The spectroscopic characteristics of intermediate-aged pre-main-sequence stars: theηChamaeleontis cluster

Abstract

We present a study of calibrated low-resolution spectra of the 18 known primaries of the ≈9-Myr-old η Chamaeleontis (η Cha) pre-main-sequence (PMS) star cluster. Using synthetic broad-band colours and narrow-band continuum-sensitive, temperature-sensitive and gravitysensitive indices derived from the spectra, we compare the η Cha stars to standard dwarfs. We find that the VRI colours of the PMS stars are indistinguishable from those of mainsequence stars, but that a B-band excess attaining ≈0.2 mag for late-M cluster stars is present, which might be an indicator of gravity, metallicity and/or activity differences between the two samples of stars. The narrow-band spectral indices for the η Cha stars possibly indicate higher metallicity and strongly indicate lower surface gravity than the dwarf indices, consistent with the elevated location of the cluster in the Hertzsprung-Russell evolutionary diagram. Using the derived synthetic colours and indices, we adopt spectral types for the late-type η Cha stars. We then produce a table of absolute optical magnitudes and colours representing the cluster isochrone for comparison with PMS evolutionary models. From our results we also conclude that the η Cha stars are unreddened, consistent with the group being a sample of older PMS stars distant from obscuring molecular clouds, except for the A1 member HD 75505 for which we confirm A V = 0.4 mag likely due to the presence of circumstellar material. the gravity-sensitive K I and Na I lines are weaker in young Pleiades brown dwarfs than in field objects of similar spectral type, while Mohanty et al. (2004) have demonstrated the gravity sensitivity of the TiO molecular bands, which increase in strength with decreasing effective gravity, in samples of PMS stars in Upper Scorpius and Taurus star-forming regions. The differences in these features are relevant to all young PMS populations, with ages several Gyr less than old disc dwarfs and elevated in the Hertzsprung-Russell (HR) diagram by several magnitudes above the main sequence.Nearby young clusters have distinct advantages for the study of fundamental properties of PMS stars. They provide a sample of stars across a wide range of spectral types at essentially uniform age, distance and metallicity, and are sufficiently bright for precise spectroscopic characterization. Our laboratory is the ≈9-Myr-old η Chamaeleontis (η Cha) cluster (Mamajek, Lawson & Feigelson 1999). The cluster is an ideal PMS group for the study of 'intermediate-aged' PMS stars owing to its well-defined distance from Hipparcos measurements (d = 97 ± 3 pc), compactness (extent ∼1 pc), the apparent high degree of coevality of the stellar C 2004 RAS

Gaia-ESO Survey: Empirical classification of VLT/Giraffe stellar spectra in the wavelength range 6440–6810 Å in theγVelorum cluster, and calibration of spectral indices

“…Several spectral indices based on molecular bands were extensively studied by e.g., Kirkpatrick et al (1991), Reid et al (1995), Briceño et al (1998), Jeffries et al (2003, for NGC 2547, Lyo et al (2004Lyo et al ( , 2008, for η Cha and Cha), Riddick et al (2007, and references therein), Lawson et al (2009), Shkolnik et al (2009. In nearly all these studies, with the exception of the "TiO 1" index defined by Reid et al (1995), the spectral region used to define interesting indices is outside the HR15n range.…”

“…To our knowledge, gravity differences between MS and PMS late-type stars have never been measured before using this wavelength range. This has been done, instead, using better-known gravity diagnostics such as the Na I lines at 8183-8195 Å (e.g., Mohanty et al 2004;Lyo et al 2004Lyo et al , 2008Riddick et al 2007;Lawson et al 2009). A thorough investigation of the diagnostic capabilities which may be offered by the "quintet" lines is therefore most interesting.…”

The unusual protoplanetary disk around the T Tauri star ET Chamaeleontis

“…Despite the age of the system, the infrared colours of the source are reminiscent of those of much younger (1-2 Myr) circumstellar disks. Furthermore, optical spectroscopy of ET Cha has shown that it is undoubtedly accreting, with a very strong and broad H α emission line (Lawson et al 2002;Lyo et al 2004;Luhman & Steeghs 2004). Based on the observed H α line, the mass accretion rate of ET Cha has been estimated to be 10 −9 M /yr , and the disk inclination (by modelling the H α line profile) to be about 60 • as measured from face-on.…”

2MASS 0213+3648 C: A wide T3 benchmark companion to an an active, old M dwarf binary

“…The Na 8200 Å doublet provides an effective gravity diagnostic for mid-late M dwarfs (Lyo, Lawson & Bessell 2004;Schlieder et al 2012). We measured the equivalent width of this feature in our IRTF spectrum and found it to be 5.8 ± 1.8 Å on the upper end of the field dwarf sequence in Schlieder et al (2012)'s fig.…”

“…3 inset). We also used the NaI spectral index derived by Lyo et al (2004) and measured a value of 1.25 ± 0.15. Fig.…”