Hot stars and cosmic abundances

TLDR: In this article, a non-LTE study of a larger sample of early B-type stars in the solar neighbourhood is presented, including new preliminary data on aluminium, sulphur and argon.

Abstract: Hot massive stars are ideal indicators for present-day cosmic abundances. We review results from a non-LTE study of a larger sample of early B-type stars in the solar neighbourhood by Nieva & Przybilla (2012) and extend the analysis. Using comprehensive models with improved microphysics, novel analysis methodologies and high-quality spectra it is shown that the present-day chemical composition in massive stars out to several hundred parsec distance from the Sun is highly homogeneous. Abundances for about a dozen astrophysically important chemical elements are presented, including new preliminary data on aluminium, sulphur and argon. This establishes a cosmic abundance standard, which serves as a reference facilitating chemical peculiarities in other stars to be identified. Similarities and differences to the solar standard, and implications are discussed.

Figures

Fig. 1. Distribution of the sample stars in the solar neighbourhood. Upper panel: Galactic plane projection. Lower panel: rotational plane projection. Numbers/letters correspond to those used by Nieva & Przybilla (2012, NP12). Open circles represent Orion stars from Nieva & Simón-Dáz (2011). The symbol size encodes the oxygen abundance according to the figure legend. The position of the Sun is marked by . From NP12.

Fig. 5. Evolution of Galactic α-process vs. iron-group elements: log (Mg/Fe) as a function of Fe abundance. Literature data on solar-type stars are displayed as grey symbols (green online) – squares: Fuhrmann (1998, 2004), triangles: Edvardsson et al. (1993). All other symbols as in Figure 3. From NP12.

Fig. 2. Example for the comparison between the global synthetic (grey, online: red) and observed spectrum (black line) for γ Peg (B2 IV) in the spectral range λλ 3900–4500 Å. Abscissa is wavelength (in Å), ordinate is relative flux. From NP12.

Fig. 4. Observational constraints on the Galactic chemical evolution (GCE) of CNO abundances: abundance ratios log (C/O) and log (N/O) vs. O abundance. Data for low-

Table 2. CAS abundances in comparison to the Orion nebula and the Suna.

Fig. 3. Abundance distributions for the astrophysically most relevant chemical species as derived from early B-stars in the solar neighbourhood. Grey (online: red) histograms: data from NP12, establishing the cosmic abundance standard. Black histograms: literature data. Photospheric and protosolar abundances from AGSS09 are also indicated, the bars representing the range spanned by the ±1σ-uncertainties. From NP12. The preliminary data on aluminium, sulphur and argon abundances show similar distributions.

Full text

Key Observations for Stellar Modelling
N. Grevesse (chair)
G. Preston (chair)
N. Przybilla
S. Bisterzo
M. Chadid
A. Korn

J. Silvester
F. Thevenin
M. Castro
J. Landstreet

New Advances in Stellar Physics: From Microscopic to Macroscopic Processes
G. Alecian, Y. Lebreton, O. Richard and G. Vauclair (eds)
EAS Publications Series, 63 (2013) 13–23
HOT STARS AND COSMIC ABUNDANCES
N. Przybilla
1
,M.F.Nieva
1,2
, A. Irrgang
2
and K. Butler
3
Abstract. Hot massive stars are ideal indicators for present-day cos-
mic abundances. We review results from a non-LTE study of a larger
sample of early B-type stars in the solar neighbourhood by Nieva &
Przybilla (2012) and extend the analysis. Using comprehensive models
with improved microphysics, novel analysis methodologies and high-
quality spectra it is shown that the present-day chemical composition
in massive stars out to several hundred parsec distance from the Sun
is highly homogeneous. Abundances for about a dozen astrophysically
important chemical elements are presented, including new preliminary
data on aluminium, sulphur and argon. This establishes a cosmic abun-
dance standard, which serves as a reference facilitating chemical pecu-
liarities in other stars to be identified. Similarities and differences to
the solar standard, and implications are discussed.
1 Introduction
Hot massive stars are ideal indicators for present-day cosmic abundances. This
is because they preserve the pristine chemical composition of their natal cloud
and typically do not migrate far beyond their birth environments over their short
lifetimes, in contrast to older stars. They are also unaffected by depletion onto
dust grains, unlike the cold/warm interstellar medium or H ii regions. From an
analysis perspective, quantitative spectroscopic studies of unevolved early B-type
stars are to be preferred when absolute abundances of highest accuracy and preci-
sion are desired. The reason is their simple photospheres, which are unaffected by
macroscopic phenomena like strong stellar winds or convection, and microscopic
atomic diffusion, which gives rise to chemical peculiarities in many other kinds
of stars. However, as a slight complication, deviations from local thermodynamic
1
Institute for Astro- and Particle Physics, University of Innsbruck, Technikerstr. 25/8,
6020 Innsbruck, Austria
2
Dr. Remeis Observatory & ECAP, University of Erlangen-Nuremberg, Sternwartstr. 7,
96049 Bamberg, Germany
3
University Observatory, University of Munich, Scheinerstr. 1, 81679 Munich, Germany
c
 EAS, EDP Sciences 2014
DOI: 10.1051/eas/1363002

