An ultraviolet spectral library of metal-poor OB stars

1. An ultraviolet spectral library of metal-poor OB stars C. J. Evans1, D. J. Lennon1, N. R. Walborn2, C. Trundle1,3, S. A. Rix1 1) Isaac Newton Group, La Palma; 2) STScI, Baltimore; 3) Queen’s University, Belfast Abstract We present new HST-STIS spectroscopy of 12 B-type stars in the Small Magellanic Cloud (SMC). The spectra are distinctly different to Galactic analogues because of the SMC metal deficiency, illustrated by a morphological comparison with archival IUE data. The intensity of the P Cygni emission in the UV resonance lines is greater, and seen to later spectral types, in the Galactic spectra than in their SMC counterparts. We attribute these effects as arising from weaker stellar winds in the SMC targets. Combined with previous HST observations of O stars, we now have metal-poor template spectra for a large part of the upper H-R diagram, of use in interpreting the stellar populations in starbursts and high redshift systems. Introduction UV observations of individual stars in the metal-deficient SMC (Z ~ 0.2Zsun) have many uses in astrophysics. They enable direct comparisons with the output of model atmosphere codes, testing our understanding of both stellar physics and stellar evolution, and their dependence on metallicity. They also offer template spectra for the creation of spectral libraries, useful when trying to disentangle the stellar populations in unresolved starbursts (observed in the UV) or in high redshift, star-forming galaxies (in which the stellar UV features are shifted in the optical). UV iron indices The SMC and Galactic spectra neatly illustrate the change in the dominant ionization stage of iron from Fe3+ at B0, to Fe2+ at B1, suggested by Vink et al. (2001, A&A, 369, 574) as a physical explanation for the so-called ‘bi-stability effect’. These spectra also let us calibrate the new 1978 iron index, advanced by Rix et al. (2004, astro-ph/0407296) as a probe of metallicity in distant (z = 1-3) star-forming galaxies. Pending new high-quality observations in this region at other metallicities, our results (tabulated in our accepted PASP paper) will permit further tests of the latest generation of model atmosphere codes – if sensible agreement is found then we can be more confident of including theoretical stellar spectra into population synthesis codes. SMC: HST Milky Way: IUE UV observations in the SMC The combined OB spectral library Together with our previous HST O star program, our UV sample of massive stars in the SMC now covers a significant part of the the upper H-R diagram, giving an unprecedented library of high resolution, metal-poor spectra. The sample is shown in the H-R diagram below. The O-type spectra have already been incorporated into STARBURST99 by Leitherer et al. (2001, ApJ, 550, 724) and work is now underway to include the B star data. By virtue of the large number of varied sightlines these spectra also provide a wealth of interstellar information, upon which we are currently focusing our efforts. (Malin/AAO) Useful in stellar analyses or as templates for starbursts & distant blue galaxies Class Ia Galactic analogues from IUE Class Ia SMC spectra Evans et al. (2004, ApJ, 607,451) NGC1705 (Hubble Heritage/M. Tosi) MS 1512-cB58 (ESO) H-R diagram of the combined HST OB-type sample • HST observations • 12 B-type stars in the SMC, comprising 9 supergiants and 3 giants, were observed with the Space Telescope Imaging Spectrograph (STIS) using two settings: • The E140M grating, λmid = 1425Å (R~45,000). • The E230M grating, λmid = 1978Å (R~30,000). • The combined spectra span 1150-2350Å, covering the principal stellar wind lines and the Fe IV and Fe III ‘forests’. Less luminous SMC stars Less luminous Galactic stars • Qualitative evidence of reduced mass-loss rates at low metallicity • The new B-type HST spectra are shown above, compared with Galactic analogues from the IUE archive. In the most luminous stars (upper panels) there are two trends as one moves from B0 to B5: • The P Cygni emission is generally weaker in the SMC targets than in their Galactic counterparts. • Wind signatures are seen at later spectral types in the Galaxy (B3 Ia) than in the SMC (B2 Ia). • Such behaviour was noted in the past (e.g., Hutchings, 1982, ApJ, 255, 70) but was explained in terms of simple abundance effects. However, we attribute these differences to weaker stellar winds in the SMC targets - the lines are saturated enough that, even over the large range of SMC to solar metallicity, the formation of the profiles is dominated by the stellar wind rather than by abundance effects. The same trends are also seen in the class Ib spectra (lower panels) in which weak P Cygni emission is seen in the C IV lines at B3 and B4 in the Galaxy, but not in the SMC. This work is largely from a paper accepted by PASP, for preprints contact cje@ing.iac.es. CJE, DJL & SAR acknowledge support from PPARC. Where available, published physical parameters have been used (solid circles), else they are interpolated from other relevant results (open circles).Evolutionary tracks (from Charbonnel et al., 1993, A&AS, 101, 415) are marked by initial stellar masses (M/Msun) .