Magnetic fields in cool stars can be investigated by measuring Zeeman line broadening and polarization in atomic and molecular lines. Similar to the Sun, these fields are complex and height-dependent. Many molecular lines dominating M-dwarf spectra (e.g., FeH, CaH, MgH, and TiO) are temperature -- and Zeeman -- sensitive and form at different atmospheric heights, which makes them excellent probes of magnetic fields on M dwarfs. Our goal is to analyze the complexity of magnetic fields in M dwarfs. We investigate how magnetic fields vary with the stellar temperature and how "surface" inhomogeneities are distributed in height -- the dimension that is usually neglected in stellar magnetic studies. We have determined effective temperatures of the photosphere and of magnetic features, magnetic field strengths and filling factors for nine M dwarfs (M1-M7). Our chi^2 analysis is based on a comparison of observed and synthetic intensity and circular polarization profiles. Stokes profiles were calculated by solving polarized radiative transfer equations. Properties of magnetic structures depend on the analyzed atomic or molecular species and their formation heights. Two types of magnetic features similar to those on the Sun have been found: a cooler (starspots) and a hotter (network) one. The magnetic field strength in both starspots and network is within 3 kG to 6 kG, on average it is 5 kG. These fields occupy a large fraction of M dwarf atmospheres at all heights, up to 100%. The plasma beta is less than one, implying highly magnetized stars. A combination of molecular and atomic species and a simultaneous analysis of intensity and circular polarization spectra have allowed us to better decipher the complexity of magnetic fields on M dwarfs, including their dependence on the atmospheric height. This work provides an opportunity to investigate a larger sample of M dwarfs and L-type brown dwarfs.
A new relative orbit solution with new dynamical masses is determined for the nearby white dwarf - red dwarf pair 40 Eri BC. The period is 230.09+/-0.68y. It is predicted to close slowly over the next half-century getting as close as 1.32" in early 2066. We determine masses of 0.575+/-0.018 $m_\odot$ for the white dwarf and 0.2041+/-0.0064 $m_\odot$ for the red dwarf companion. The inconsistency of the masses determined by gravitational redshift and dynamical techniques, due to a premature orbit calculation, no longer exists.
Based on detailed spectral synthesis we carry out quantitative measurements of the strength and complexity of surface magnetic fields in the four well-known M-dwarfs GJ 388, GJ 729, GJ 285, and GJ 406 populating the mass regime around the boundary between partially and fully convective stars. Very high resolution R=100000, high signal-to-noise (up to 400) near-infrared Stokes I spectra were obtained with CRIRES at ESO's Very Large Telescope covering regions of the FeH Wing-Ford transitions at 1mum. The field distributions in all four stars are characterized by three distinct groups of field co…
Modern databases and rows of observations allow to make progress in study of magnetic activity of solar-type stars and cold dwarfs. Digitizing of wide-filed plates in the large archives give historical data expanding the investigated time intervals up to century. Photometric sources for a seek and study of cycles produced by starspots are considered and results for several red dwarfs are presented.
