Spectral Type M5 Research Articles

Fundamental Parameters of ∼30,000 M dwarfs in LAMOST DR1 Using Data-driven Spectral Modeling

Abstract M dwarfs are the most common type of star in the Galaxy, and because of their small size are favored targets for searches of Earth-sized transiting exoplanets. Current and upcoming all-sky spectroscopic surveys, such as the Large Sky Area Multi Fiber Spectroscopic Telescope (LAMOST), offer an opportunity to systematically determine physical properties of many more M dwarfs than has been previously possible. Here, we present new effective temperatures, radii, masses, and luminosities for 29,678 M dwarfs with spectral types M0–M6 in the first data release (DR1) of LAMOST. We derived these parameters from the supervised machine-learning code, The Cannon, trained with 1388 M dwarfs in the Transiting Exoplanet Survey Satellite Cool Dwarf Catalog that were also present in LAMOST with high signal-to-noise ratio (>250) spectra. Our validation tests show that the output parameter uncertainties are strongly correlated with the signal-to-noise of the LAMOST spectra, and we achieve typical uncertainties of 110 K in (∼3%), 0.065 (∼14%) in radius, 0.054 (∼12%) in mass, and 0.012 (∼20%) in luminosity. The model presented here can be rapidly applied to future LAMOST data releases, significantly extending the samples of well-characterized M dwarfs across the sky using new and exclusively data-based modeling methods.

2MASS J15460752−6258042: a mid-M dwarf hosting a prolonged accretion disc

ABSTRACT We report the discovery of the oldest (∼55 Myr) mid-M type star known to host ongoing accretion. 2MASS J15460752–6258042 (2M1546, spectral type M5, 59.2 pc) shows spectroscopic signs of accretion such as strong H α, He i, and [O i] emission lines, from which we estimate an accretion rate of ∼10−10 M⊙ yr−1. Considering the clearly detected infrared excess in all WISE bands, the shape of its spectral energy distribution (SED) and its age, we believe that the star is surrounded by a transitional disc, clearly with some gas still present at inner radii. The position and kinematics of the star from Gaia DR2 and our own radial-velocity measurements suggest membership in the nearby ∼55 Myr-old Argus moving group. At only 59 pc from Earth, 2M1546 is one of the nearest accreting mid-M dwarfs, making it an ideal target for studying the upper limit on the lifetimes of gas-rich discs around low-mass stars.

Spectral library of age-benchmark low-mass stars and brown dwarfs

ABSTRACT In recent years, some extremely red brown dwarfs have been discovered. They were believed to have a low surface gravity, but many of their spectral characteristics are similar to those of high-surface-gravity brown dwarfs, showing that the spectral characteristics of young brown dwarfs are poorly understood. We aim to test surface-gravity indicators in late-M and early-L brown dwarf spectra using data obtained with the X-shooter spectrograph at the Very Large Telescope. We select a benchmark sample of brown dwarf members of Chamaeleon I (∼2 Myr), Upper Scorpius (5−10 Myr), the Pleiades (132 ± 27 Myr) and Praesepe (590−790 Myr) with well-constrained ages and similar metallicities. We provide a consistent spectral classification of the sample in the optical and in the near-infrared. We measure the equivalent widths of their alkali lines, finding that they have a moderate correlation with age, especially for objects with spectral types M8 and later. We use spectral indices defined in the literature to estimate surface gravity, finding that their gravity assignment is accurate for 75 per cent of our sample. We investigate the correlation between red colour and age, finding that after ∼10 Myr, the colour does not change significantly for our sample with spectral types M6.0–L3.0. In this case, the red colours might be associated with circumstellar discs, ring structures, extinction, or viewing angle. Finally, we calculate the bolometric luminosity, and J and K bolometric corrections for our sample. We find that six objects are overluminous compared with other members of the same association. These objects are flagged as binary candidates by the Gaia survey.

