Hot Massive Stars in the Far-UV and UV
ranges
M. Garcı́a
Instituto de Astrofı́sica de Canarias, Vı́a Láctea s/n, E38200 La Laguna, Tenerife,
Spain. mgg@iac.es
Summary. Massive stars play a crucial role in the chemical and mechanical evolution of structures in the Universe, through their feedback of momentum and energy
to the interstellar medium and their contribution to the cosmic and galactic nucleosynthesis. This work is a summary of my PhD dissertation, focused on the quantitative analysis of FUSE (∆λ905-1187Å) and IUE/HST-STIS (∆λ ∼1150-1800Å)
spectra of Milky Way O-type stars to determine their physical properties. For the
anlysis we used synthetic spectra calculated with the WM-basic code [8], which applies an accurate treatment of non-LTE and line-blocking to the entire atmosphere
and solves the hydrodynamical structure of the spherically expanding envelope. A
grid of models was built as an aid and is described here. By fitting simultaneously
the spectral features in the joint FUSE and IUE range, we determined consistently
both the photospheric and wind parameters. We additionally quantified shocks in
the wind using the O V I λλ1031.9,1037.6 doublet in the FUSE range. The X-ray
emission produced in the cooling zones of shocks alters the ionization equilibrium
and is accounted for by WM-basic. Our most important result is a new temperature
scale for the spectral types spanned by the sample, O3-O7. The temperature values
derived for the sample of stars are consistently lower than previously determined in
the literature or assigned by the calibrations of Vacca et al.(1996) and Martins et
al. (2005) to their spectral type.
1 INTRODUCTION
Understanding hot massive stars, their physical properties and their radiation
field is essential for a number of astrophysical branches, ranging from studies
of H II regions to the chemical evolution of galaxies and the Universe. However, constraining their parameters is not simple and requires quantitative
spectroscopic analyses with models that provide an accurate physical description of their expanding envelope. The latter task is most complicated, since
radiative processes dominate the atmosphere and both non-LTE effects and
the radiation driven wind must be accounted for.
The fact of the matter is that there is still lack of consensus on their most
fundamental properties. For instance, the temperature scale of OB-stars has
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M. Garcı́a
changed dramatically over the past decade, following the improvement of the
codes that model their atmospheres. The inclusion of line blanketing (a sideeffect of metallic lines blocking the UV radiation) and mass loss has led to
cooler effective temperatures (compare e.g. the calibrations of [9] and [6]).
The wind parameters are no exception. Different approaches to determine the
mass loss rate, relying on wind effects at different regions of the expanding atmosphere, provide discrepant results for some targets. Additionally, stars with
thin winds do not follow the theoretical relationship between the momentum
of the wind and the stellar luminosity (see [2], [5] and [7]).
This work contributes to the on-going efforts to improve our knowledge
of hot massive stars with a spectroscopic study in the Far-UV–UV range
(∆λ ∼900-1800Å), wealthy in metallic spectral transitions formed in different
layers of the atmosphere. In Sect. 2 we describe briefly the Far-UV–UV range,
the analysis and the tool we used. In Sect. 3 we present the main results: a
new temperature scale for OB-stars and mass loss rate measurements. More
details about the sample of stars and the analytical procedure are provided
in [1], [3] and [4], where our results are discussed in depth.
2 THE FAR-UV–UV RANGE
The Far-UV–UV spectra of an O4 supergiant star is shown in Fig. 1. The
absorption bands in the FUSE range are caused by interstellar (atomic and
molecular) hydrogen and have been modeled to avoid confusion with stellar
features. The genuine stellar lines form in different parts of the atmosphere
and display a variety of profiles, including pure photospheric absorptions and
P Cygni profiles. We found that the variation of the UV and FUV spectral
morphology of O-stars correlates with their spectral classification, based on
the relative strength of optical lines.
We derive the stellar parameters of the target stars by fitting their FUSE
and IUE spectra with spherical, line blanketed, non-LTE models with mass
loss, calculated with WM-basic [8]. The followed strategy allowed us to determine consistently both the photospheric and the wind parameters of the analyzed stars for two reasons. Firstly, because the combined FUSE+IUE range
contains both photospheric and wind lines. Secondly, because WM-basic simulates the conditions in the atmosphere from the inner parts (in hydrostatic
equilibrium) to the outer layers (where the outflow reaches its maximum velocity). FUSE data played a crucial role in our analysis, by enlarging the
list of spectral transitions accessible to IUE or STIS/FOS. The most important feature in the FUSE range is O V I λ1031.9,1037.6, used to constrain the
amount of shock induced soft X-rays in the wind (log LX /Lbol ), responsible
for the production of such a high ionization state of oxygen (see Sect 2.1).
The analysis of the wealth of lines in the joined spectral range with WMbasic models allowed us to solve the wind ionization equilibrium, consistently
with the ionization caused by the soft X-rays.
Hot Massive Stars in the Far-UV and UV ranges
3
Fig. 1. Best fit model (green) to the FUSE and IUE spectra (black) of HD 190429A
(O4 If+ ). The interstellar hydrogen absorption model calculated for the line of sight
of this object (pink-dotted line) has been included.