There are 88 stars which lack colours, but have measured parallaxes in \textit{Gaia} EDR3 that place them within 20\,pc from the sun. Among them we found two new common parallax and proper motion (CPPM) companions separated from their primaries by about 3\,arcsec. The CPPM companion of a nearby ($d=14.98$\,pc) F1 star, \object{HD 105452 B}, was already imaged with the \textit{Hubble Space Telescope} and is now confirmed with \textit{Gaia} data and photometrically classified by us as M4 dwarf. The other CPPM companion, \object{SCR J1214-2345 B} orbiting an M4.5 dwarf at $d=10.77$\,pc, represent…
We examine the hypothesis that the red near-infrared colors of some L dwarfs could be explained by a "dust haze" of small particles in their upper atmospheres. This dust haze would exist in conjunction with the clouds found in dwarfs with more typical colors. We developed a model which uses Mie theory and the Hansen particle size distributions to reproduce the extinction due to the proposed dust haze. We apply our method to 23 young L dwarfs and 23 red field L dwarfs. We constrain the properties of the dust haze including particle size distribution and column density using Markov-Chain Monte C…
We present the completion of a program to cross-correlate the SDSS Data Release 1 and 2MASS Point Source Catalog in search for extremely red L and T dwarfs. The program was initiated by Metchev and collaborators, who presented the findings on all newly identified T dwarfs in SDSS DR1, and estimated the space density of isolated T0--T8 dwarfs in the solar neighbourhood. In the current work we present most of the L dwarf discoveries. Our red-sensitive (z-J > 2.75 mag) cross-match proves to be efficient in detecting peculiarly red L dwarfs, adding two new ones, including one of the reddest known …
We report the discovery of an unusually red brown dwarf found in a search for high proper motion objects using WISE and 2MASS data. WISEP J004701.06+680352.1 is moving at 0.44$ arcsec/yr and lies relatively close to the Galactic Plane (b=5.2 degrees). Near-infrared photometry and spectroscopy reveals that this is one of the reddest (2MASS J-K_s = 2.55 +/- 0.08 mag) field L dwarfs yet detected, making this object an important member of the class of unusually red L dwarfs. We discuss evidence for thick condensate clouds and speculate on the age of the object. Although models by different researc…
We present three new nearby L dwarf candidates, found in a continued combined color/proper motion search using WISE, 2MASS, and other survey data, where we included extended WISE sources and looked closer to the Galactic plane region. Their spectral types and distances were estimated from photometric comparisons to well-known L dwarfs with trigonometric parallaxes. The first object, 2MASS J07555430-3259589, is an extremely red L7.5p dwarf candidate at a photometric distance of about 16 pc. Its position, proper motion and distance are consistent with membership in the Carina-Near young moving g…
Early-type R stars and J stars are a special type of carbon star, having enhanced nitrogen ($\rm [N/Fe]\approx 0.5$), lithium, a low \iso{12}{C}/\iso{13}{C} ratio ($<15$) and no s-element enhancements. The merger of a helium white dwarf with a red giant is regarded to be a possible model for the origin of early-type R stars, but the details of nucleosynthesis are not clear. In this paper we investigate three possible channels for helium white-dwarf + red-giant mergers, and find that, amongst the three, only a high-mass helium white dwarf subducted into a low core-mass red giant can make an ear…
Using high resolution UVES spectra of the eclipsing Post Common Envelope Binary QS Vir we detect material along the line of sight to the white dwarf at orbital phase $ϕ=0.16$. We ascribe this to a stellar prominence originating from the M dwarf secondary star which passes in front of the white dwarf at this phase. This creates sharp absorption features in the hydrogen Balmer series and Ca II H and K lines. The small size of the white dwarf allows us to place tight constraints on the column density of hydrogen in the n=2 level of log_(10)(N_2) = 14.10 +/- 0.03 cm^(-2) and, assuming local thermo…
We report the discovery of a peculiar L dwarf from the UKIDSS LAS, ULAS J222711-004547. The very red infrared photometry (MKO J-K = 2.79$\pm$0.06, WISE W1-W2 = 0.65$\pm$0.05) of ULAS J222711-004547 makes it one of the reddest brown dwarfs discovered so far. We obtained a moderate resolution spectrum of this target, and classify it as L7pec, confirming its very red nature. Comparison to theoretical models suggests that the object could be a low-gravity L dwarf with a solar or higher than solar metallicity. Nonetheless, the match of such fits to the spectrum is rather poor and this and other pec…
