Measuring physical properties of pre-main sequence stars using high resolution infrared spectroscopy
Physical properties of pre–main sequence stars and their disks are measured from high resolution near–infrared spectra. A new fitting technique using spectral synthesis models has been developed to find the best fit to fully resolved K band absorption lines, allowing measurements of the stellar effective temperature, rate of rotation, and amount of non–stellar continuum excess from infrared emission by the circumstellar disk. An IDL fitting routine selects the best spectral synthesis model by the RMS minimum to the residuals of the fits, and has been validated by observations of MK standards. This new technique has been applied to a sample of pre–main sequence stars in the ρ Ophiuchi dark cloud to obtain accurate effective temperatures, continuum veilings, and some of the first measurements of vsini rotations in highly extincted young stars from 2.2 µm spectra. In two sources, a new spectroscopic technique to measure surface gravity has been applied using line flux ratios in high resolution spectra at 2.2 µm and 2.3 µm, leading to the first luminosity measurements that are independent of extinction and only weakly dependent on continuum excess. Previous assumptions of no continuum excess at J (1.2 µm) upon which the photometrically determined luminosities are based, are called into question with these new results. In the absence of strong magnetic fields, the ages of these objects inferred from stellar evolutionary model tracks on the HR diagram reveals an older age (6–9 Myr) from spectroscopically determined luminosities than from the photometry of the same objects. Another useful application of high resolution spectroscopy is the detection of binary systems. Characterization of binary systems offers a direct way to obtain mass and relative mass information. With a sensitivity down to ~0.5 km s-1 , we have measured precision radial velocities of the sources in our Ophiuchus sample, resulting in the discovery of one short period (P < 1 yr) pre–main sequence binary (GSS29). This system has been subsequently monitored to determine its orbital characteristics (P = 145 d, v2 = 29 km s -1 ) and constrain the mass (M1 + M2 ~1.8 M⊙ ). We find the dominant spectral line component to be the less massive (and cooler) star. The more massive star is mostly featureless because it is hotter and rotating fast. More observations are required to better constrain these preliminary results.