D. J. Hillier

Quantitative Spectroscopy of O Stars at Low Metallicity: O Dwarfs in NGC 346

We present the results of a detailed analysis of the properties of dwarf O-type stars in a metal-poor environment. High-resolution, high-quality ultraviolet and optical spectra of six O-type stars in the H II region NGC 346 have been obtained from a spectroscopic survey of O stars in the SMC. Stellar parameters and chemical abundances have been determined using non-LTE (NLTE) line-blanketed photospheric models calculated with TLUSTY. Additionally, we have modeled the spectra with the NLTE line-blanketed wind code CMFGEN to derive wind parameters. Stellar parameters, chemical abundances, and in particular iron abundances obtained with the two NLTE codes compare quite favorably. This consistency demonstrates that basic photospheric parameters of main-sequence O stars can be reliably determined using NLTE static model atmospheres. With the two NLTE codes, we need to introduce a microturbulent velocity to match the observed spectra. Our results hint at a decrease of the required microturbulent velocity from a value close to the sonic velocity in early O stars to a low value in late O stars. As in several recent studies of Galactic, LMC, and SMC stars, we derive effective temperatures lower than predicted from the widely used relation between spectral type and Teff, resulting in lower stellar luminosities and lower ionizing fluxes. From evolutionary tracks in the H-R diagram, we find the age 3 × 106 yr for NGC 346. A majority of the stars in our sample reveal CNO cycle-processed material at their surface during the main-sequence stage, thus indicating fast stellar rotation and/or very efficient mixing processes. We obtain an overall metallicity Z = 0.2 Z☉, in good agreement with other recent analyses of SMC stars. We study the dependence of the mass-loss rate on the stellar metallicity and find a satisfactory agreement with recent theoretical predictions for the three most luminous stars of the sample. The wind momentum-luminosity relation for our sample stars derived for these stars agrees with previous studies. However, the three other stars of our sample reveal very weak signatures of mass loss. We obtain mass-loss rates that are significantly lower than 10-8 M☉ yr-1, below the predictions of radiative line-driven wind theory by an order of magnitude or more. Furthermore, evidence of clumping in the wind of main-sequence O stars is provided by O V λ1371. As in previous studies of O star winds, we are unable to reproduce this line with homogeneous-wind models, but we have achieved very good fits with clumped models. Clumped-wind models systematically yield lower mass-loss rates than theoretical predictions.

Lower mass loss rates in O-type stars : Spectral signatures of dense clumps in the wind of two Galactic O4 stars

We have analyzed the far-ultraviolet spectrum of two Galactic O4 stars, the O4If+ supergiant HD 190429A and the O4V((f)) dwarf HD 96715, using archival FUSE and IUE data. We have conducted a quantitative analysis using the two NLTE model atmosphere and wind codes,  and , which incorporate a detailed treatment of NLTE metal line blanketing. From the far-UV spectrum, we have derived the stellar and wind parameters and the surface composition of the two stars. The surface of HD 190429A has a composition typical of an evolved O supergiant (nitrogen-rich, carbon and oxygen- poor), while HD 96715 exhibits surface nitrogen enhancement similar to the enrichment found in SMC O dwarfs which has been attributed to rotationally-induced mixing. Following studies of Magellanic Cloud O stars, we find that homogeneous wind mod- els could not match the observed profile of O λ1371 and require very low phosphorus abundance to fit the P λλ1118−1128 resonance lines. We show, on the other hand, that we are able to match the O  and P  lines using clumped wind models. In addition to these lines, we find that N λ1718 is also sensitive to wind clumping. For both stars, we have calculated clumped wind models that match well all these lines from different species and that remain consistent with Hα data. In particular, we have achieved an excellent match of the P  resonance doublet, indicating that our physical description of clumping is adequate. These fits therefore provide a coherent and thus much stronger evidence of wind clumping in O stars than earlier claims. We show that the success of the clumped wind models in matching these lines results from increased recombination in the clumps, hence from a better description of the wind ionization structure. We find that the wind of these two stars is highly clumped, as expressed by very small volume filling factors, namely f∞ = 0.04 for HD 190429A and f∞ = 0.02 for HD 96715. In agreement with our analysis of SMC stars, clumping starts deep in the wind, just above the sonic point. The most crucial consequence of our analysis is that the mass loss rates of O stars need to be revised downward significantly, by a factor of 3 and more. These lower mass loss rates will affect substantially the evolution of massive stars. Accounting for wind clumping is essential when determining the wind properties of O stars. Our study therefore calls for a fundamental revision in our understanding of mass loss and of O-type star stellar winds.

