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Hot Massive Stars: The Impact of HST

We review the contribution of Hubble Space Telescope to the study of hot, luminous stars. Optical and IR imaging have permitted spatially resolved observations of young, massive clusters within Local Group galaxies, such as R136, NGC 3603 and Arches, revealing unprecedented concentrations of very massive O stars. UV spectroscopy of field OB stars in the Magellanic Clouds have provided suitable templates for interpretation of metal-poor star-forming galaxies at high-redshift. Spectacular imaging provides the detailed structure of ejecta nebulae from individual stars, including the Homunculus associated with eta Carinae and M1-67, associated with a Wolf-Rayet star. HST has permitted individual massive stars to be spatially resolved in giant HII regions located beyond the Local Group, such as NGC 604, plus individual clusters, dominated by the light of massive stars within starburst galaxies at larger distances, such as NGC 3125. UV spectroscopy of young, massive clusters in the extremely metal-poor HII galaxy I Zw 18 include signatures of large numbers of Wolf-Rayet stars.


Introduction
Massive stars distinguish themselves from their lower mass siblings by their exceptionally high main-sequence luminosities, such that their lifetimes are measured in Myr rather than Gyr. Individual stars may be readily studied in detail in external galaxies -it was not be accident that the first post-servicing mission WFPC2 image was obtained of the Wolf-Rayet star Melnick 34 in the 30 Doradus star-forming region of the LMC (News Release: STScI-1994-05). Here, selected HST results are discussed, together with their impact upon European astronomy.

Stellar Winds -Metallicity dependent winds
The first balloon and satellite missions provided the means of studying winds from hot stars, via ultraviolet P Cygni profiles, for which IUE provided a comprehensive sample of Milky Way OB stars (e.g. Howarth & Prinja 1989). Comparable quality spectroscopy of OB stars in the metal-poor Magellanic Clouds required the superior throughput and spatial resolution of HST , Evans et al. 2004).
Winds of early-type stars are predicted to be driven by radiation pressure through (CNO, Fe-peak) metal-lines (e.g. Vink et al. 2001), from which metalpoor stars are expected to possess lower density, slower winds. Indeed, wind velocities of early O stars in the SMC were established to be slower than those of comparable Galactic stars (e.g. Prinja & Crowther 1998), from CIV 1550 observations, although it has taken large ground-based surveys, such as the VLT/FLAMES survey of massive stars in Milky Way, LMC and SMC clusters to quantify the metallicity-dependence of mass-loss rates, revealing dM/dt ∝ Z 0.78±0.17 (Mokiem et al. 2007), in good agreement with theory Z 0.69±0.10 according to Vink et al. (2001).

Ejecta nebulae -Signatures of mass ejections
Amongst the many images obtained with HST, one of the most breathtaking has been the WFPC2 images of Morse et al. (1998) of the Homunculus reflection nebula, produced by the 'eruption' of the prototype Luminous Blue Variable (LBV) η Carinae during the mid-19th Century, and now illuminated from within. The expanding emission lobes extend 8.5 ′′ (0.1 pc) from the central star, now known to be a 5.5 yr period binary system, for which WFPC2 achieved a spatial resolution of ∼115 AU, comparable with the size of our Solar System. A physical mechanism for the eruption, in which in excess of 10M ⊙ were ejected over 20 years, remains unclear. STIS long-slit spectroscopy of the central star, reveals exceptional properties, with current mass-loss rates of 10 −3 M ⊙ yr −1 for an adopted (infrared) luminosity of 5 × 10 6 L ⊙ (Hillier et al. 2001).
WFPC2 has also provided an unprecedented view of the young ejecta (or ring) nebula M1-67 associated with the Galactic Wolf-Rayet star WR124 (Grosdidier et al. 1998). The radial density distribution of this nebula, approximately 90 ′′ (∼2 pc) in diameter with a r −0.7 dependence, has enabled photo-ionization modelling of the central WR star (Crowther et al. 1999) and may represent the immediate environment into which cosmological Gamma-Ray Bursts (GRBs) explode. M1-67 is compared to η Carinae on a common physical scale in Figure 1.

