CHEMICAL ABUNDANCES OF WEAK T TAURI STARS

Transcription

1 CHEMICAL ABUNDANCES OF WEAK T TAURI STARS Gustavo de Araujo Rojas Universidade de São Paulo Instituto de Astronomia, Geofísica e Ciências Atmosféricas rojas@astro.iag.usp.br Jane Gregorio-Hetem Universidade de São Paulo Instituto de Astronomia, Geofísica e Ciências Atmosféricas jane@astro.iag.usp.br Abstract This work is a spectroscopic study aiming to derive chemical abundances of a sample of low mass pre-main sequence stars, previously identified as Weak T Tauri stars. High resolution optical spectra were obtained at the Observatório do Pico dos Dias and European Southern Observatory. A classification was developed based on the equivalent widths of the Li I 6708 Å and Hα lines and the spectral type of the studied stars. This classification allowed the identification of them into categories that correspond to different evolutive phases in the pre-main sequence. The position of these stars in the H-R Diagram was also inspected in order to estimate their ages and to confirm their classification in the assumed evolutive scenario. Among the 24 objects of the sample, it was verified that 6 are actually Weak T Tauri stars, 7 are Classical T Tauri, 1 is Post T Tauri, and 10 are Fe/Ge Pre-Main Sequence stars. The stellar fundamental parameters effective temperature and gravity were calculated by excitation and ionization equilibria of Iron absorption lines. Chemical abundances were obtained by spectral synthesis. The first results indicate that the studied sample shows solar abundances. 1

2 2 Introduction The identification of large samples of young stars and detailed analysis of the distribution of their chemical abundances, comparing pre-main sequence stars with young clusters, is of great interest to galactic chemical evolution models. Among pre-main sequence stars, the Weak T Tauri stars (WTTS) are specially suited for spectroscopic abundance analysis, as the veiling effect on absorption lines (which have made difficult the chemical analysis of classical T Tauri stars) is not enhanced. This can be interpreted as a less intense accretion activity. There are very few results in the literature concerning abundances in pre-main sequence stars, most of them focused on Lithium abundances. One of the few examples presenting metallicities of T Tauri stars was published by Padgett (1996), who calculated the Iron abundances of 30 stars of nearby star forming regions. 1. Target Selection and Observations 1.1 Target Selection Our sample consists of 22 candidates previously classified as WTTS, selected from the young stellar objects catalogues Pico dos Dias Survey (Gregorio-Hetem et al. 1992; Torres et al. 1995) and Herbig-Bell Catalogue (Herbig & Bell 1988). We selected all stars with Hα equivalent widths in emission inferior to 20 Å and a visual magnitude brighter than 12 mag. We also included 2 WTTS selected from the literature, HD 8558 (Torres et al. 2000) and SAO 76411A (Walter et al. 1988). 1.2 Observations The observations were performed at the Observatório do Pico dos Dias (OPD) in Brasópolis, Brazil, and at the European Southern Observatory (ESO) in La Silla, Chile, in six runs between October, 2000 and November, At the OPD we used the Coudé spectrograph and 1,60 m. telescope, obtaining spectra with resolution R=20000 in 4 spectral regions. At ESO we used the FEROS spectrograph and 1,52 m. telescope, obtaining high-resolution spectra (R=48000) covering the whole optical range (360 to 920 nm). Table 1 lists the stars of the sample and the observational data from the literature.

3 3 Table 1. Published data for the sample stars. The equivalent widths (W) of emission lines are represented by negative values. Object V Spectral Type W(Hα) (Å) HBC 400 a 12.1 K7V -3.2 HD 8558 b 8.5 G6V -3.3 PDS G9V PDS G5V PDS F6V -6.0 PDS M2V -2.1 PDS G0V PDS K5V 0.0 PDS M3V PDS K2V -5.0 PDS K7V -5.0 PDS F5V -3.0 PDS G0V PDS F7V 3.0 PDS F5V -6.0 PDS G3V -0.5 PDS G5V -9.0 PDS 117S 11.1 F6V -8.0 PDS F6V -7.0 PDS F5V -2.0 PDS F5+F8 IV -1.0 PDS 390S 8.7 F6V PDS F6IV -5.0 SAO 76411A c 8.9 G0V 0.1 References : a) Mundt et al. 1983; b) Torres et al. 2000; c) Walter et al The data for the remaining were obtained by Torres (1998) 2. Spectroscopic Classification Spectral types were determined by comparing the observed spectra with the digital libraries published by Danks & Dennefeld (1994) and Jacoby et al. (1984). We found no spectral type variations with the types published in previous works. 2.1 Emission Line Profiles By definition, the WTTS are distinguished from the Classical T Tauri stars (CTTS) by the equivalent width of the Hα emission, adopting W(Hα) < 10 Å for WTTS. However, the Hα emission is variable and should not be used as the only distinguishing criterion between these