THOR 42: A touchstone ∼24 Myr-old eclipsing binary spanning the fully-convective boundary

Abstract We present the characterization of CRTS J055255.7−004426 (=THOR 42), a young eclipsing binary comprising two pre-main sequence M dwarfs (combined spectral type M3.5). This nearby (103 pc), short-period (0.859 d) system was recently proposed as a member of the ∼24 Myr-old 32 Orionis Moving Group. Using ground- and space-based photometry in combination with medium- and high-resolution spectroscopy, we model the light and radial velocity curves to derive precise system parameters. The resulting component masses and radii are 0.497 ± 0.005 and 0.205 ± 0.002 $\rm {M}_{\odot }$, and 0.659 ± 0.003 and 0.424 ± 0.002 $\rm {R}_{\odot }$, respectively. With mass and radius uncertainties of ∼1 per cent and ∼0.5 per cent, respectively, THOR 42 is one of the most precisely characterized pre-main sequence eclipsing binaries known. Its systemic velocity, parallax, proper motion, colour–magnitude diagram placement and enlarged radii are all consistent with membership in the 32 Ori Group. The system provides a unique opportunity to test pre-main sequence evolutionary models at an age and mass range not well constrained by observation. From the radius and mass measurements we derive ages of 22–26 Myr using standard (non-magnetic) models, in excellent agreement with the age of the group. However, none of the models can simultaneously reproduce the observed mass, radius, temperature and luminosity of the coeval components. In particular, their H–R diagram ages are 2–4 times younger and we infer masses ∼50 per cent smaller than the dynamical values.

Photometric flaring fraction of M dwarf stars from the SkyMapper Southern Survey

ABSTRACT We present our search for flares from M dwarf stars in the SkyMapper Southern Survey DR1, which covers nearly the full Southern hemisphere with six-filter sequences that are repeatedly observed in the passbands uvgriz. This allows us to identify bona fide flares in single-epoch observations on time-scales of less than four minutes. Using a correlation-based outlier search algorithm we find 254 flare events in the amplitude range of Δu ∼ 0.1 to 5 mag. In agreement with previous work, we observe the flaring fraction of M dwarfs to increase from ∼30 to ∼1000 per million stars for spectral types M0 to M5. We also confirm the decrease in flare fraction with larger vertical distance from the Galactic plane which is expected from declining stellar activity with age. Based on precise distances from Gaia DR2, we find a steep decline in the flare fraction from the plane to 150 pc vertical distance and a significant flattening towards larger distances. We then reassess the strong type dependence in the flaring fraction with a volume-limited sample within a distance of 50 pc from the Sun: in this sample the trend disappears and we find instead a constant fraction of ∼1 650 per million stars for spectral types M1 to M5. Finally, large-amplitude flares with Δi > 1 mag are very rare with a fraction of ∼0.5 per million M dwarfs. Hence, we expect that M-dwarf flares will not confuse SkyMapper’s search for kilonovae from gravitational-wave events. proper references for those databases (or follow their guideline on citation).

Evidence for disc regulation in the lowest mass stars of the young stellar cluster NGC 2264

In the pre-main-sequence stage, star-disc interactions have been shown to remove stellar angular momentum and regulate the rotation periods of stars with M2 and earlier spectral types. Whether disc regulation also extends to stars with later spectral types still remains a matter of debate. Here we present a star-disc interaction study in a sample of over 180 stars with spectral types M3 and later (corresponding to stellar masses $\leq 0.3 M_\odot$) in young stellar cluster NGC 2264. Combining rotation periods from the literature, new and literature spectral types, and newly presented deep Spitzer observations, we show that stars with masses below 0.3 $M_\odot$ with discs also rotate slower than stars without a disc in the same mass regime. Our results demonstrate that disc-regulation still operates in these low-mass stars, although the efficiency of this process might be lower than in higher-mass objects. We confirm that stars with spectral types earlier and later than M2 have distinct period distributions and that stars with spectral types M5 and later rotate even faster M3 and M4-type stars.

