INTERACTION BETWEEN MASSIVE STARS AND
THE ISM IN GALACTIC H ii REGIONS
Chapter I: Oxygen abundances in Orion nebula cluster
stars
S. Simón-Díaz
Instituto de Astrofísica de Canarias
Isaac Newton Group
[email protected]
A. Herrero
Instituto de Astrofísica de Canarias
Departamento de Astrofísica, Universidad de La Laguna
[email protected]
C. Esteban
Instituto de Astrofísica de Canarias
Departamento de Astrofísica, Universidad de La Laguna
[email protected]
F. Najarro
Instituto de Estructura de la Materia, CSIC
[email protected]
Abstract
Using Fastwind, a state-of-the-art model atmosphere program, we derive new
stellar oxygen abundances for the brightest stars of Orion nebula cluster that
point towards values lower than presently assumed and very close to those obtained for the ionized gas phase. This has strong consequences for the dust
content and the assumed total oxygen abundance of the nebula.
These results are part of an ongoing project aimed at the study of ionising massive stars and the surrounding ionized gas in Galactic H ii regions.
2
Keywords:
Stars: abundances – Stars: early-type – Stars: fundamental parameters – Stars:
atmospheres – ISM: abundances – ISM: HII regions – ISM: Orion nebula
Introduction
Since their birth, massive stars, are closely related to the interstellar medium
surrounding them and from where they are born. Being coeval, both objects
must share the same abundance pattern, reflecting the very recent chemical
composition of the zones where they are located.
Until a few years ago, only nebular studies allow us to reach extragalactic distances for abundance determinations due to their big sizes and confined radiation in very luminous emission lines. Nowadays, the construction of very large
telescopes has made possible to use the massive stars for abundance studies
even beyond the Local Group.
Some work has been done to show if the results given by both methodologies,
nebular and stellar, give coherent results (see Urbaneja et al., ? and Trundle et
al., ? as examples of the kind of studies that has been done). However, until
now, there have not been systematic studies combining results obtained from
nebular and stellar studies.
In this paper we present new determinations of the oxygen abundance of the
brightest stars inside the nearest H ii region, the Orion nebula.
1.
The Orion nebula and its associated cluster
Located in the Orion arm of the Galaxy, the Orion nebula is projected just
toward the south of the three stars of the Orion’s Belt. It is an H ii region
that appears like a thin skin of ionized material on the surface of a molecular
complex. The Orion nebula has been extensively studied (see O’Dell, ?, for a
review of the characteristics of this H ii region and its stellar population). Recently, Esteban et al. (?) have worked on a reappraisal study of the chemical
composition of the ionised gas phase of the nebula.
The main ionising source of the Nebula, θ1 Ori C (O7V) star is one of the
brightest stars in the Orion nebula cluster (onc). It can be found in the socalled Trapezium Cluster, the central part of the onc, together with another
three massive stars (θ1 Ori A, B and D). Near the so-called Bright Bar of the
nebula there are two other massive stars, θ2 Ori A and B. Three of those stars
(θ1 Ori A, D and θ2 Ori B) were included in the B stars survey associated with
the Orion complex by Cunha & Lambert (?, ?). These authors obtained C, N,
O, Si and Fe abundances in the stars of the survey.
3
Oxygen abundances in the ONC
Table 1. Orion nebula cluster stars and the two standard stars selected for our study arranged
by spectral type. Projected rotational velocities were derived through Fourier method. Stellar
parameters were derived by means of an HHe analysis of the optical spectra with the latest
version of Fastwind (see Simón-Díaz et al., ?).
HD number
Name
SpT
v sin i (km s−1 )
Teff (K)
log g (dex)
HD37022
HD37041
HD37020
HD37023
HD37042
θ1 Ori C
θ2 Ori A
θ1 Ori A
θ1 Ori D
θ2 Ori B
O7V
O9V
B0.5V
B0.5V
B0.5V
24.0 ± 2.0
131 ± 4
55.0 ± 0.6
49.0 ± 0.9
34.0 ± 0.5
38000
35000
30000
32000
29000
4.1
4.2
4.0
4.2
4.2
HD214680
HD149438
10 Lac
τ Sco
O9V
B0.2V
30.0 ± 0.8
13.0 ± 1.0
35500
32000
3.9
4.0
a
This is the faintest of the sample stars, it has a high v sin i and is a binary. Its INT-IDS spectrum does not
have enough quality for a good spectral study.
2.
Spectroscopic observations
The stellar spectra were obtained with the Intermediate Dispersion Spectrograph (IDS) attached to the Isaac Newton Telescope (INT) in the Roque de Los
Muchachos Observatory. Two spectral regions were observed: from 4000 to
5000 A and the Hα region (Figure ?? shows the 4000-5000 region of the stars
observed for this study). The hydrogen Balmer lines, together with the optical
He i and He ii lines allow us to determine the stellar parameters.