4 4 two classes. Additional information about the circumstellar structure and activity can be obtained by the Hα emission profiles. In this work these line profiles were classified in three types : Type 1 for absorption lines, Type 2 for symmetric emission profiles and Type 3 for complex emission profiles. Figure 1 shows examples of each profile type. Figure 1. Examples of the three profile types of the Hα line. PDS 54 (upper left panel) shows a Type 1 profile, in absorption. HBC 400 (upper right panel) presents a Type 2 profile, with symmetric emission. PDS 111 (lower panel) shows a complex Hα emission, classified as Type 3.

5 2.2 Ages Ages were estimated by comparing the positions of the stars in the HR Diagram. The pre-main sequence evolutionary tracks published by D Antonna & Mazzitelli (1994) were adopted. The luminosities were derived using distances estimated from the Hipparcos parallaxes when avaiable, or the distance of the parent molecular cloud. Figure 2 shows the observed HR diagram and the evolutionary tracks. 5 Figure 2. Position in the HR diagram of the stars of the sample. The pre-main sequence evolutionary tracks are from D Antonna & Mazitelli (1994).

6 6 2.3 Spectroscopic Classification According to emission features, spectral type, and derived age, the stars of the sample were classified in four categories : Classical T Tauri (CTTS) : spectral type K or M, stronger emission lines with complex profiles, ages < 10 Myr Weak T Tauri (WTTS): spectral type K or M, weak emission lines with symmetric profiles, ages < 20 Myr Post T Tauri (PTTS) : spectral type K or M, Hα in absorption, ages > 20 Myr Fe/Ge Pre-Main Sequence (Fe/Ge): spectral type F or G, stronger emission lines with complex profiles, ages < 10 Myr Table 2 presents the final obtained stellar parameters and classification. Table 2. Spectroscopic classification of the sample Object T eff (K) W LiI (Å) W(Hα) (Å) Hα Profile Class HBC WTT HD PTT PDS CTT PDS CTT PDS Fe/Ge PDS WTT PDS CTT PDS WTT PDS WTT PDS WTT PDS CTT PDS Fe/Ge PDS CTT PDS Fe/Ge PDS Fe/Ge PDS CTT PDS CTT PDS 117S Fe/Ge PDS Fe/Ge PDS Fe/Ge PDS Fe/Ge PDS 390S Fe/Ge PDS Fe/Ge SAO 76411A WTT

7 3. Results and Discussion 3.1 Effective Temperatures The effective temperature (T eff ) was derived from three independent criteria: calibration with spectral type, excitation equilibrium of Fe I and equivalent width ratio of metallic absorption lines. A first estimative of the T eff was obtained using the empirical spectral type calibration published by de Jager & Nieuwenhuijzen (1987). This method was employed to all the stars of the sample. Excellent quality spectra were obtained for seven stars, which have small rotational velocities and low veiling. In this case, hundreds of Iron absorption lines were accuratelly measured and used in the code Renoir (kindly provided by M. Spite) to determine effective temperatures based on the excitation equilibrium (more details are presented in section 3.2). An even more precise method uses the equivalent width ratio of selected absorption lines showing different excitation potentials, which are very sensitive to temperature changes. The adopted calibration was published by Padgett (1996), who lists six pairs of absorption lines of Fe I and V I. This method was only applied to the seven stars mentioned above. Table 3 lists the obtained results, showing a good agreement among the three different methods. 7 Table 3. Comparision of the effective temperatures obtained from three independent criteria: spectral type calibration (T ST ), excitation equilibrium (T EE ), and equivalent width ratio (T EW ). Object T ST (K) T EE(K) T EW (K) HBC HD PDS PDS PDS PDS SAO 76411A