Massive binaries -Colliding winds
High spatial resolution radio surveys of massive binaries reveal thermal (stellar wind) and non-thermal (colliding wind) components (e.g. Williams et al. 1997). Positions of stars within such systems have been established with WFPC2 imaging, enabling their relative wind strengths to be established (Niemela et al. 1998).
Indeed, FGS has enabled searches for hitherto unknown massive binaries in regions of parameter space inaccessible to ground-based techniques. One To date, the current heavyweight record holder is WR20a, a 3.7 day eclipsing binary system composed of two H-rich WN-type stars each of ∼ 82M ⊙ (Rauw et al. 2005).  HST has also spatially resolved the Milky Way cluster NGC 3603, again revealing many early O stars (Drissen et al. 1995). The central cluster is comparable to R136a within a radius of ∼1pc (Crowther & Dessart 1998). Elsewhere in the Milky Way, NICMOS has been used to spatially resolve the compact (<0.5 pc) Arches cluster close to the Galactic centre. The Arches cluster is apparently sufficiently young that the present-day mass function approximates the true IMF, from which an upper stellar mass limit of ∼150 M ⊙ has been proposed (Figer 2005).

Young star clusters -A plethora of hot stars
At present, the most massive young Milky Way cluster known is Westerlund 1 (Wd 1), in which ∼ 5×10 4 M ⊙ are contained within a radius of ∼1.5 pc (Clark et al. 2005). This cluster hosts a full menagerie of rare, massive stars, Such high mass, compact clusters are unusual for normal spiral galaxies. Large, scaled-up OB associations are more typical, for which WFPC2 imaging provided an excellent example of NGC 604 in M33 (see Yang et al. 1996), several hundred pc in diameter, containing the equivalent of several hundred O7 V stars. This is compared to ACS imaging of NGC 346 in the SMC (Nota et al. 2006) in Fig. 3, somewhat smaller and with the equivalent of 50 equivalent O7 V stars (Kennicutt 1984). The high spatial resolution achieved with WFPC2 permitted Drissen et al. (1993) to identify individual Of and Wolf-Rayet emission line stars in NGC 604 to be established. A similar ground-based study of Wolf-Rayet stars within a bright giant HII region in the southern barred spiral galaxy NGC 1313 by Hadfield & Crowther (2007), comparable in size and ionizing output to NGC 604 demonstrates the invaluable role of ACS in disentangling the stellar content. Field Wolf-Rayet stars at the 4 Mpc distance of NGC 1313 can be detected from ground-based telescopes, but HST has proved decisive in detection core-collapse supernova progenitors from pre-SN imaging (e.g. Smartt et al. 2004). 6 Starburst knots -Templates for high-z galaxies Local starbursts, such as NGC 3125 (11 Mpc) host young massive clusters, significantly more massive than R136, as presented in Figure 4. Starbursts from such galaxies provide an order of magnitude higher ionizing output than NGC 604 from within a much smaller region, e.g. the equivalent of 2,500 equivalent O7 V stars from knot A of NGC 3125. STIS ultraviolet spectroscopy of individual clusters enables robust age estimates of such clusters, based on comparisons between the OB P Cygni features and spectral synthesis predictions (e.g. Chandar et al. 2004), as shown in Figure 5 for NGC 3125-A1 from . Host galaxies typically possess Magellanic Cloud metallicities, such that the use of template LMC/SMC OB stars (recall Sect.2) has proved to be invaluable. In extreme cases, far lower metallicities are sampled, such as the HII galaxy I Zw 18, possessing 1/30 Z ⊙ based upon the recently reduced Solar oxygen abundance. Brown et al. (2002) undertook STIS ultraviolet spectroscopy across I Zw 18, revealing multiple clusters in which carbon-sequence Wolf-Rayet stars were observed. Such prominent WR signatures at low metallicity led  to conclude that I Zw 18 hosts substantial WR populations, wholly unexpected for single star evolutionary models at such low metallicity.
The composite rest-frame UV spectra of z∼3 Lyman break galaxies (LBG) ) also includes spectral signatures of O and WR stars. Amongst the brightest (lensed) LBGs, MS1512-cB58 (z∼2.7) has been observed at rest-frame UV wavelengths with Keck, with which Pettini et al. (2002) analysed the interstellar lines, suggesting a Magellanic Cloud metallicity. Indeed, Pettini et al. (2003) could reproduce its stellar CIV 1550 feature using Magellanic Cloud template OB stars, where they had previously been unsuccessful using Milky Way template stars.

Future Issues?
Perhaps the most significant unresolved aspect relating to high mass stars is their formation, for which future high-spatial resolution infrared and radio may prove decisive. Violent mass ejections, such as that experienced by η Carinae, producing the Homunculus nebula, also remains ill-explained. A further puzzle is how large numbers of Wolf-Rayet stars form in young, massive clusters within very metal-poor galaxies such as I Zw 18.