Free Access
Volume 543, July 2012
Article Number A30
Number of page(s) 20
Section Interstellar and circumstellar matter
Published online 22 June 2012

Online material

Appendix A: Additional material

A.1. VISIR transmission spectra

Figure A.1 provides the VISIR transmission spectra for both our targets and the telluric standard.

A.2. Radial velocities used in this study

Since the stellar radial velocity is a key parameter in determining the centroid of the emission lines, it is worth to explain the choice of radial velocities used in this study.

For the stars located in the Taurus region with no measurement of the radial velocity available in the literature, we adopted the average radial velocity of Taurus, vrad = 16.0 ± 0.4 km s-1 (Bertout & Genova 2006). We also use this value for the Herbig Be star V892 Tau. The 12CO J = 2 − 1 observations by Panić & Hogerheijde (2009) were likely dominated by extended emission from the cloud. A search through the Herbig & Bell (1988) catalog within 1′ of V892 Tau for radial velocities of good quality also argued in favor of a radial velocity around 15 − 16 km s-1 for V892 Tau.

In addition, the radial velocities used for two stars (L1551 IRS5 and EC 92) are reported in the literature using the frame of the Local Standard of Rest (LSR), vLSR (Covey et al. 2006). In these cases the radial velocities were converted to vrad using the expression vrad = vLSR − (Ucosl + Vsinl)cosb − W, where l and b are the coordinates of the star in the Galactic coordinate system, and U = 10.3 km s-1, V = 15.3 km s-1, and W = 7.7 km s-1 come from Niinuma et al. (2011).

A.2.1. Determination of the stellar radial velocity from UVES spectra

To determine the radial velocity of our program stars, we used the optical spectra obtained by UVES. Optical spectra are populated by a series of absorption lines from the stellar photosphere, where the lines are shifted due to the stellar motion with respect to the observer, the radial velocity. In order to determine the line shift, each spectrum was compared with a high-resolution synthetic spectrum (Bertone et al. 2008) of a star having the same spectral type as the star observed. Where an appropriate synthetic spectrum was not available, we used a synthetic spectrum of similar spectral type as the star observed. For this

purpose we used the interactive IDL-based software described in Carmona et al. (2010). This tool allows the synthetic and target spectra to be displayed, and to calculate the radial velocity of the observed spectrum using the cross-correlation technique. The maximization of the cross-correlation function is a widely-used procedure to determine radial velocities (e.g., Tonry & Davis 1979; Allende Prieto 2007). If we have two arrays, S corresponding to the stellar spectrum and T the template spectrum, the cross-correlation between the two will be a new array C defined by the following expression: (A.1)The maximum value of the cross-correlation function C will correspond to the element i = p where p is the shift in pixels between both the stellar spectra S and the template spectrum T. The position of the maximum of the function (the radial velocity) and its error were calculated with a Gaussian fit. The radial velocity measured by cross-correlation was corrected by the barycentric motion of the earth at the moment of the observations to obtain the final radial velocity measurement. The radial velocities obtained with this method are presented in Table A.1, where we also included the radial velocities from the literature, when available. Errors are reported between parentheses. For one object (V853 Oph) there is no previous measurement of the radial velocity, while for FS Tau A there are two measurements available in the literature, both having uncertainties larger than 5 km s-1. For FS Tau A and V853 Oph the precision achieved in this work is < 1 km s-1. Finally, due to the lower signal-to-noise ratio obtained in the spectrum of CoKu Tau 1, fewer photospheric lines were detected. Therefore, the precision achieved in the radial velocity is lower for this star (17.5 ± 2.4 km s-1) than that derived by White & Hillenbrand (2004), although our value is consistent. Therefore we used their radial velocity in this paper.

In addition, we obtained the vsin i for the three stars by comparing the observed spectra with rotationally broadened synthetic spectra (e.g., Bertone et al. 2008; Müller et al. 2011) of Teff = 3850 K and log g = 4.5 for CoKu Tau 1 and FS Tau A. For V853 Oph the values used were Teff = 3370 K and log g = 4.5, the lines are spectrally unresolved. The results are reported in Table A.1.

thumbnail Fig. A.1

VISIR transmission spectra. In black the spectra of the star and in red the spectrum of the telluric standard. We included one spectrum for the stars observed in two days. The spectra are presented before the barycentric and radial velocity correction to visualize the telluric absorption lines and the observed science and standard star/asteroid spectra.

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Table A.1

Radial velocities obtained from UVES spectra.

thumbnail Fig. A.2

Lines detected in the UVES spectrum of CoKu Tau 1. The spectrum was normalized to the continuum level. The fitted Gaussian profile is overplotted in green solid line.

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thumbnail Fig. A.3

Lines detected in the UVES spectrum of FS Tau A. The absorption feature next to [S II] (λ6716) is the absorption line of Ca I (λ6717).

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thumbnail Fig. A.4

Lines detected in the UVES spectrum of V853 Oph. The profile of [O I] λ6300 was fitted with two Gaussians to account for the high and low-velocity components observed.

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© ESO, 2012

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