We present an analysis of long-term photometric variability for nearby red dwarfs at optical wavelengths. The sample consists of 264 M dwarfs south of DEC = +30 with V-K = 3.96-9.16 and Mv~10-20 (spectral types M2V-M8V), most of which are within 25 pc. The stars have been observed in the VRI filters for ~4-14 years at the CTIO/SMARTS 0.9m telescope. Of the 238 red dwarfs within 25 pc, we find that only ~8% are photometrically variable by at least 20 mmag (~2%) in the VRI bands. We find that high variability at optical wavelengths over the long-term can be used to identify young stars. Overall,…
We present the results of a radial velocity survey of 20 white dwarf plus M dwarf binaries selected as a follow up to a \textit{Hubble Space Telescope} study that aimed to spatially resolve suspected binaries. Our candidates are taken from the list of targets that were spatially unresolved with \textit{Hubble}. We have determined the orbital periods for 16 of these compact binary candidates. The period distribution ranges from 0.14 to 9.16\,d and peaks near 0.6\,d. The original sample therefore contains two sets of binaries, wide orbits ($\approx100-1000$\,au) and close orbits ($\lesssim1-10$\…
We present i and z photometry for 25 T dwarfs and one L dwarf. Combined with published photometry, the data show that the i - z, z - Y and z - J colors of T dwarfs are very red, and continue to increase through to the late-type T dwarfs, with a hint of a saturation for the latest types with T_eff ~ 600 K. We present new 0.7-1.0 um and 2.8-4.2 um spectra for the very late-type T dwarf UGPS J072227.51-054031.2, as well as improved astrometry for this dwarf. Examination of the spectral energy distribution using the new and published data, with Saumon & Marley models, shows that the dwarf has T_ef…
Some gaps in the mosaic of binary star evolution have recently been filled by the discoveries of helium-core white dwarf progenitors (often called extremely low mass (ELM) white dwarfs) as stripped cores of first-giant branch objects. Two varieties can be distinguished. One class is made up by SB1 binaries, companions being white dwarfs as well, another class, the so-called EL CVn stars, are composite spectrum binaries, with A-Type companions. Pulsating stars are found among both classes. A riddle is posed by the apparently single objects. There is a one-to-one correspondence of the phenomena …
Results are presented for a Hubble Space Telescope Advanced Camera for Surveys high-resolution imaging campaign of 90 white dwarfs with known or suspected low mass stellar and substellar companions. Of the 72 targets which remain candidate and confirmed white dwarfs with near-infrared excess, 43 are spatially resolved into two or more components, and a total of 12 systems are potentially triples. There is a possible, slight deficit of earlier spectral types (bluer colors) among the spatially unresolved companions, exactly the opposite of expectations if significant mass is transferred to the c…
We present first results from a targeted search for brown dwarfs with unusual red colors indicative of peculiar atmospheric characteristics. These include objects with low surface gravities or with unusual dust content or cloud properties. From a positional cross-match of SDSS, 2MASS and WISE, we have identified 40 candidate peculiar early L to early T dwarfs that are either new objects or have not been identified as peculiar through prior spectroscopy. Using low resolution spectra, we confirm that 10 of the candidates are either peculiar or potential L/T binaries. With a J-Ks color of 2.62 +/…
Magneto-Archeology of White Dwarfs. Revisiting the fossil field scenario with observational constraints during the red giant branch
Institute of Science and Technology Austria (ISTA)（奥地利科学技术研究所）；Grupo de Evolución Estelar y Pulsaciones, Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata（拉普拉塔国立大学天文学与地球物理科学学院恒星演化与脉动研究组）；Instituto de Astrofísica La Plata, CONICET-UNLP（拉普拉塔天文研究所（CONICET-UNLP））；Center for Astrophysics | Harvard & Smithsonian（哈佛-史密森天体物理中心）
Red Dwarf (dM) stars are overwhelmingly the most numerous stars in our Galaxy. These cool, faint and low mass stars make up more than 80% of all stars. Also dM stars have extremely long life times (longer than 50-100 Gyr). Determining the number of red dwarfs with planets and assessing planetary habitability (a planet's potential to develop and sustain life) is critically important because such studies would indicate how common life is in the universe. Our program - "Living with a Red Dwarf" - addresses these questions by investigating the long-term nuclear evolution and the coronal and chromo…
