X-ray band, the stellar XMMSL2 sources ought to have relatively bright optical counterparts. Therefore the stellar identifications were obtained by an automatic crossmatch of the XMMSL2 catalog with the first Gaia data release (Gaia DR1), 2MASS, and Tycho2 catalogs. The reliability of this procedure was verified by a comparison with the individually classified Einstein Observatory medium sensitivity survey X-ray sources and by a crossmatch with the Chandra Source Catalog. Results. We identify 6815 of the 23 252 unique XMMSL2 sources to be stellar sources, while 893 sources are flagged as unreliable. For every counterpart a matching probability is estimated based upon the distance between the XMMSL2 source and the counterpart. Given this matching probability the sample is expected to be reliable to 96.7 % and complete to 96.3 % . The sample contains stars of all spectral types and luminosity classes, and late-type dwarfs have the largest share. For many stellar sources the fractional contribution of the X-ray band to the total energy output is found above the saturation limit of previous studies (Lx/Lbol = 10−3), because the XMMSL2 sources are more affected by flares owing to their short exposure times of typically 6 s. A comparison with the second ROSAT all-sky survey (2RXS) source catalog shows that about 25 % of the stellar XMMSL2 sources are previously unknown X-ray sources. The results of our identification procedure can be accessed via VizieR.

Spitzer Light Curves of the Young, Planetary-mass TW Hya Members 2MASS J11193254–1137466AB and WISEA J114724.10–204021.3

Abstract We present Spitzer Space Telescope time-series photometry at 3.6 and 4.5 μm of 2MASS J11193254−1137466AB and WISEA J114724.10−204021.3, two planetary-mass, late-type (∼L7) brown dwarf members of the ∼10 Myr old TW Hya Association. These observations were taken in order to investigate whether or not a tentative trend of increasing variability amplitude with decreasing surface gravity seen for L3–L5.5 dwarfs extends to later-L spectral types and to explore the angular momentum evolution of low-mass objects. We examine each light curve for variability and find a rotation period of 19.39+0.33 −0.28 hr and semi-amplitudes of 0.798+0.081 −0.083% at 3.6 μm and 1.108+0.093 −0.094% at 4.5 μm for WISEA J114724.10−204021.3. For 2MASS J11193254−1137466AB, we find a single period of 3.02+0.04 −0.03 hr with semi-amplitudes of 0.230+0.036 −0.035% at 3.6 μm and 0.453 ± 0.037% at 4.5 μm, which we find is possibly due to the rotation of one component of the binary. Combining our results with 12 other late-type L dwarfs observed with Spitzer from the literature, we find no significant differences between the 3.6 μm amplitudes of low surface gravity and field gravity late-type L brown dwarfs at Spitzer wavelengths, and find tentative evidence (75% confidence) of higher amplitude variability at 4.5 μm for young, late-type Ls. We also find a median rotation period of young brown dwarfs (10–300 Myr) of ∼10 hr, more than twice the value of the median rotation period of field-age brown dwarfs (∼4 hr), a clear signature of brown dwarf rotational evolution.

The UV Emission of Stars in the LAMOST Survey. I. Catalogs

Abstract We present the ultraviolet magnitudes for over three million stars in the LAMOST survey, in which 2,202,116 stars are detected by GALEX. For 889,235 undetected stars, we develop a method to estimate their upper limit magnitudes. The distribution of (FUV − NUV) shows that the color declines with increasing effective temperature for stars hotter than 7000 K in our sample, while the trend disappears for the cooler stars due to upper atmosphere emission from the regions higher than their photospheres. For stars with valid stellar parameters, we calculate the UV excesses with synthetic model spectra, and find that the (FUV − NUV) versus can be fitted with a linear relation and late-type dwarfs tend to have high UV excesses. There are 87,178 and 1,498,103 stars detected more than once in the visit exposures of GALEX in the FUV and NUV, respectively. We make use of the quantified photometric errors to determine statistical properties of the UV variation, including intrinsic variability and the structure function on the timescale of days. The overall occurrence of possible false positives is below 1.3% in our sample. UV absolute magnitudes are calculated for stars with valid parallaxes, which could serve as a possible reference frame in the NUV. We conclude that the colors related to UV provide good criteria to distinguish between M giants and M dwarfs, and the variability of RR Lyrae stars in our sample is stronger than that of other A and F stars.

Cracking the Conundrum of F-supergiant Coronae

Abstract Chandra X-ray and HST far-ultraviolet (FUV) observations of three early-F supergiants have shed new light on a previous puzzle involving a prominent member of the class: α Persei (HD 20902: F5 Ib). The warm supergiant is a moderately strong, hard coronal ( K) X-ray source, but has 10 times weaker “subcoronal” Si iv 1393 Å ( K) emissions than early-G supergiants of similar high-energy properties. The α Per X-ray excess was speculatively ascribed to a close-in hyperactive G-dwarf companion, which could have escaped previous notice, lost in the glare of the bright star. However, a subsequent dedicated multi-wavelength imaging campaign failed to find any evidence for a resolved secondary. The origin of the α Per high-energy dichotomy then devolved to (1) an unresolved companion or (2) intrinsic coronal behavior. Exploring the second possibility, the present program has found that early-F supergiants do appear to belong to a distinct coronal class, characterized by elevated X-ray/FUV ratios, although sharing some similarities with Cepheid variables in their transitory X-ray “high states.” Remarkably, the early-F supergiants now are seen to align with the low-activity end of the X-ray/FUV sequence defined by late-type dwarfs, suggesting that the disjoint behavior relative to the G supergiants might be attributed to thinner outer atmospheres on the F types, as in dwarfs, but in this case perhaps caused by a weakened “ionization valve” effect due to overly warm photospheres.

Observations and light curve solutions of three ultrashort-period W UMa binaries

Photometric observations in V, Rc and Ic bands of the ultrashort-period W UMa binaries 1SWASP J044132.96+440613.7, 1SWASP J052926.88+461147.5 and NSVS 2175434 are presented. The results from the modeling and analysis of our observations reveal that: (i) All targets undergo total eclipses and their photometric mass ratios should be accepted with confidence; (ii) All stellar components are late-type dwarfs; (iii) The temperature difference of target components does not exceed 150 K; (iv) All targets have overcontact configurations with fill-out factor around 0.24; (v) The orbit of NSVS 2175434 is slightly eccentric which is unusual for such an ultrashort-period binary; (vi) The orbital periods of all targets were improved. Masses, radii and luminosities of the stellar components were estimated by the empirical relation “period, orbital axis” for short- and ultrashort-period binaries.