O stars with weak winds: The Galactic case

We study the stellar and wind properties of a sample of Galactic O dwarfs to track the conditions under which weak winds (i.e mass loss rates lower than � 10 −8 M⊙ yr −1 ) appear. The sample is composed of low and high luminosity dwarfs including Vz stars and stars known to display qualitatively weak winds. Atmosphere models including non-LTE treatment, spherical expansion and line blanketing are computed with the code CMFGEN (Hillier & Miller 1998). Both UV and Hlines are used to derive wind properties while optical H and He lines give the stellar parameters. We find that the stars of our sample are usually 1 to 4 Myr old. Mass loss rates of all stars are found to be lower than expected from the hydrodynamical predictions of Vink et al. (2001). For stars with log L L⊙ > 5.2, the reduction is by less than a factor 5 and is mainly due to the inclusion of clumping in the models. For stars with log L L⊙ < 5.2 the reduction can be as high as a factor 100. The inclusion of X-ray emission (possibly due to magnetic mechanisms) in models with low density is crucial to derive accurate mass loss rates from UV lines, while it is found to be unimportant for high density winds. The modified wind momentum - luminosity relation shows a significant change of slope around this transition luminosity. Terminal velocities of low luminosity stars are also found to be low. Both mass loss rates and terminal velocities of low L stars are consistent with a reduced line force parameter �. However, the physical reason for such a reduction is still not clear although the finding of weak winds in Galactic stars excludes the role of a reduced metallicity. There may be a link between an early evolutionary state and a weak wind, but this has to be confirmed by further studies of Vz stars. X-rays, through the change in the ionisation structure they imply, may be at the origin of a reduction of the radiative acceleration, leading to lower mass loss rates. A better understanding of the origin of X-rays is of crucial importance for the study of the physics of weak winds

ON THE NATURE OF THE PROTOTYPE LUMINOUS BLUE VARIABLE AG CARINAE. I. FUNDAMENTAL PARAMETERS DURING VISUAL MINIMUM PHASES AND CHANGES IN THE BOLOMETRIC LUMINOSITY DURING THE S-Dor CYCLE

We present a detailed spectroscopic analysis of the luminous blue variable AG Carinae during the last two visual minimum phases of its S-Dor cycle (1985-1990 and 2000-2003). The analysis reveals an overabundance of He, N, and Na, and a depletion of H, C, and O, on the surface of AG Car, indicating the presence of CNO-processed material. Furthermore, the ratio N/O is higher on the stellar surface than in the nebula. We found that the minimum phases of AG Car are not equal to each other, since we derived a noticeable difference between the maximum effective temperature achieved during 1985-1990 (22,800 K) and 2000-2001 (17,000 K). While the wind terminal velocity was 300 km/s in 1985-1990, it was as low as 105 km/s in 2001. The mass-loss rate, however, was lower from 1985-1990 (1.5 x 10^(-5) Msun/yr) than from 2000-2001 (3.7 x 10^(-5) Msun/yr). We found that the wind of AG Car is significantly clumped (f=0.10 - 0.25) and that clumps must be formed deep in the wind. We derived a bolometric luminosity of 1.5 x 10^6 Lsun during both minimum phases which, contrary to the common assumption, decreases to 1.0 x 10^6 Lsun as the star moves towards maximum flux in the V band. Assuming that the decrease in the bolometric luminosity of AG Car is due to the energy used to expand the outer layers of the star (Lamers 1995), we found that the expanding layers contain roughly 0.6 - 2 Msun. Such an amount of mass is an order of magnitude lower than the nebular mass around AG Car, but is comparable to the nebular mass found around lower-luminosity LBVs and to that of the Little Homunculus of Eta Car. If such a large amount of mass is indeed involved in the S Dor-type variability, we speculate that such instability could be a failed Giant Eruption, with several solar masses never becoming unbound from the star.(abridged)

The periodicity of the η Carinae events

Extensive spectral observations of η Carinae over the last cycle, and particularly around the 2003.5 low-excitation event, have been obtained. The variability of both narrow and broad lines, when combined with data taken from two earlier cycles, reveal a common and well-defined period. We have combined the cycle lengths derived from the many lines in the optical spectrum with those from broad-band X-rays, optical and near-infrared observations, and obtained a period length of P pres = 2022.7 ± 1.3 d. Spectroscopic data collected during the last 60 yr yield an average period of P avg = 2020 ± 4 d, consistent with the present-day period. The period cannot have changed by more than AP/P = 0.0007 since 1948. This confirms the previous claims of a true, stable periodicity, and gives strong support to the binary scenario. We have used the disappearance of the narrow component of He I 6678 to define the epoch of the Cycle 11 minimum, To = JD 245 2819.8. The next event is predicted to occur on 2009 January 11 (±2 d). The dates for the start of the minimum in other spectral features and broad-bands are very close to this date, and have well-determined time-delays from the He I epoch.