We present new 5.2-14.5 micron low-resolution spectra of 14 mid-L to mid-T dwarfs, as well as 3.0-4.1 micron spectra for five of these dwarfs. These data are supplemented by existing red and near-infrared spectra (0.6-2.5 micron), as well as red through mid-infrared spectroscopy of seven other L and T dwarfs presented by Cushing et al. (2008). We compare these spectra to those generated from the model atmospheres of Saumon & Marley (2008). The models reproduce the observed spectra well, except in the case of one very red L3.5 dwarf, 2MASS J22244381-0158521. The broad wavelength coverage allows…
KOI-3890 is a highly eccentric, 153-day period eclipsing, single-lined spectroscopic binary system containing a red-giant star showing solar-like oscillations alongside tidal interactions. The combination of transit photometry, radial velocity observations, and asteroseismology have enabled the detailed characterisation of both the red-giant primary and the M-dwarf companion, along with the tidal interaction and the geometry of the system. The stellar parameters of the red-giant primary are determined through the use of asteroseismology and grid-based modelling to give a mass and radius of $M_…
We scrutinize the red dwarf component in the eclipsing binary system V471 Tau in order to unravel relations between different activity layers from the stellar surface through the chromosphere up to the corona. We aim at studying how the magnetic dynamo in the late-type component is affected by the close white dwarf companion. We use space photometry, high resolution spectroscopy and X-ray observations from different space instruments to explore the main characteristics of magnetic activity. From K2 photomery we find that 5-10 per cent of the apparent surface of the red dwarf is covered by cool…
V471 Tauri, a white dwarf--red dwarf eclipsing binary in the Hyades, is well known for stimulating development of common envelope theory, whereby novae and other cataclysmic variables form from much wider binaries by catastrophic orbit shrinkage. Our evaluation of a recent imaging search that reported negative results for a much postulated third body shows that the object could have escaped detection or may have actually been seen. The balance of evidence continues to favor a brown dwarf companion about 12 AU from the eclipsing binary. A recently developed algorithm finds unified solutions fro…
The presence of a white dwarf in a resolved binary system, such as Sirius, provides an opportunity to combine dynamical information about the masses, from astrometry and spectroscopy, with a gravitational red-shift measurement and spectrophotometry of the white dwarf atmosphere to provide a test of theoretical mass-radius relations of unprecedented accuracy. We demonstrated this with the first Balmer line spectrum of Sirius B to be obtained free of contamination from the primary, with STIS on HST. However, we also found an unexplained discrepancy between the spectroscopic and gravitational red…
We have identified a sample of cool field brown dwarf candidates using IRAC data from the Spitzer Deep, Wide-Field Survey (SDWFS). The candidates were selected from 400,000 SDWFS sources with [4.5] <= 18.5 mag and required to have [3.6]-[4.5] >= 1.5 and [4.5] - [8.0] <= 2.0 on the Vega system. The first color requirement selects objects redder than all but a handful of presently known brown dwarfs with spectral classes later than T7, while the second eliminates 14 probable reddened AGN. Optical detection of 4 of the remaining 18 sources implies they are likely also AGN, leaving 14 brown dwarf …
Near-infrared photometry and spectroscopy is obtained for a heterogeneous sample of nearby white dwarfs with possible excess flux as identified primarily in the Two Micron All Sky Survey. Among the sample of 43 stars are a number of white dwarfs that are either metal-rich, magnetic, or binary suspects. With a few notable exceptions in four (or possibly five) distinct categories, the newly obtained JHK photometric data fail to corroborate the putative excesses, with K_IRTF - K_2MASS = +0.31 mag. Where available, GALEX photometric data are used to better constrain the overall spectral energy dis…
The analysis of the localization regions of TeV gamma-radiation sources in the X-ray and optical spectral ranges is carried out. The distances from the position of the maxima in the distribution of high-energy fluxes to the probable candidates for identification with red dwarfs are indicated. Possible identifications of weaker TeV sources and other field objects are also considered.