The CARMENES search for exoplanets around M dwarfs

Aims. We determine the radii and masses of 293 nearby, bright M dwarfs of the CARMENES survey. This is the first time that such a large and homogeneous high-resolution (R > 80 000) spectroscopic survey has been used to derive these fundamental stellar parameters. Methods. We derived the radii using Stefan–Boltzmann’s law. We obtained the required effective temperatures Teff from a spectral analysis and we obtained the required luminosities L from integrated broadband photometry together with the Gaia DR2 parallaxes. The mass was then determined using a mass-radius relation that we derived from eclipsing binaries known in the literature. We compared this method with three other methods: (1) We calculated the mass from the radius and the surface gravity log g, which was obtained from the same spectral analysis as Teff. (2) We used a widely used infrared mass-magnitude relation. (3) We used a Bayesian approach to infer stellar parameters from the comparison of the absolute magnitudes and colors of our targets with evolutionary models. Results. Between spectral types M0 V and M7 V our radii cover the range 0.1 R⊙ < R < 0.6 R⊙ with an error of 2–3% and our masses cover 0.09 ℳ⊙ < ℳ< 0.6ℳ⊙ with an error of 3–5%. We find good agreement between the masses determined with these different methods for most of our targets. Only the masses of very young objects show discrepancies. This can be well explained with the assumptions that we used for our methods.

Resolved Young Binary Systems and Their Disks

Abstract We have conducted a survey of young single and multiple systems in the Taurus–Auriga star-forming region with the Atacama Large Millimeter Array (ALMA), substantially improving both the spatial resolution and sensitivity with which individual protoplanetary disks in these systems have been observed. These ALMA observations can resolve binary separations as small as 25–30 au and have an average 3σ detection level of 0.35 mJy, equivalent to a disk mass of 4 × 10−5 M ⊙ for an M3 star. Our sample was constructed from stars that have an infrared excess and/or signs of accretion and have been classified as Class II. For the binary and higher-order multiple systems observed, we detect λ = 1.3 mm continuum emission from one or more stars in all of our target systems. Combined with previous surveys of Taurus, our 21 new detections increase the fraction of millimeter-detected disks to over 75% in all categories of stars (singles, primaries, and companions) earlier than spectral type M6 in the Class II sample. Given the wealth of other information available for these stars, this has allowed us to study the impact of multiplicity with a much larger sample. While millimeter flux and disk mass are related to stellar mass as seen in previous studies, we find that both primary and secondary stars in binary systems with separations of 30–4200 au have lower values of millimeter flux as a function of stellar mass than single stars. We also find that for these systems, the circumstellar disk around the primary star does not dominate the total disk mass in the system and contains on average 62% of the total mass.

Exploring the stellar properties of M dwarfs with high-resolution spectroscopy from the optical to the near-infrared (Corrigendum)

Context. Being the most numerous and oldest stars in the galaxy, M dwarfs are objects of great interest for exoplanet searches. The presence of molecules in their atmosphere complicates our understanding of their atmospheric properties. But great advances have recently been made in the modeling of M dwarfs due to the revision of solar abundances. Aims. We aim to determine stellar parameters of M dwarfs using high resolution spectra (R = 90 000) simultaneously in the visible and the near-infrared. The high resolution spectra and broad wavelength coverage provide an unique opportunity to understand the onset of dust and cloud formation at cool temperatures. Furthermore, this study will help in understanding the physical processes which occur in a cool atmospheres, particularly, the redistribution of energy from the optical to the near-infrared. Methods. The stellar parameters of M dwarfs in our sample have been determined by comparing the high resolution spectra both in the optical and in the near-infrared simultaneously observed by CARMENES with the synthetic spectra obtained from the BT-Settl model atmosphere. The detailed spectral synthesis of these observed spectra both in the optical and in the near-infrared helps to understand the missing continuum opacity. Results. For the first time, we derive fundamental stellar parameters of M dwarfs using the high resolution optical and near-infrared spectra simultaneously. We determine Teff , log g and [M/H] for 292 M dwarfs of spectral type M0 to M9, where the formation of dust and clouds are important. The derived Teff for the sample ranges from 2300 to 4000 K, values of log g ranges from 4.5 geq log g leq 5.5 and the resulting metallicity ranges from -0.5 geq [M/H] leq +0.5. We have also explored the possible differences in Teff , log g and [M/H] by comparing them with other studies of the same sample of M dwarfs.