Quantitative spectroscopy of photospheric-phase type II supernovae

We present first results on the quantitative spectroscopic analysis of the photospheric-phase of type II supernovae (SN). The analyses are based on the model atmosphere code, CMFGEN, of Hillier & Miller (1998) which solves the radiative transfer and statistical equilibrium equations in expanding outflows under the constraint of radiative equilibrium. A key asset of CMFGEN is its thorough treatment of line-blanketing due to metal species. From its applicability to hot star environments, the main modifications to the source code were to allow a linear velocity law, a power-law density distribution, an adaptive grid to handle the steep H recombination/ionization front occurring in some SN models, and a routine to compute the gray temperature structure in the presence of large velocities. In this first paper we demonstrate the ability of CMFGEN to reproduce, with a high level of accuracy, the UV and optical observations of a sample of well observed type II SN, i.e. SN1987A and SN1999em, at representative stages of their photospheric evolution. Two principal stages of SN are modeled - that where hydrogen is fully ionized, and that in which H is only partially ionized. For models with an effective temperature below ∼8000 K, hydrogen recombines and gives rise to a steep ionization front. The effect of varying the location of the outer grid radius on the spectral energy distribution (SED) is investigated. We find that going to 5−6 times the optically-thick base radius is optimal, since above that, the model becomes prohibitively large, while below this, significant differences appear because of the reduced line-blanketing (which persists even far above the photosphere) and the truncation of line-formation regions. To constrain the metallicity and the reddening of SN, the UV spectral region of early-time spectra is essential. We find that the density of the photosphere and effect of line blanketing decline as the spatial scale of the SN increases. The density distribution is found to have a strong impact on the overall flux distribution as well as line profiles. For a given base density, the faster the density drops, the higher the effective temperature of the model. We also find in cool models that the set of Ca  lines, near 8500 A is strongly sensitive to the density gradient. They show a weaker and narrower profile for steeper density distributions. Hydrogen Balmer lines are very well reproduced in fully or partially ionized models, but underestimated when hydrogen recombines. A reduced turbulent velocity or a flatter density layout are found to partially, but not fully, cure this persistent problem in studies of type II SN. He  lines observed in early-time spectra are very well reproduced, even for very modest helium enrichments, likely resulting from treatment of important non-LTE effects. At similar early epochs CMFGEN predicts, unambiguously, the presence of N  lines in the blue-wing of both Hβ and He  5875 A. These lines have been observed but so far have generally been associated with peculiar emission, from locations far above the photosphere, in the strong adjacent lines. Finally, we present a pedagogical investigation on P-Cygni profile formation in type II SN. Hα is found to form very close to the photosphere and thus presents a significant flux-deficit in the red, made greater by the rapidly declining density distribution. This provides a clear explanation for the noticeable blue-shift of P-Cygni profiles observed in early-time spectra of type II SN. Future studies based on CMFGEN modeling will focus on using type II SN for the calibration of distances in the Universe, as well as on detailed spectroscopic analyses for the determination of progenitor properties.

Properties of Galactic early-type O-supergiants - A combined FUV-UV and optical analysis

We aim to constrain the properties and evolutionary status of early and mid-spectral type supergiants (from O4 to O7.5). These posses the highest mass-loss rates among the O stars, and exhibit conspicuous wind profiles. Using the non-LTE wind code CMFGEN, we simultaneously analyzed the FUV-UV and optical spectral range to determine the photospheric properties and wind parameters. We derived effective temperatures, luminosities, surface gravities, surface abundances, mass-loss rates, wind terminal velocities, and clumping filling factors. The supergiants define a very clear evolutionary sequence, in terms of ages and masses, from younger and more massive stars to older stars with lower initial masses. O4 supergiants cluster around the 3 Myr isochrone and are more massive than 60 Msun, while the O5 to O7.5 stars have masses in the range 50 - 40 Msun and are 4 +/- 0.3 Myr old. The surface chemical composition is typical of evolved O supergiants (nitrogen-rich, carbon- and oxygen-poor). While the observed ranges of carbon and nitrogen mass-fractions are compatible with those expected from evolutionary models for the measured stellar masses, the N/C ratios as a function of age are inconsistent with the theoretical predictions for the four earliest (O4 spectral type) stars of the sample. We question the efficiency of rotational mixing as a function of age for these stars and suggest that another mechanism may be needed to explain the observed abundance patterns. Mass-loss rates derived with clumped-models range within a factor of three of the theoretical mass-loss rates. The corresponding volume-filling factors associated with small-scale clumping are 0.05 +/- 0.02. Clumping is found to start close to the photosphere for all but three stars, two of which are fast rotators.