Table ?? resumes the spectral type of the five onc stars as well as another
two standard stars selected for comparison. We have derived the v sin i and
stellar parameters for all the stars of the table, however only three of them have
been selected for the abundance analysis.
3.
B0.5V stars. The perfect targets
The optical spectrum of the early-B, late-O Main Sequence stars shows
many C, N, O, Si and Mg lines. These lines become weaker when we go
through earlier spectral types, as it can be see in Figure ??. So it is better if we
consider B1 - 09 stars for the stellar abundance determinations. However, if
the projected rotational velocity of the star is high, then metal lines will appear
blended, making more difficult the abundance analysis. This is, for example
the case of θ2 Ori A, an O9V star, similar to 10 Lac, but with higher v sin i.
Metal lines, that can be perfectly distinguished in 10 Lac, appear blended in θ2
Ori A.
4
Figure 1.
INT+IDS spectra of the sample stars.
The best targets for the abundance analysis in the onc stars are: θ1 Ori A,
D and θ2 Ori B, three B0.5V. Finally, we have selected the B0.2V star τ Sco as
standard; its Teff is very close to those of the B0.5V onc stars.
4.
The code: FASTWIND
The analysis has been performed by means of the latest version of Fastwind (an acronym for Fast analysis of stellar atmospheres with winds), a
code which was originally described by Santolaya-Rey et al. (?). See Repolust
et al. (?) for the newest description of the code. The newest version uses a
temperature correction method based on the energy balance of electrons. The
technique used for the derivation of the stellar parameters is already standard
and has been described elsewhere (Herrero et al., ?; Repolust et al., ?).
Table ?? resumes the stellar parameters derived through the H - He i / He ii
analysis. The details of the analysis will be published soon (Simón-Díaz et al.,
?).
Oxygen abundances in the ONC
5
Figure 2. Squematic representation of how does the curve of growth method work. See text
for details. (Left) Curves of growth for the O ii λ4641 line in θ1 Ori A; the observed EW and its
uncertainties are represented as horizontal lines. (Right) Each O ii line is represented by a point
in these graphs; the microturbulence that makes all the lines give the same abundance (zero
slope), as well as the abundance associated with that microturbulence is what we are looking
for.
5.
Stellar abundances
Once the stellar parameters have been determined, we proceed to the abundance analysis by means of the curve of growth method. A set of well determined, not blended O ii lines are selected and their equivalent widths are
measured (see Table ??). Then using Fastwind models we proceed as follows: fixing four values of the microturbulence, we construct equivalent width
curves for each line varying the assumed O/ H ratio.
Figure ?? illustrates the curve of growth method. By comparing the observed
EW with the theoretical ones we can derive an abundance for each line and
for each microturbulence. Then we construct an abundance - equivalent width
graph for each microturbulence. We adopt the microturbulence that gives the
same abundance for all the oxygen lines (e.g. the zero slope graph). Finally
the oxygen abundance is calculated statistically with all the lines for that microturbulence.
Several uncertainties have to be taken into account for deriving the final abundance uncertainty. Errors in the measurement of the observed EWs are due to
the SNR of the spectrum. These affect the microturbulence determined through
the zero slope and the individual line abundances. The uncertainty in the microturbulence propagate to the final abundance error. The uncertainties in the
stellar parameters have to be taken also into account. All this sources of un-
6
Line
EWo
²(O)a
τ Sco
(ξt = 9)
O ii
O ii
O ii
O ii
O ii
O ii
O ii
O ii
O ii
O ii
O ii
O ii
O ii
a
4638
4641
4661
4676
4319
4366
4416
4452
4941
4943
4956
4891
4906
85
127
90
79
86
80
100
37
38
60
17
24
34
8.80
8.70
8.76
8.77
8.73
8.65
8.67
8.74
8.68
8.66
8.78
8.79
8.67
EWo
²(O)
EWo
²(O)
EWo
²(O)
θ1 Ori A
(ξt = 6)
θ1 Ori D
(ξt = 8)
θ2 Ori B
(ξt = 6)
97
132
80
82
100
96
121
54
50
63
20
29
42
–
–
74
68
71
77
93
32
35
55
13
18
31
93
—
88
82
95
103
117
60
44
62
21
27
43
8.74
8.63
8.44
8.59
8.67
8.64
8.68
8.72
8.68
8.54
8.66
8.70
8.62
—
—
8.45
8.51
8.45
8.49
8.46
8.47
8.51
8.49
8.51
8.56
8.50
8.72
—
8.56
8.61
8.62
8.74
8.67
8.78
8.59
8.54
8.67
8.64
8.63
²(O) = log(O/H) + 12
Table 2.