Exploring the stellar properties of M dwarfs with high-resolution spectroscopy from the optical to the near-infrared

Context. Being the most numerous and oldest stars in the galaxy, M dwarfs are objects of great interest for exoplanet searches. The presence of molecules in their atmosphere complicates our understanding of their atmospheric properties. But great advances have recently been made in the modeling of M dwarfs due to the revision of solar abundances. Aims. We aim to determine stellar parameters of M dwarfs using high resolution spectra (R ∼ 90 000) simultaneously in the visible and the near-infrared. The high resolution spectra and broad wavelength coverage provide an unique opportunity to understand the onset of dust and cloud formation at cool temperatures. Furthermore, this study will help in understanding the physical processes which occur in a cool atmospheres, particularly, the redistribution of energy from the optical to the near-infrared. Methods. The stellar parameters of M dwarfs in our sample have been determined by comparing the high resolution spectra both in the optical and in the near-infrared simultaneously observed by CARMENES with the synthetic spectra obtained from the BT-Settl model atmosphere. The detailed spectral synthesis of these observed spectra both in the optical and in the near-infrared helps to understand the missing continuum opacity. Results. For the first time, we derive fundamental stellar parameters of M dwarfs using the high resolution optical and near-infrared spectra simultaneously. We determine Teff, log g and [M/H] for 292 M dwarfs of spectral type M0 to M9, where the formation of dust and clouds are important. The derived Teff for the sample ranges from 2300 to 4000 K, values of log g ranges from 4.5 ≤ logg ≤ 5.5 and the resulting metallicity ranges from −0.5 ≤ [M/H] ≤ +0.5. We have also explored the possible differences in Teff, log g and [M/H] by comparing them with other studies of the same sample of M dwarfs.

Understanding Fundamental Properties and Atmospheric Features of Subdwarfs via a Case Study of SDSS J125637.13–022452.4∗

Abstract We present the distance-calibrated spectral energy distribution (SED) of the sdL3.5 subdwarf SDSS J125637.13−022452.4 (J1256−0224) using its Gaia DR2 parallax. We report the bolometric luminosity and semi-empirical fundamental parameters, as well as updated UVW velocities. The SED of J1256−0224 is compared to field-age and low-gravity dwarfs of the same effective temperature (T eff) and bolometric luminosity. In the former comparison, we find that the SED of J1256−0224 is brighter than the field source in the optical, but dims in comparison beyond the J band, where it becomes fainter than the field from the H through W2 bands. Compared to the young source, it is fainter at all wavelengths. We conclude that J1256−0224 is depleted of condensates compared to both objects. A near-infrared band-by-band analysis of the spectral features of J1256−0224 is done and is compared to the equivalent T eff sample. From this analysis, we find a peculiar behavior of the J-band K i doublets whereby the 1.17 μm doublet is stronger than the field or young source, as expected, while the 1.25 μm doublet shows indications of low gravity. In examining a sample of four other subdwarfs with comparable data, we confirm this trend across different subtypes indicating that the 1.25 μm doublet is a poor indicator of gravity for low-metallicity objects. In the K-band analysis of J1256−0224, we detect the 2.29 μm CO line of J1256−0224, previously unseen in the low-resolution SpeX data. We also present fundamental parameters using Gaia parallaxes for nine additional subdwarfs with spectral types M7–L7 for comparison. The 10 subdwarfs are placed in a temperature sequence, and we find a poor linear correlation with spectral type. We present polynomial relations for absolute magnitude in JHKW1W2, effective temperature, and bolometric luminosity versus spectral type for subdwarfs.

