X-shooter spectroscopy of young stellar objects

Abstract

Context. Traditionally, the chromospheres of late-type stars are studied through their strongest emission lines, Hα and Ca ii HK emission. Our knowledge on the whole emission line spectrum is more elusive as a result of the limited spectral range and sensitivity of most available spectrographs. Aims. We intend to reduce this gap with a comprehensive spectroscopic study of the chromospheric emission line spectrum of a sample of non-accreting pre-main sequence stars (Class III sources). Methods. We analyzed X-shooter/VLT spectra of 24 Class III sources from three nearby star-forming regions (σ Orionis, Lupus III, and TW Hya). We determined the effective temperature, surface gravity, rotational velocity, and radial velocity by comparing the observed spectra with synthetic BT-Settl model spectra. We investigated in detail the emission lines emerging from the stellar chromospheres and combined these data with archival X-ray data to allow for a comparison between chromospheric and coronal emissions. Results. For some objects in the sample the atmospheric and kinematic parameters are presented here for the first time. The effective temperatures are consistent with those derived for the same stars from an empirical calibration with spectral types. Small differences in the surface gravity found between the stars can be attributed to differences in the average age of the three star-forming regions. The strength of lithium absorption and radial velocities confirm the young age of all but one object in the sample (Sz 94). Both X-ray and Hα luminosity as measured in terms of the bolometric luminosity are independent of the effective temperature for early-M stars but decline toward the end of the spectral M sequence. For the saturated early-M stars the average emission level is almost one dex higher for X-rays than for Hα: log (L x /L bol ) = −2.85 ± 0.36 vs. log (L Hα /L bol ) = −3.72 ± 0.21. When all chromospheric emission lines (including the Balmer series up to H11, Ca ii HK, the Ca ii infrared triplet, and several He i lines) are summed up the coronal flux still dominates that of the chromosphere, typically by a factor 2−5. Flux-flux relations between activity diagnostics that probe different atmospheric layers (from the lower chromosphere to the corona) separate our sample of active pre-main sequence stars from the bulk of field M dwarfs studied in the literature. Flux ratios between individual optical emission lines show a smooth dependence on the effective temperature. The Balmer decrements can roughly be reproduced by an NLTE radiative transfer model devised for another young star of similar age. Future, more complete chromospheric model grids can be tested against this data set.

HADES RV Programme with HARPS-N at TNG

“…11). Stelzer et al (2013) found a similar trend analyzing a sample of pre-MS low-mass stars in the 2500-4500 K temperature range. Conversely, Kendall's correlation test cannot reject the hypothesis that the slope of the flux-flux relationship is independent of the average level of activity, or the metallicity of the star.…”

Evidence from stellar rotation of enhanced disc dispersal

“…This behavior is more evident in the B component. The fluxes we measured are also close to the maximum values found by Stelzer et al (2013) in a sample of Class III stars belonging to three young associations.…”

“…The Balmer decrement, F Hα /F Hβ , is lower than two for both stars, like the minimum values observed by Stelzer et al (2013) for stars of comparable T eff , and it is close to the solar plage values (see their Fig. 13).…”

“…13). More interesting is the behavior of the flux ratio F Ca iiK /F Hα , which is about 0.25-0.40 for both components and nearly equal to the value measured by Stelzer et al (2013) for three members of TW Hya association (8 Myr; Torres et al 2008) of similar temperature and resembling the "saturated" stars.…”

“…Thus, we derived the chromospheric losses by means of the "spectral subtraction" technique, which is based on the comparison between the target spectrum, and a non-active template, which can be a synthetic spectrum or that of slowly-rotating, low-activity star. The difference between the observed and the template removes the photospheric lines and leaves the net chromospheric line emission, from which we can get the total radiative losses in the line (see, e.g., Barden 1985;Frasca & Catalano 1994;Stelzer et al 2013). The residual equivalent width (EW) of the lines was measured by integrating the emission profile in the difference spectrum, while the error (σ EW ) was estimated by multiplying the integration range by the photometric error on each point.…”