An extensive VLT/X-shooter library of photospheric templates of pre-main sequence stars

Abstract

Context. Studies of the formation and evolution of young stars and their disks rely on knowledge of the stellar parameters of the young stars. The derivation of these parameters is commonly based on comparison with photospheric template spectra. Furthermore, chromospheric emission in young active stars impacts the measurement of mass accretion rates, a key quantity for studying disk evolution. Aims. Here we derive stellar properties of low-mass (M 2 M ) pre-main sequence stars without disks, which represent ideal photospheric templates for studies of young stars. We also use these spectra to constrain the impact of chromospheric emission on the measurements of mass accretion rates. The spectra are reduced, flux-calibrated, and corrected for telluric absorption, and are made available to the community. Methods. We derive the spectral type for our targets by analyzing the photospheric molecular features present in their VLT/X-shooter spectra by means of spectral indices and comparison of the relative strength of photospheric absorption features. We also measure effective temperature, gravity, projected rotational velocity, and radial velocity from our spectra by fitting them with synthetic spectra with the ROTFIT tool. The targets have negligible extinction (A V < 0.5 mag) and spectral type from G5 to K6, and from M6.5 to M8. They thus complement the library of photospheric templates presented in our previous publication. We perform synthetic photometry on the spectra to derive the typical colors of young stars in different filters. We measure the luminosity of the emission lines present in the spectra and estimate the noise due to chromospheric emission in the measurements of accretion luminosity in accreting stars. Results. We provide a calibration of the photospheric colors of young pre-main sequence stars as a function of their spectral type in a set of standard broad-band optical and near-infrared filters. The logarithm of the noise on the accretion luminosity normalized to the stellar luminosity is roughly constant and equal to ∼−2.3 for targets with masses larger than 1 solar mass, and decreases with decreasing temperatures for lower-mass stars. For stars with masses of ∼1.5 M and ages of ∼1−5 Myr, the chromospheric noise converts to a limit of measurable mass accretion rates of ∼3 × 10 −10 M /yr. The limit on the mass accretion rate set by the chromospheric noise is of the order of the lowest measured values of mass accretion rates in Class II objects.

The little dippers: transits of star-grazing exocomets?

“…This process is rapid ( 50 Myr) for mid-to-late M dwarfs with fully convective envelopes, but is slower for early-M and K dwarfs, as illustrated in Figure 8. X-Shooter is routinely used to detect Li I absorption in young stars (e.g., Manara et al 2017). As shown in Figure 7, our X-Shooter spectra show no signs of significant Li I absorption for either target: we find equivalent widths of EW Li = −0.07 ± 0.05 for EPIC 205718330 and EW Li = 0.00 ± 0.02Å for EPIC 235240266.…”

A New Look at T Tauri Star Forbidden Lines: MHD-driven Winds from the Inner Disk

“…Interestingly, the spectra of DoAr 21 and DoAr 24E do not show any [O I] λ6300 emission while that of SR 21A only a marginal detection, see Appendix C for more details. To clarify which sources are truly accreting, we also compare the derived accretion luminosities with typical chromospheric values (see Manara et al 2017). Only TWA 3A has an accretion luminosity below the chromospheric emission, suggesting that chromospheric activity dominates the line emission.…”

Discovery of a Low-mass Companion Embedded in the Disk of the Young Massive Star MWC 297 with VLT/SPHERE*

“…Next, we considered two libraries of young stellar objects (YSOs) template spectra: (i) all 76 pre-main sequence stars spectra compiled in Alcalá et al (2014) and Manara et al (2013Manara et al ( , 2017, which are members of the TW Hya, σ Ori, Lupus I, III and IV star forming regions, spanning G5 to M8.5 spectral types; and (ii) all young dwarfs from the SpeX library (Burgasser 2014), identified based on their gravity class or their membership to young (< 10 Myr old) clusters. In either cases, we considered two free parameters to account for dif-ferent A V and distance between observed and template spectra.…”