A Gap in the Lower Main Sequence Revealed by Gaia Data Release 2

Abstract We present the discovery of a gap near M G ≈ 10 in the main sequence on the Hertzsprung–Russell Diagram (HRD) based on measurements presented in Gaia Data Release 2 (DR2). Using an observational form of the HRD with M G representing luminosity and G BP − G RP representing temperature, the gap presents a diagonal feature that dips toward lower luminosities at redder colors. The gap is seen in samples extracted from DR2 with various distances, and is not unique to the Gaia photometry—it also appears when using near-IR (NIR) photometry (J − K s versus ). The gap is very narrow (∼0.05 mag) and is near the luminosity–temperature regime where M dwarf stars transition from partially to fully convective, i.e., near spectral type M3.0V. This gap provides a new feature in the HRD that hints at an underlying astrophysical cause, and we propose that it is linked to the onset of full convection in M dwarfs.

K2 Ultracool Dwarfs Survey. III. White Light Flares Are Ubiquitous in M6-L0 Dwarfs

Abstract We report the white light flare rates for 10 ultracool dwarfs using Kepler K2 short-cadence data. Among our sample stars, two have spectral type M6, three are M7, three are M8, and two are L0. Most of our targets are old low-mass stars. We identify a total of 283 flares in all of the stars in our sample, with Kepler energies in the range log E Kp ∼ (29–33.5) erg. Using the maximum-likelihood method of line fitting, we find that the flare frequency distribution (FFD) for each star in our sample follows a power law with slope −α in the range −(1.3–2.0). We find that cooler objects tend to have shallower slopes. For some of our targets, the FFD follows either a broken power law, or a power law with an exponential cutoff. For the L0 dwarf 2MASS J12321827-0951502, we find a very shallow slope (−α = −1.3) in the Kepler energy range (0.82–130) × 1030 erg: this L0 dwarf has flare rates which are comparable to those of high-energy flares in stars of earlier spectral types. In addition, we report photometry of two superflares: one on the L0 dwarf 2MASS J12321827-0951502 and another on the M7 dwarf 2MASS J08352366+1029318. In the case of 2MASS J12321827-0951502, we report a flare brightening by a factor of ∼144 relative to the quiescent photospheric level. Likewise, for 2MASS J08352366+1029318, we report a flare brightening by a factor of ∼60 relative to the quiescent photospheric level. These two superflares have bolometric (ultraviolet/optical/infrared) energies 3.6 × 1033 erg and 8.9 × 1033 erg respectively, while the full width half maximum timescales are very short, ∼2 min. We find that the M8 star TRAPPIST-1 is more active than the M8.5 dwarf 2M03264453+1919309, but less active than another M8 dwarf (2M12215066-0843197).

The CARMENES search for exoplanets around M dwarfs

The CARMENES radial velocity (RV) survey is observing 324 M dwarfs to search for any orbiting planets. In this paper, we present the survey sample by publishing one CARMENES spectrum for each M dwarf. These spectra cover the wavelength range 520–1710 nm at a resolution of at least R >80 000, and we measure its RV, Hα emission, and projected rotation velocity. We present an atlas of high-resolution M-dwarf spectra and compare the spectra to atmospheric models. To quantify the RV precision that can be achieved in low-mass stars over the CARMENES wavelength range, we analyze our empirical information on the RV precision from more than 6500 observations. We compare our high-resolution M-dwarf spectra to atmospheric models where we determine the spectroscopic RV information content, Q, and signal-to-noise ratio. We find that for all M-type dwarfs, the highest RV precision can be reached in the wavelength range 700–900 nm. Observations at longer wavelengths are equally precise only at the very latest spectral types (M8 and M9). We demonstrate that in this spectroscopic range, the large amount of absorption features compensates for the intrinsic faintness of an M7 star. To reach an RV precision of 1 m s−1 in very low mass M dwarfs at longer wavelengths likely requires the use of a 10 m class telescope. For spectral types M6 and earlier, the combination of a red visual and a near-infrared spectrograph is ideal to search for low-mass planets and to distinguish between planets and stellar variability. At a 4 m class telescope, an instrument like CARMENES has the potential to push the RV precision well below the typical jitter level of 3–4 m s−1.