14 New Advances in Stellar Physics
equilibrium (non-LTE effects) need to be accounted for properly in line-formation
calculations.
Several rather comprehensive non-LTE studies have targeted nearby early-
B main-sequence stars in the past to derive present-day cosmic abundances, e.g.
Kilian (1992), Gies & Lambert (1992), Cunha & Lambert (1994), Morel et al. (2006).
Using standard analysis techniques, these have found large uncertainties in basic
stellar parameters, a tendency towards a metal-poor composition with respect to
older stars like the Sun and an overall enormous range in derived elemental abun-
dances. The latter are contrasted by a chemically homogeneous ISM gas (Sofia &
Meyer 2001) – the material the stars were formed from.
These discrepancies have recently been resolved by Nieva & Przybilla (2012,
henceforth abbreviated to NP12). Using comprehensive models with improved mi-
crophysics, novel analysis methodologies and high-quality spectra for a carefully
selected star sample it was shown that the present-day chemical composition in
massive stars out to several hundred parsec distance from the Sun is highly ho-
mogeneous, to the 10%-level. This allowed a cosmic abundance standard to be
established, which is an important complement to the only other standard abun-
dances available, the solar values. Here we summarise the major steps in the work
of NP12, present new (preliminary) results on additional chemical species, and
discuss implications.
2 Observations and analysis
The sample of NP12 consists of bright, sharp-lined, apparently single and chemi-
cally inconspicuous stars that show no indications for the presence of circumstel-
lar emission. An initial sample of 20 objects in associations and the field out to
∼400 pc distance from the Sun was complemented by 9 additional stars in the
Orion OB1 association analysed by Nieva & Sim´on-D´ıaz (2011, NS11). The dis-
tribution of the sample stars in the solar neighbourhood is displayed in Figure 1,
essentially delineating Gould’s Belt.
High-S/N and high-resolution echelle spectra (with S/N > 250–500 typically,
and resolving power R = λ/∆λ ≥ 40 000) were obtained with FEROS on the
ESO/MPG 2.2 m telescope at La Silla, FOCES on the 2.2 m telescope on Calar
Alto, FIES on the 2.5 m Nordic Optical Telescope on La Palma, or taken from the
ELODIE (on the 1.93 m telescope at Observatoire de Haute-Provence) archive. All
spectra have a wide wavelength coverage (∼3900–7000
˚
A, and often out to 9000
˚
A),
which is required to access all important spectroscopic indicators.
Model calculations were carried out using a hybrid non-LTE approach as dis-
cussed in detail by Nieva & Przybilla (2007, 2008) and Przybilla et al. (2011).
In brief, hydrostatic, plane-parallel and line-blanketed LTE model atmospheres
were computed with Atlas9 (Kurucz 1993). Non-LTE line formation compu-
tations were then performed on these model structures using updated versions
of the Detail and Surface codes (Giddings 1981; Butler & Giddings 1985).
State-of-the-art non-LTE model atoms (see Table 1) relying on data from ab-
initio computations – avoiding rough approximations wherever possible – were