8 8 3.2 Surface Gravities Surface gravities (log g) were calculated based on the ionization equilibrium of Iron lines, using the code Renoir mentioned in section 3.1. This code calculate Iron abundances from measured equivalent widths of Fe I and Fe II lines, where the gravity is a free parameter. The adopted value of log g is the one which provides the same abundance for both Fe I and Fe II lines. For the calculations solar metallicities and a microturbulent velocity of 2 km/s were assumed. The atmospheric models are from Kurucz (1993) and the atomic line data are from the Vienna Atomic Line Database (Kupka et al. 1999). Table 4 lists the results, showing typical subgiant gravities (as expected), with values of log g ranging from 4.0 to 4.8 dex. Table 4. Surface gravities (log g) calculated from the ionization equilibrium of Fe I and Fe II lines. Object log g (dex) HBC HD PDS PDS PDS PDS SAO 76411A Abundances Elemental abundances were calculated for 22 chemical elements by spectral synthesis of selected absorption lines of 22 elements. The code Spectroscopy Made Easy (Valenti & Piskunov 1996) was adopted, which calculates abundances in the local thermodynamical equilibrium. As in the case of excitation and ionization equilibrium, we were only able to perform the spectral synthesis for the seven stars of excellent spectra quality. Examples of synthetic spectra are presented in Figure 3, and the calculated abundances are listed in Table 5.

9 Figure 3. Examples of the fitted synthetic spectra for the star HD Blue : synthetic spectrum. Red : observed spectrum. 9

10 10 Table 5. Chemical abundances [X/Fe] relative to the Sun calculated for the stars of the sample for which the spectral synthesis were performed. Element Li C O Na Mg Al Si S Ca Sc Ti HBC HD PDS PDS PDS PDS SAO76411A Element V Cr Mn Fe Co Ni Sr Y Ba La Eu HBC HD PDS PDS PDS PDS SAO76411A Conclusions Spectroscopic criteria were applied for 24 objects previously identified as Weak T Tauri stars in order to separate them into different pre-main sequence categories. Our re-classification showed that only six of the stars are actually WTTS, the remaining being CTTS, PTTS or Fe/Ge Pre-Main Sequence stars. Stellar parameters and chemical abundances of 22 elements were obtained for seven stars of the sample. For all of them the abundances, with the exception of Lithium, are similar to the solar values. Acknowledgments GAR acknowledges the grant provided by the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) for the MSc research. Both authors thank the Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) for partial finnancial support (Proc. No. 2001/ ).

11 References Danks, A. C., Dennefeld, M. 1994, PASP 106, 382 D Antona, F., Mazzitelli, I. 1994, ApJS 90, 467 de Jager, C., Nieuwenhuijzen, H. 1987, A&A 177, 217 Gregorio-Hetem, J., Lépine, J., Quast, G., Torres, C.A., de la Reza, R. 1992, AJ 103, 549 Herbig, G. H., Bell, K. R Lick Obs. Bull Jacoby, G. H., Hunter, D. A., Christian, C. A. 1984, ApJS 56, 257 Kupka, F., Ryabchikova, T. A., Stemples, H. C., Weiss, W. W. 1999, A&AS 138, 119 Kurucz, R. L CD-ROM No. 18 Mundt, R., Walter, F. M., Feigelson, E. D., Finkenzeller, U., Herbig, G. H., Odell, A. P ApJ 269, 229 Padgett, D. L. 1996, ApJ 471, 847 Torres, C. A. O PhD. Thesis, CNPq/Observatório Nacional, Brasil Torres, C.A., Quast, G., de la Reza, R., Lépine, J., Gregorio-Hetem, J. 1995, AJ 109, 2146 Torres, C. A. O., Da Silva, l., Quast, G. R., De La Reza, R., Jilinski, E AJ 120, 1410 Valenti, J. A., Piskunov, N A&ASS 118, 595 Walter, F. M., Brown, A., Mathieu, R. D., Myers, P. C., Vrba, F. J AJ 96,