IPTF Archival Search for Fast Optical Transients

Abstract There has been speculation about a class of relativistic explosions with an initial Lorentz factor Γinit smaller than that of classical gamma-ray bursts (GRBs). These “dirty fireballs” would lack prompt GRB emission but could be pursued via their optical afterglow, appearing as transients that fade overnight. Here we report a search for such transients (that fade by 5-σ in magnitude overnight) in four years of archival photometric data from the intermediate Palomar Transient Factory (iPTF). Our search criteria yielded 50 candidates. Of these, two were afterglows to GRBs that had been found in dedicated follow-up observations to triggers from the Fermi GRB Monitor. Another (iPTF14yb) was a GRB afterglow discovered serendipitously. Eight were spurious artifacts of reference image subtraction, and one was an asteroid. The remaining 38 candidates have red stellar counterparts in external catalogs. The photometric and spectroscopic properties of the counterparts identify these transients as strong flares from M dwarfs of spectral type M3–M7 at distances of d ≈ 0.15–2.1 kpc; three counterparts were already spectroscopically classified as late-type M stars. With iPTF14yb as the only confirmed relativistic outflow discovered independently of a high-energy trigger, we constrain the all-sky rate of transients that peak at m = 18 and fade by Δm = 2 mag in Δt = 3 hr to be , with a 68% confidence interval of . This implies that the rate of visible dirty fireballs is at most comparable to that of the known population of long-duration GRBs.

The Solar Neighborhood. XXXX. Parallax Results from the CTIOPI 0.9 m Program: New Young Stars Near the Sun

Abstract As a step toward completing and characterizing the census of the solar neighborhood, we present astrometric, photometric, and spectroscopic observations of 32 systems observed with the Cerro Tololo Inter-American Observatory 0.9 m and 1.5 m telescopes. Astrometry from the 0.9 m indicates that among the 17 systems that had no previous published trigonometric parallaxes, 14 are within 25 pc. In the full sample, nine systems have proper motions larger than 0.″5 yr−1, including 2MASS J02511490-0352459, which exceeds 2.″0 yr−1. VRI photometry from the 0.9 m and optical spectra from the 1.5 m indicate that the targets have V = 11–22 mag and spectral types M3.0V–L3.0V. For 2MASS J23062928-0502285 (TRAPPIST-1), we present updated astrometry and photometric variability based on over 12 years of observations. Of the nine binaries in the sample, two promise mass determinations in the next decade: LHS 6167AB, an M4.5V system for which we present an accurate parallax placing the binary at 9.7 pc, and 2MASS J23515048-2537367AB, an M8.5V system at 21.1 pc for which we present the first evidence of an unseen, low-mass companion. Most importantly, Na i and K i gravity indicators, Hα measurements, long-term photometric variability, locations on the H-R diagram, and kinematic assessments indicate that as many as 13 of the systems are young, including candidate members of young moving groups, with ages less than ∼120 Myr.

Individual Dynamical Masses of Ultracool Dwarfs* † ‡

Abstract We present the full results of our decade-long astrometric monitoring programs targeting 31 ultracool binaries with component spectral types M7–T5. Joint analysis of resolved imaging from Keck Observatory and Hubble Space Telescope and unresolved astrometry from CFHT/WIRCam yields parallactic distances for all systems, robust orbit determinations for 23 systems, and photocenter orbits for 19 systems. As a result, we measure 38 precise individual masses spanning 30–115 . We determine a model-independent substellar boundary that is ≈70 in mass (≈L4 in spectral type), and we validate Baraffe et al. evolutionary model predictions for the lithium-depletion boundary (60 at field ages). Assuming each binary is coeval, we test models of the substellar mass–luminosity relation and find that in the L/T transition, only the Saumon & Marley “hybrid” models accounting for cloud clearing match our data. We derive a precise, mass-calibrated spectral type–effective temperature relation covering 1100–2800 K. Our masses enable a novel direct determination of the age distribution of field brown dwarfs spanning L4–T5 and 30–70 . We determine a median age of 1.3 Gyr, and our population synthesis modeling indicates our sample is consistent with a constant star formation history modulated by dynamical heating in the Galactic disk. We discover two triple-brown-dwarf systems, the first with directly measured masses and eccentricities. We examine the eccentricity distribution, carefully considering biases and completeness, and find that low-eccentricity orbits are significantly more common among ultracool binaries than solar-type binaries, possibly indicating the early influence of long-lived dissipative gas disks. Overall, this work represents a major advance in the empirical view of very low-mass stars and brown dwarfs.