η Carinae across the 2003.5 minimum: Spectroscopic evidence for massive binary interactions

We have analyzed high spatial, moderate spectral resolution observations of η Carinae (η Car) obtained with the STIS from 1998.0 to 2004.3. The data were obtained at discrete times covering an entire 2024 day spectroscopic cycle, with focus on the X-ray/ionization low state that began in 2003 June. The spectra show prominent P Cygni lines in H I, Fe II, and He I, which are complicated by blends and contamination by nebular emission and absorption. All lines show phase- and species-dependent variations in emission and absorption. For most of the cycle the He I emission is blueshifted relative to the H I and Fe II P Cygni emission lines, which are centered at approximately system velocity. The blueshifted He I absorption components vary in intensity and velocity throughout the 2024 day period. We construct radial velocity curves for the absorption component of the He I and H I lines. The He I absorption shows significant radial velocity variations throughout the cycle, with a rapid change of over 200 km s-1 near the 2003.5 event. The H I velocity curve is similar to that of the He I absorption, although offset in phase and reduced in amplitude. We interpret the complex line profile variations in He I, H I, and Fe II to be a consequence of the dynamic interaction of the dense wind of η Car A with the less dense, faster wind plus the radiation field of a hot companion star, η Car B. We use the variations seen in He I and the other P Cygni lines to constrain the geometry of the orbit and the character of η Car B.

Analysis of Galactic late-type O dwarfs: more constraints on the weak wind problem

Aims. We investigate the stellar and wind properties of a sample of late-type O dwarfs. Previous analyses of such stars have found very low mass-loss rates; rates much lower than predicted by theory (the weak wind problem). Methods. Far-UV to optical spectra of five Galactic O stars were analyzed: HD 216898 (O9IV/O8.5V), HD 326329 (O9V), HD 66788 (O8V/O9V), ζ Oph (O9.5Vnn), and HD 216532 (O8.5V((n))). We used a grid of TLUSTY models to obtain effective temperatures, gravities, rotational velocities, and to identify wind lines. Wind parameters for each object were obtained using expanding atmosphere models calculated with the CMFGEN code. Results. The spectra of our sample have primarily a photospheric origin. A weak wind signature is seen in C iv λλ1548, 1551, from which mass-loss rates consistent with previous CMFGEN results for O8-O9V stars were derived (∼10 −10 −10 −9 Myr −1 ). A discrepancy of roughly two orders of magnitude is found between these mass-loss rates and the values predicted by theory ( u MVink), confirming a breakdown or a steepening of the modified wind momentum-luminosity relation at log L� /L� −3.5). The validity of the models used in our analyses is discussed.

On the He II Emission in η Carinae and the Origin of Its Spectroscopic Events

We describe and analyze Hubble Space Telescope (HST) observations of transient emission near 4680 8 inCar, reportedearlierbySteiner&Damineli.If,asseemsprobable,thisisHeiik4687,thenitisauniquecluetoCars5.5yr cycle. According to our analysis, several aspects of this feature support a mass-ejection model of the observed spectro- scopicevents,andnotaneclipsemodel.TheHeiiemissionappearedinearly2003,grewtoabriefmaximumduringthe 2003.5spectroscopicevent,andthenabruptlydisappeared.ItdidnotappearinanyotherHSTspectrabeforeorafter the event. The peak brightness was larger than previously reported and is difficult to explain even if one allows for an un- certaintyfactoroforder3.Thestellarwindmustprovideatemporarylarger-than-normalenergysupply,andwedescribe a special form of radiative amplification that may also be needed. These characteristics are consistent with a class of mass-ejectionorwind-disturbancescenarios,whichhaveimplicationsforthephysicalstructureandstabilityofCar. Subject headingg binaries: general — line: profiles — stars: individual (� Car) — stars: variables: other — stars: winds, outflows