N. Przybilla et al.: Hot Stars and Cosmic Abundances 15
Fig. 1. Distribution of the sample stars in the solar neighbourhood. Upper panel:
Galactic plane projection. Lower panel: rotational plane projection. Numbers/letters
correspond to those used by Nieva & Przybilla (2012, NP12). Open circles represent
Orion stars from Nieva & Sim´on-D´az (2011). The symbol size encodes the oxygen abun-
dance according to the figure legend. The position of the Sun is marked by .From
NP12.
Tabl e 1. Model atoms for non-LTE calculations.
Ion Model atom
H Przybilla & Butler (2004)
He i/ii Przybilla (2005)
C ii-iv Nieva & Przybilla (2006, 2008)
N ii Przybilla & Butler (2001), updated
O i/ii Przybilla et al. (2000), Becker & Butler (1988), updated
Ne i/ii Morel & Butler (2008), updated
Mg ii Przybilla et al. (2001)
Al ii/iii Przybilla (in prep.)
Si iii/iv Becker & Butler (1990), updated
S ii/iii Vrancken et al. (1996), updated
Ar i/ii Butler (in prep.)
Fe ii/iii Becker (1998), Morel et al. (2006), corrected
utilised in this step, see Przybilla (2010) for an overview of the importance of these
input data.
The stellar parameter and abundance determination for early B-type
main-sequence to giant stars employs an iterative analysis methodology

Frequently asked questions

Q1. What contributions have the authors mentioned in the paper "Key observations for stellar modelling" ?

The authors review results from a non-LTE study of a larger sample of early B-type stars in the solar neighbourhood by Nieva & Przybilla ( 2012 ) and extend the analysis. Similarities and differences to the solar standard, and implications are discussed. 

Q2. What is the CAS requirement for GCE models?

GCE models not only have to reproduce the behaviour of the observation but also have to match the present-day composition as a boundary condition. 

Q3. What is the wavelength coverage of NP12?

All spectra have a wide wavelength coverage (∼3900–7000 Å, and often out to 9000 Å), which is required to access all important spectroscopic indicators. 

Q4. How many telescopes were used to obtain NP12?

High-S/N and high-resolution echelle spectra (with S/N > 250–500 typically, and resolving power R = λ/∆λ ≥ 40 000) were obtained with FEROS on the ESO/MPG 2.2m telescope at La Silla, FOCES on the 2.2m telescope on Calar Alto, FIES on the 2.5m Nordic Optical Telescope on La Palma, or taken from the ELODIE (on the 1.93m telescope at Observatoire de Haute-Provence) archive. 

Q5. What is the significance of the abundances in the histograms?

Note that only 20 stars are considered in the histograms for N – the atmospheres of nine stars are mixed with CN-processed material – as pristine abundances are of interest for constraining a Cosmic Abundance Standard (CAS). 

Q6. Why is the dust phase in the ISM so large?

This is due to the large density variations of the gas, generated by a complex interaction of many factors such as momentum injection by stellar winds and supernova shocks, magnetic fields and self-gravity, which is supported by recent theoretical investigations. 

Q7. Why are hot massive stars ideal indicators for present-day cosmic abundances?

The reason is their simple photospheres, which are unaffected by macroscopic phenomena like strong stellar winds or convection, and microscopic atomic diffusion, which gives rise to chemical peculiarities in many other kinds of stars. 

Q8. What are the main characteristics of hot massive stars?

Using standard analysis techniques, these have found large uncertainties in basic stellar parameters, a tendency towards a metal-poor composition with respect to older stars like the Sun and an overall enormous range in derived elemental abundances. 

Q9. What is the reason for the existence of a cosmic abundance standard?

This allowed a cosmic abundance standard to be established, which is an important complement to the only other standard abundances available, the solar values. 

Q10. Why is the dust phase in the ISM so inaccessible?

The huge advantage of studying early-type stars is that the entire metal content can be determined using quantitative spectroscopy, with no material hidden in an observationally inaccessible reservoir like the dust-phase in the ISM. 

Q11. How was the stellar parameter determination verified?

The quality of the stellar parameter determination was verified in addition by simultaneously matching the measured stellar spectral energy distributions (employing UV spectrophotometry obtained with the IUE satellite and ground-based optical and near-IR photometry) with the model fluxes and by comparing the spectroscopic with Hipparcos distances. 

Q12. What is the way to study the early B-type stars?

From an analysis perspective, quantitative spectroscopic studies of unevolved early B-type stars are to be preferred when absolute abundances of highest accuracy and precision are desired. 

Q13. What is the amount of money MFN receives from the Elite Network of Bavaria?

MFN acknowledges a FFL stipend from the University of Erlangen-Nuremberg and AI support by a research scholarship from the Elite Network of Bavaria, and from the German Research Foundation (DFG) through grant He 1356/45-2.