LSPM J1314+1320: An Oversized Magnetic Star with Constraints on the Radio Emission Mechanism

Abstract LSPM J1314+1320 (=NLTT 33370) is a binary star system consisting of two nearly identical pre-main-sequence stars of spectral type M7. The system is remarkable among ultracool dwarfs for being the most luminous radio emitter over the widest frequency range. Masses and luminosities are at first sight consistent with the system being coeval at age ∼80 Myr according to standard (nonmagnetic) evolutionary models. However, these models predict an average effective temperature of ∼2950 K, which is 180 K hotter than the empirical value. Thus, the empirical radii are oversized relative to the standard models by ≈13%. We demonstrate that magnetic stellar models can quantitatively account for the oversizing. As a check on our models, we note that the radio emission limits the surface magnetic field strengths: the limits depend on identifying the radio emission mechanism. We find that the field strengths required by our magnetic models are too strong to be consistent with gyrosynchrotron emission but are consistent with electron cyclotron maser emission.

Surface Gravities for 228 M, L, and T Dwarfs in the NIRSPEC Brown Dwarf Spectroscopic Survey∗

Abstract We combine 131 new medium-resolution (R ∼ 2000) J-band spectra of M, L, and T dwarfs from the Keck NIRSPEC Brown Dwarf Spectroscopic Survey (BDSS) with 97 previously published BDSS spectra to study surface-gravity-sensitive indices for 228 low-mass stars and brown dwarfs spanning spectral types M5–T9. Specifically, we use an established set of spectral indices to determine surface gravity classifications for all of the M6–L7 objects in our sample by measuring the equivalent widths (EW) of the K i lines at 1.1692, 1.1778, and 1.2529 μm, and the 1.2 μm FeH J absorption index. Our results are consistent with previous surface gravity measurements, showing a distinct double peak—at ∼L5 and T5—in K i EW as a function of spectral type. We analyze the K i EWs of 73 objects of known ages and find a linear trend between log(Age) and EW. From this relationship, we assign age ranges to the very low gravity, intermediate gravity, and field gravity designations for spectral types M6–L0. Interestingly, the ages probed by these designations remain broad, change with spectral type, and depend on the gravity-sensitive index used. Gravity designations are useful indicators of the possibility of youth, but current data sets cannot be used to provide a precise age estimate.

Change in the activity character of the coronae of low-mass stars of various spectral types

We study the dependence of the coronal activity index on the star's rotation rate. This question was considered earlier for 824 late-type stars on the basis of a consolidated catalogue of the soft X-ray fluxes. We carry out a more refined analysis separately for G, K and M dwarfs. They distinctively exhibit two modes of activity. The first one is the saturation mode, it is characteristic of young stars and is practically not related to their rotation. The second one refers to the solar-type activity the level of which strongly depends on the rotation period. We show that the transition from one mode to another takes place at the rotation periods of 1.1, 3.3 and 7.2 days for the stars of spectral types G2, K4 and M3 respectively. In the light of the discovery of superflares on G and K stars on the Kepler spacecraft there arises a question of how these objects differ from other active late-type stars. We analyse the location of superflare stars relative to the stars observed by Kepler on the "amplitude of rotational brightness modulation (ARM) -- rotation period" diagram. The value of ARM reflects the relative spots area on a star and characterises the activity level in the whole atmosphere. It is shown that G and K superflare stars are basically fast rotating young objects, but some of them belong to the stars with the solar type of activity.