Equivalent widths and derived line abundances for the suitable set of O ii lines in the four
studied stars. Line abundances refer to the microturbulence given in brackets for each star (ξt in km s−1 ).
Uncertainties in the observed EW are 5 mA (τ Sco) and 10 mA (Orion stars). Some lines in θ1 Ori B and
θ2 Ori B were not used because are blended
certainties (due to microturbulence, stellar parameters and statistics) are added
quadratically for giving the final uncertainty.
The resulting oxygen abundances for the three B0.5V stars of the onc, as well
as the standard B0.2V star τ Sco, are presented in Table ??. There is agreement between the oxygen abundances derived for the Orion stars taking into
account the uncertainties.
6.
Comparison with nebular abundances
Esteban et al. (?) have recently published a reappraisal study of the chemical
composition of the Orion nebula. They derived a total oxygen gas phase abundance log(O/ H) = 8.65 ± 0.12. However, some oxygen could be depleted onto
dust grains, so the total gas+dust oxygen abundance should have this depletion
into account. In a previous work (Esteban et al. ?), these authors estimate the
depletion onto dust grains by comparing Si and Fe nebular abundances with
those obtained by Cunha & Lambert (?, 1994) in B stars in Orion (see Table
7
Oxygen abundances in the ONC
Object
N
φt (km/s)
log(O/H)+12
This work
HD 149438
1
θ Ori A
θ1 Ori D
θ2 Ori B
15
8.9 ± 1.2
8.72 ± 0.06
14
13
13
6.7 ± 2.5
7.0 ± 4.5
6.0 ± 2.7
8.66 ± 0.12
8.58 ± 0.11
8.62 ± 0.12
Cunha & Lambert
1
θ Ori A
θ1 Ori D
θ2 Ori B
7
6
6
5.0
7.0
6.0
8.83 ± 0.12
8.87 ± 0.08
8.85 ± 0.06
Esteban et al.
Gas
Gas + dust
8.65 ± 0.12
8.73 ± 0.12
Table 3.
Oxygen abundances for the three B0.5V stars inside Orion nebula and the reference star τ Sco.
Oxygen NLTE abundances derived by Cunha & Lambert (?) for the Orion stars as well as the those
calculated by Esteban et al. (?) for the nebula are also presented for comparison.
??). Taking into account that correction, the final gas+dust oxygen abundance
Esteban et al. (?) propose is log(O/ H) = 8.73 ± 0.12.
How well do these values compare with the oxygen abundance we derived?
There are two points we may discuss:
1 Our O/ H ratios are systematically lower than those determined by Cunha
& Lambert (?).
2 Our stellar O/ H ratios are in agreement with the gas phase oxygen abundance of Esteban et al. (?).
Cunha & Lambert obtain higher values of Teff for these stars, and this produces larger oxygen abundances (the theoretical O ii lines become fainter when
a higher Teff is considered, so we need a higher oxygen abundance for fitting
the observed O ii lines equivalent widths). As gas phase nebular oxygen abundance is very close to what we derived, this suggest that the fraction of oxygen
depleted onto dust grains could be lower than the previous estimation. However, the uncertainties in the stellar and nebular gas phase oxygen abundances
do not allow us to estimate accurately the oxygen depletion factor. We are
working in the Si and Fe analysis to further explore this possibility.
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7.
Conclusions
We have performed an oxygen abundance analysis of three B0.5V stars located in the core of the Orion nebula:
Our oxygen abundances are ∼ 0.2 dex lower than those derived by Cunha
& Lambert (?)
Our abundances are in agreement with those gas phase abundances derived by Esteban et al. (?)
Our values suggest that the amount of oxygen depleted onto dust grains
is lower than that assumed by Esteban et al. (?) taking into account the
stellar Si, and Fe abundances in Orion B stars by Cunha & Lambert.
8.
Discussion during the meeting
J. Maíz Apellaniz: There is some confusion regarding θ1 Ori A. It is really
an early-B star but in some places it is mistaken with θ1 Ori C, which is really
the only O star.
S. Simón-Díaz: This confusion can be easily solved if you have a look to
the spectra. The optical spectrum of θ1 Ori C, classified as O7V is very different compared with the optical spectrum of θ1 Ori A.
Acknowledgments
SSD wish to thank M. A. Urbaneja, M. R. Villamariz, C. Trundle and D.
Lennon for their help and useful comments on this study. This work has been
funded by the Spanish MCyT under project AYA2001-0436 and the Spanish
MEC under project AYA2004-08271-C02-00.
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Oxygen abundances in the ONC
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