CHEMICAL ABUNDANCES OF WEAK T
TAURI STARS
Gustavo de Araujo Rojas
Universidade de São Paulo
Instituto de Astronomia, Geofı́sica e Ciências Atmosféricas
rojas@astro.iag.usp.br
Jane Gregorio-Hetem
Universidade de São Paulo
Instituto de Astronomia, Geofı́sica 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
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.
1.1
Target Selection and Observations
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, 2002. 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
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 400a
HD 8558b
PDS 008
PDS 013
PDS 014
PDS 045
PDS 046
PDS 054
PDS 055
PDS 073
PDS 083
PDS 093
PDS 108
PDS 109
PDS 110
PDS 111
PDS 115
PDS 117S
PDS 118
PDS 119
PDS 177
PDS 390S
PDS 401
SAO 76411Ac
12.1
8.5
10.0
9.8
10.4
11.1
10.7
8.9
11.4
10.4
11.7
8.8
10.7
10.2
10.4
9.9
11.0
11.1
9.9
10.5
9.1
8.7
8.3
8.9
K7V
G6V
G9V
G5V
F6V
M2V
G0V
K5V
M3V
K2V
K7V
F5V
G0V
F7V
F5V
G3V
G5V
F6V
F6V
F5V
F5+F8 IV
F6V
F6IV
G0V
-3.2
-3.3
-12.0
-28.0
-6.0
-2.1
-21.0
0.0
-11.0
-5.0
-5.0
-3.0
-14.0
3.0
-6.0
-0.5
-9.0
-8.0
-7.0
-2.0
-1.0
-14.5
-5.0
0.1
References : a) Mundt et al. 1983; b) Torres et al. 2000; c) Walter et al. 1988. 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
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.
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
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
Tef f (K)
WLiI (Å)
W(Hα) (Å)
Hα Profile
Class
HBC 400
HD 8558
PDS 008
PDS 013
PDS 014
PDS 045
PDS 046
PDS 054
PDS 055
PDS 073
PDS 083
PDS 093
PDS 108
PDS 109
PDS 110
PDS 111
PDS 115
PDS 117S
PDS 118
PDS 119
PDS 177
PDS 390S
PDS 401
SAO 76411A
4405
5635
5310
4690
6530
3520
5945
4405
3405
4990
4150
6655
5945
6535
6655
5795
5795
6530
6530
6650
6650
6530
6530
5945
0.54
0.20
0.26
0.27
0.16
0.52
0.25
0.35
0.71
0.73
0.51
0.05
0.16
0.15
0.12
0.26
0.27
0.12
0.11
0.11
0.18
0.13
0.15
0.10
-1.9
-3.3
-12.8
-23.4
-10.5
-1.7
-21.0
0.5
-8.9
-9.2
-12.8
-1.4
-9.1
3.5
-4.4
-1.2
-8.6
-2.7
-5.4
-3.1
-1.0
-14.5
-5.0
0.1
2
2
3
3
3
2
3
1
2
3
3
3
3
3
3
3
3
3
3
3
3
2
3
1
WTT
PTT
CTT
CTT
Fe/Ge
WTT
CTT
WTT
WTT
WTT
CTT
Fe/Ge
CTT
Fe/Ge
Fe/Ge
CTT
CTT
Fe/Ge
Fe/Ge
Fe/Ge
Fe/Ge
Fe/Ge
Fe/Ge
WTT
7
3.
Results and Discussion
3.1
Effective Temperatures
The effective temperature (Tef f ) 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 Tef f 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.
Table 3. Comparision of the effective temperatures obtained from three independent
criteria: spectral type calibration (TST ), excitation equilibrium (TEE ), and equivalent
width ratio (TEW ).
Object
TST (K)
TEE (K)
TEW (K)
HBC 400
HD 8558
PDS 045
PDS 054
PDS 055
PDS 083
SAO 76411A
4405
5635
3520
4405
3405
4150
5945
4400
5600
3600
4400
3400
4200
6000
4560
5715
3700
4500
3510
4220
6020
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.
3.3
Object
log g (dex)
HBC 400
HD 8558
PDS 045
PDS 054
PDS 055
PDS 083
SAO 76411A
4.2
4.4
4.0
4.4
4.2
4.0
4.8
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 8558. Blue :
synthetic spectrum. Red : observed spectrum.
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 400
HD 8558
PDS 045
PDS 054
PDS 055
PDS 073
SAO76411A
2.2
2.0
3.0
3.0
2.0
2.6
1.8
0.0
0.1
-0.1
0.2
0.0
-0.1
0.0
0.0
0.2
-0.1
0.0
0.1
0.0
0.1
0.1
0.0
0.2
0.1
0.0
0.1
0.0
0.2
0.0
0.2
0.0
0.2
0.0
0.1
0.0
0.0
0.1
0.0
0.1
-0.1
0.0
0.0
0.0
0.1
0.0
0.0
0.1
0.0
0.0
0.0
0.0
0.1
0.0
0.0
0.0
0.2
0.1
0.1
0.0
0.1
0.0
0.0
0.2
0.2
0.2
0.0
0.2
0.0
0.0
0.1
0.0
0.2
0.0
0.0
0.1
0.1
Element
V
Cr
Mn
Fe
Co
Ni
Sr
Y
Ba
La
Eu
HBC 400
HD8558
PDS 045
PDS 054
PDS 055
PDS 073
SAO76411A
0.2
0.2
0.2
0.1
0.2
0.2
0.0
0.0
0.1
0.0
0.0
0.1
0.0
0.0
0.2
0.0
0.0
0.1
0.0
0.0
0.0
0.2
0.2
0.1
0.1
0.1
0.1
0.0
0.2
0.1
0.1
0.1
0.0
0.0
0.0
0.0
0.0
0.0
0.2
0.0
0.0
0.0
0.1
0.1
0.1
0.0
0.1
0.0
0.0
0.1
0.1
0.0
0.1
0.1
0.0
0.0
0.2
0.2
0.1
0.0
0.1
0.1
0.0
0.1
0.0
-0.1
-0.1
0.0
0.0
-0.1
0.0
0.0
0.0
-0.1
0.0
0.0
0.0
4.
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 Cientı́fico 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.
2001/09018-2).
11
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549
Herbig, G. H., Bell, K. R. 1988 Lick Obs. Bull. 1111
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Kupka, F., Ryabchikova, T. A., Stemples, H. C., Weiss, W. W. 1999, A&AS 138, 119
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