The formation histories of lithium-rich and carbon-rich red giants are not yet understood. It has been proposed that the merger of a helium-core white dwarf with a red giant branch star might provide a solution. We have computed an extended grid of post-merger evolution models and combined these with predictions of binary star population synthesis. The results strongly support the proposal that the merger of a helium white dwarf with a red giant branch star can provide the progenitors of both lithium-rich red clump stars and early-R carbon stars. The distribution of post-merger models in $T_{\…
We present the discovery of CWISE J050626.96$+$073842.4 (CWISE J0506$+$0738), an L/T transition dwarf with extremely red near-infrared colors discovered through the Backyard Worlds: Planet 9 citizen science project. Photometry from UKIRT and CatWISE give a $(J-K)_{\rm MKO}$ color of 2.97$\pm$0.03 mag and a $J_{\rm MKO}-$W2 color of 4.93$\pm$0.02 mag, making CWISE J0506$+$0738 the reddest known free-floating L/T dwarf in both colors. We confirm the extremely red nature of CWISE J0506$+$0738 using Keck/NIRES near-infrared spectroscopy and establish that it is a low-gravity late-type L/T transiti…
Condensate clouds fundamentally impact the atmospheric structure and spectra of exoplanets and brown dwarfs but the connections between surface gravity, cloud structure, dust in the upper atmosphere, and the red colors of some brown dwarfs remain poorly understood. Rotational modulations enable the study of different clouds in the same atmosphere, thereby providing a method to isolate the effects of clouds. Here we present the discovery of high peak-to-peak amplitude (8%) rotational modulations in a low-gravity, extremely red (J-Ks=2.55) L6 dwarf WISEP J004701.06+680352.1 (W0047). Using the Hu…
The paper presents the identification of the stars in the lower part of the Main Sequence among the objects of the deep survey of the equatorial region of the sky, performed by the eROSITA telescope at the SRG orbital observatory. Of the 27,910 X-ray objects, 110 are possibly candidates for identification with red dwarfs. 12 stars were previously classified as X-ray sources. In the area of identification of two stars, galaxies fall. Several identifications contain closely spaced objects. The list of identified objects, comments to it and search maps are given in the appendices.
Dark matter could have a dissipative asymmetric subcomponent in the form of atomic dark matter (aDM). This arises in many scenarios of dark complexity, and is a prediction of neutral naturalness, such as the Mirror Twin Higgs model. We show for the first time how White Dwarf cooling provides strong bounds on aDM. In the presence of a small kinetic mixing between the dark and SM photon, stars are expected to accumulate atomic dark matter in their cores, which then radiates away energy in the form of dark photons. In the case of white dwarfs, this energy loss can have a detectable impact on thei…
Generic scalar-tensor theories of gravity predict deviations from Newtonian physics inside astrophysical bodies. In this paper, we point out that low mass stellar objects, red and brown dwarf stars, are excellent probes of these theories. We calculate two important and potentially observable quantities: the radius of brown dwarfs and the minimum mass for hydrogen burning in red dwarfs. The brown dwarf radius can differ significantly from the GR prediction and upcoming surveys that probe the mass-radius relation for stars with masses $<\mathcal{O}(0.1M_\odot)$ have the potential to place new co…
Rotational modulations of emission spectra in brown dwarf and exoplanet atmospheres show that clouds are often distributed non-uniformly in these ultracool atmospheres. The spatial heterogeneity in cloud distribution demonstrates the impact of atmospheric dynamics on cloud formation and evolution. In this study, we update the Hubble Space Telescope (HST) time-series data analysis of the previously reported rotational modulations of WISEP J004701+680352 -- an unusually red late-L brown dwarf with a spectrum similar to that of the directly imaged planet HR8799e. We construct a self-consistent sp…