2.1 The Grid of WM-basic Models
We developed a grid of WM-basic models to aid in the analysis of O- and
early-B Galactic stars. Upper and lower limits to the physical properties of
a given star are established by contrasting the synthetic Far-UV–UV spectra
and the observations. The stellar parameters are determined by subsequent
refinement of the grid within those limits, until a general good fit to the
observed spectrum is achieved.
We built a graphical scheme showing most of the grid models in the spectral transitions of interest, as the one shown in Fig. 2, and we learned how
their profile changes as a function of the stellar parameters that we let vary
across the lattice (the dependence is described extensively in [4]). Similar
graphics for the ionization fractions enabled its interpretation in terms of the
ionization equilibrium.
Most importantly, we found indicators for constraining the stellar parameters. For instance, aided by the grid we showed that the O V I λλ1031.9,1037.6
P Cygni profile observed in the FUV spectra of O-type stars is reproduced
only in models with shocks (see Fig. 2).
Other applications of the grid of WM-basic models include the calculation
of ionizing fluxes for the study of H II regions, the determination of the FarUV–UV extinction law towards an object or its use as input stellar library for
stellar population synthesis codes, such as STARBURST99.
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M. Garcı́a
Fig. 2. Variation of the O V I λλ1031.9,1037.6 doublet as a function of Tef f , log g,
R∗ , Ṁ and log LX /Lbol .
3 RESULTS
The most robust results obtained in this work are the effective temperature
of the stars, their mass loss rate and the parameter log LX /Lbol .
We obtained a new calibration of effective temperature with spectral type
for Galactic early- and mid-O type stars. The scale is consistently lower than
all existing calibrations, including those based on optical spectroscopic analyses using models that apply a physical treatment of a degree of sophistication
comparable to WM-basic, as shown in Fig. 3.
It is essential to ascertain whether our method produces consistently lower
effective temperatures in an extended sample of stars. It is also necessary to
assess how the adopted simplifications (the most retrictive being (i) considering only smooth unclumped winds and (ii) using solar abundances) can
Hot Massive Stars in the Far-UV and UV ranges
5
Fig. 3. The effective temperatures (left plot) and luminosities (right plot) derived
in this analysis (denoted with symbols, see figure for explanation) are plotted as a
function of their spectral type. For comparison, the empirical calibration of [6] is
included. The calibrations for supergiant and dwarf stars have been plotted as thick
and thin lines respectively. Our derived effective temperatures are consistently lower
than the calibration, for all spectral types and luminosity classes. The luminosities
derived for the stars are consequently lower than calibrated for their types (although
in agreement within the error bars for some cases).
account for the difference. If the Far-UV–UV lower temperature scale holds
after these tests, there are two possible explanations for the discrepancy. In
the first place, thanks to the variety of spectral transitions of different ionization stages of the same atom available in the merged FUSE+IUE spectra, the
ionization equilibrium of metals, which are responsible for line blocking and
line blanketing, is better constrained and it is constrained consistently with
the X-rays in the wind. In the second place, the wind layers probed by the
lines in the FUSE and IUE ranges, sampling wind layers at different depths,
may provide better constraints for the temperature structure in the wind (and
ultimately for the effective temperature).
Finally, we present our mass loss rate determinations for the sample stars
in Fig. 4. There is good agreement with the theoretical predictions made using
the parameterization of [10]. There is a small discrepancy between our values
and those determined from other methods, namely the analysis of Hα or the
radio continuum. The largest differences are found for dwarf stars. A possible
explanation is that the Far-UV and UV lines are more sensitive to Ṁ therefore
they may provide more accurate diagnostics for stars with weak winds.
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M. Garcı́a
Fig. 4. Comparison of the mass loss rate determinatined in this and previous works
(using different methods). Symbol shapes indicate different luminosity classes: dwarf
(cricles), giant (triangles) and supergiant (rhombs) stars. Different methods are represented by different colors as explained in the plot. The predictions were made with
the “recipe” of [10], using the photospheric parameters derived in this work.
References
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
Bianchi, L. & Garcia, M. 2002, ApJ, 581, 610
Bouret, J.-C., Lanz, T., Hillier, D. J., et al. 2003, ApJ, 595, 1182
Garcia, M. & Bianchi, L. 2004, ApJ, 606, 497
M. Garcia: A Study of Massive Stars from High Resolution Spectroscopy in the
Far-Ultraviolet, Ultraviolet and Optical Ranges. PhD Thesis, Universidad de
La Laguna, La Laguna (2005).
Martins, F., Schaerer, D., Hillier, D. J., & Heydari-Malayeri, M. 2004, A&A,
420, 1087
Martins, F., Schaerer, D., & Hillier, D. J. 2005, A&A, 436, 1049
Martins, F., Schaerer, D., Hillier, D. J., et al. 2005, A&A, 441, 735
Pauldrach, A. W. A., Hoffmann, T. L., & Lennon, M. 2001, A&A, 375, 161
Vacca, W. D., Garmany, C. D., & Shull, J. M. 1996, ApJ, 460, 914
Vink, J. S., de Koter, A., & Lamers, H. J. G. L. M. 2000, A&A, 362, 295