We present here new medium resolution spectra (λ/∆λ ∼ 1200) of K−M giants covering wavelength range 1.50−1.80 and 1.95−2.45 µm. The sample includes 72 K0−M8 giants from our TIRSPEC observations and all available 35 giants in that spectral range from archival IRTF spectral library. We have calibrated here the empirical relations between fundamental parameters (e.g., effective temperature, surface gravity) and equivalent widths of some important spectral features like Si I, Na I, Ca I, 12 CO. We find that the 12 CO first overtone band at 2.29 µm and second overtone band at 1.62 µm are a reasonably good indicator of temperature above 3400 K and surface gravity. We show that the dispersion of empirical relations between 12 CO and T ef f significantly improve considering the effect of surface gravity.
We present here medium resolution (λ/∆λ ∼1200) H-and K-band spectra of M type dwarf stars covering the wavelength range 1.50 -1.80 µm and 1.95 -2.45 µm. The sample includes 53 dwarf stars (M0V-M7V) from new observations using the TIFR Near-Infrared Spectrometer and Imager (TIRSPEC) instrument on the 2-m Himalayan Chandra Telescope (HCT). Using interferometrically-measured effective temperature (T e f f ), radius and luminosity of nearby bright calibrator stars, we have created new empirical relationships among those fundamental parameters and spectral indices. The equivalent widths of H-band spectral features like Mg (1.57 µm), Al (1.67 µm) and Mg (1.71 µm), and the H 2 O-H index are found to be good indicators of T e f f , radius and luminosity and we establish the linear functions using these features relating to those stellar parameters. The root mean squared error (RMSE) of our best fits are 102K, 0.027R ⊙ and 0.12dex respectively. Using spectral type standards along with known parallaxes, we calibrate both H and K-band H 2 O indices as a tracer of spectral type and absolute K s magnitude. Metallicities of M-dwarf samples are estimated using the K-band calibration relationships. The mass of M-dwarfs could be determined using the luminosity (L/L ⊙ ) and we establish a new empirical relation for this. We also compare and contrast our results with other similar work from the literature.
We present here optical I-band photometric variability study down to ≃ 19 mag of a young (∼2-3 Myr) star-forming region IC 348 in the Perseus molecular cloud. We aim to explore the fast rotation (in the time-scales of hours) in Very Low Mass stars (VLMs) including Brown Dwarfs (BDs). From a sample of 177 light-curves using our new I-band observations, we detect new photometric variability in 22 young M-dwarfs including 6 BDs, which are bonafide members in IC 348 and well-characterized in the spectral type of M-dwarfs. Out of 22 variables, 11 M dwarfs including one BD show hour-scale periodic variability in the period range 3.5 - 11 hours and rest are aperiodic in nature. Interestingly, an optical flare is detected in a young M2.75 dwarf in one night data on 20 December 2016. From the flare light curve, we estimate the emitted flared energy of 1.48 × 1035 ergs. The observed flared energy with an uncertainty of tens of per cent is close to the super-flare range (∼ 1034 ergs), which is rarely observed in active M dwarfs.
We estimate effective temperature (Teff), stellar radius, and luminosity for a sample of 271 M-dwarf stars (M0V-M7V) observed as a part of CARMENES (Calar Alto high-Resolution search for M dwarfs with Exo-earths with Near-infrared and optical Echelle Spectrographs) radial-velocity planet survey. For the first time, using the simultaneously observed high resolution (R∼90000) spectra in the optical (0.52–0.96 μm) and near-infrared (0.96–1.71 μm) bands, we derive empirical calibration relationships to estimate the fundamental parameters of these low-mass stars. We select a sample of nearby and bright M-dwarfs as our calibrators for which the physical parameters are acquired from high-precision interferometric measurements. To identify the most suitable indicators of Teff, radius, and luminosity (log L/L⊙), we inspect a range of spectral features and assess them for reliable correlations. We perform multivariate linear regression and find that the combination of pseudo equivalent widths and equivalent width ratios of the Ca II at 0.854 μm and Ca II at 0.866 μm lines in the optical and the Mg I line at 1.57 μm in the NIR give the best fitting linear functional relations for the stellar parameters with root mean square errors (RMSE) of 99K, 0.06 R⊙ and 0.22 dex respectively. We also explore and compare our results with literature values obtained using other different methods for the same sample of M dwarfs.
We report on photometric variability studies of a L3.5 brown dwarf 2MASS J00361617+1821104 (2M0036+18) in the field and of four young brown dwarfs in the star-forming region IC 348. From muti-epoch observations, we found significant periodic variability in 2M0036+18 with a period of 2.66 ± 0.55 hours on one occasion while it seemed to be non-variable on three other occasions. An evolving dust cloud might cause such a scenario. Among four young brown dwarfs of IC 348 in the spectral range M7.25 - M8, one brown dwarf 2MASS J03443921+3208138 shows significant variability. The K-band spectra (2.0-2.4 m) of nine very low mass stars (M1 - M9 V) are used to characterize the water band index (H20-K2). We found that it is strongly correlated with the surface temperature of M dwarfs.
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