Most stars in the Universe are red dwarfs. They outnumber stars like our Sun by a factor of 5 and outlive them by another factor of 20 (population-weighted mean). When combined with recent observations uncovering an abundance of temperate, rocky planets around these diminutive stars, we're faced with an apparent logical contradiction - why don't we see a red dwarf in our sky? To address this "Red Sky paradox", we formulate a Bayesian probability function concerning the odds of finding oneself around a F/G/K-spectral type (Sun-like) star. If the development of intelligent life from prebiotic ch…
We present the third discovery from the COol Companions ON Ultrawide orbiTS (COCONUTS) program, the COCONUTS-3 system, composed of a young M5 primary star UCAC4 374-046899 and a very red L6 dwarf WISEA J081322.19$-$152203.2. These two objects have a projected separation of 61$''$ (1891 au) and are physically associated given their common proper motions and estimated distances. The primary star, COCONUTS-3A, has a mass of $0.123\pm0.006$ M$_{\odot}$ and we estimate its age as 100 Myr to 1 Gyr based on its kinematics and spectrophotometric properties. We derive its metallicity as $0.21 \pm 0.07$…
As part of Villanova's Living with a Red Dwarf program, we have obtained UV, X-ray and optical data of the Population II red dwarf -- Kapteyn's Star. Kapteyn's Star is noteworthy for its large proper motions and high RV of ~+245 km s^-1. As the nearest Pop II red dwarf, it serves as an old age anchor for calibrating Activity/Irradiance-Rotation-Age relations, and an important test bed for stellar dynamos and the resulting X-ray -- UV emissions of slowly rotating, near-fully convective red dwarf stars. Adding to the notoriety, Kapteyn's Star has recently been reported to host two super-Earth ca…
High-Time-Cadence Spectroscopy and Photometry of Stellar Flares on M-dwarf YZ Canis Minoris with Seimei Telescope and TESS. II. Statistical Properties of Blue/Red Asymmetries in the H$α$ Line
KIC 5773205 is the least luminous eclipsing M dwarf found in the Villanova catalog of eclipsing binaries detected by the {\it Kepler} mission. We processed and analyzed the three available quarters of mission data for this star and discovered a persistent periodic variation of the light curve with a period, which is in exact 4:5 commensurability to the orbital period. Three routes of interpretation are considered: 1) non-radial pulsations excited by the tidal interaction at a specific eigenfrequency; 2) a high-order spin-orbit resonance caused by the tides; 3) an ellipsoidal deformation caused…
We have used Pan-STARRS1 to discover an extremely red late-L dwarf, which has (J-K)_MKO = 2.84 and (J-K)_2MASS = 2.78, making it the reddest known field dwarf and second only to 2MASS J1207-39b among substellar companions. Near-IR spectroscopy shows a spectral type of L7 and reveals a triangular H-band continuum and weak alkali (K I and Na I) lines, hallmarks of low surface gravity. Near-IR astrometry from the Hawaii Infrared Parallax Program gives a distance of 24.6+/-1.4 pc and indicates a much fainter J-band absolute magnitude than field L dwarfs. The position and kinematics of PSO J318-22 …
We present a kinematic analysis of 152 low surface gravity M7-L8 dwarfs by adding 8 parallaxes, 38 radial velocities, and 19 proper motions. We find 39 objects to be high-likelihood or bona fide members of nearby moving groups, 92 objects to be ambiguous members and 21 objects that are non-members. We find that gravity classification and photometric color separate 5-150 Myr sources from > 3 Gyr field objects, but they do not correlate one-to-one with the narrower 5 -150 Myr age range. The absolute magnitudes of low-gravity sources from J band through W3 show a flux redistribution when compared…
