arXiv subjects

P. A. Crowther

Publications and source records attributed to P. A. Crowther.

122 records · Page 7Linked to original sources

arXiv2004

WR20a: a massive cornerstone binary system comprising two extreme early-type stars

We analyse spectroscopic observations of WR20a revealing that this star is a massive early-type binary system with a most probable orbital period of \sim 3.675 days. Our spectra indicate that both components are most likely of WN6ha or O3If^*/WN6ha spectral type. The orbital solution for a period of 3.675 days yields extremely large minimum masses of 70.7 \pm 4.0 and 68.8 \pm 3.8 M_{\odot} for the two stars. These properties make WR20a a cornerstone system for the study of massive star evolution.

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arXiv2004

Quantitative studies of the Far-UV, UV and optical spectra of late O and early B-type supergiants in the Magellanic Clouds

We present quantitative studies of 8 late O and early B-type supergiants in the Magellanic Clouds using far-ultraviolet FUSE, ultraviolet IUE/HST and optical VLT-UVES spectroscopy. Temperatures, mass-loss rates and CNO abundances are obtained using the non-LTE, spherical, line-blanketed model atmosphere code of Hillier & Miller (1998). We support recent results for lower temperatures of OB-type supergiants as a result of stellar winds and blanketing, which amounts to ~2000 K at B0 Ia. In general, H$α$ derived mass-loss rates are consistent with UV and far-UV spectroscopy, although from consideration of the SIV $λλ$1063-1073 doublet, clumped winds are preferred over homogenous models. AV 235 (B0 Iaw) is a notable exception, which has an unusually strong H$α$ profile that is inconsistent with the other Balmer lines and UV wind diagnostics. We also derive CNO abundances for our sample, revealing substantial nitrogen enrichment, with carbon and oxygen depletion. Our results are supported by comparison with the Galactic supergiant HD 2905 (BC0.7 Ia) for which near-solar CNO abundances are obtained. This bolsters previous suggestions that ``normal'' OB-type supergiants exhibit atmospheric compositions indicative of partical CNO processing.

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arXiv2003

Gemini observations of Wolf-Rayet Stars in the Local Group Starburst Galaxy IC10

We present Gemini-N GMOS and CFHT MOS spectroscopy of Wolf-Rayet candidates in the Local Group dwarf galaxy IC10 that were previously identified by Massey et al. and Royer et al. From the present spectroscopic survey, the WC/WN ratio for IC10 remains unusually high, given its low metallicity, although none of the WC9 stars suspected from narrow-band imaging are confirmed. Our spectroscopy confirms 9 newly discovered Wolf-Rayet candidates from Royer et al., whilst spectral types of 14 Wolf-Rayet stars previously observed by Massey & Armandroff are refined here. In total, there are 26 spectroscopically confirmed Wolf-Rayet stars in IC~10. All but one of the fourteen WC stars are WC4-6 stars, the exception being # 10 from Massey et al., a broad-lined, apparently single WC7 star. There are a total of eleven WN stars, which are predominantly early WN3-4 stars, but include a rare WN10 star, #8 from Royer et al. #5 from Massey et al. is newly identified as a transition WN/C star. Consequently, the WC/WN ratio for IC10 is 14/11=1.3, unusually high for a metal-poor galaxy. Re-evaluating recent photometric data of Massey & Holmes, we suggest that the true WC/WN ratio may not be as low as 0.3. Finally, we present ground-based finding charts for all confirmed WR stars, plus HST/WFPC2 charts for twelve cases.

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arXiv2002

A large Wolf-Rayet population in NGC300 uncovered by VLT-FORS2

We have detected 58 Wolf-Rayet candidates in the central region of the nearby spiral galaxy NGC 300, based on deep VLT-FORS2 narrow-band imaging. Our survey is close to complete except for heavily reddened WR stars. Of the objects in our list, 16 stars were already spectroscopically confirmed as WR stars by Schild & Testor and Breysacher et al., to which 4 stars are added using low resolution FORS2 datasets. The WR population of NGC300 now totals 60,a threefold increase over previous surveys, with WC/WN>1/3, in reasonable agreement with Local Group galaxies for a moderately sub-solar metallicity. We also discuss the WR surface density in the central region of NGC 300. Finally, analyses are presented for two apparently single WC stars - #29 (alias WR3, WC5) and #48 (alias WR13, WC4) located close to the nucleus, and at a deprojected radius of 2.5 kpc, respectively. These are among the first models of WR stars in galaxies beyond the Local Group, and are compared with early WC stars in our Galaxy and LMC.

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arXiv2002

Physical parameters of the high-mass X-ray binary 4U1700-37

We present the results of a detailed non-LTE analysis of the UV and optical spectrum of the O6.5Iaf+ star HD153919 - the mass donor in the high-mass X-ray binary 4U1700-37. Given the eclipsing nature of the system these results allow us to determine the most likely masses of both components of the binary via Monte Carlo simulations. These suggest a mass for HD153919 of 58+/-11M_sun - implying the initial mass of the companion was rather high (>60 M_sun). The most likely mass for the compact companion is found to be 2.44+/-0.27M_sun, with only 3.5 per cent of the trials resulting in a mass less than 2.0M_sun and none less than 1.65M_sun. Our observational data is inconsistent with the canonical neutron star mass and the lowest black hole mass observed (>4.4M_sun; Nova Vel). Significantly changing observational parameters can force the compact object mass into either of these regimes but this results in the O-star mass changing by factors of greater than 2, well beyond the limits determined from its evolutionary state and surface gravity. The low mass of the compact object implies that it is difficult to form high mass black holes through both the Case A & B mass transfer channels and, if the compact object is a neutron star, would significantly constrain the high density nuclear EoS

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arXiv2002

An Atlas of Galactic Spectra Observed with the Far Ultraviolet Spectroscopic Explorer

An atlas of far-ultraviolet spectra of 45 Galactic OB stars observed with the Far Ultraviolet Spectroscopic Explorer is presented. The atlas covers the wavelength region between 912 and 1185A with an effective spectral resolution of 0.12A. Systematic trends in the morphology and strength of stellar features are discussed. Particular attention is drawn to the variations of the CIII$λ$1176, SIV$λλ$1063, 1073, and PV$λλ$1118, 1128 line profiles as a function of temperature and luminosity class; and the lack of a luminosity dependence associated with OVI$λλ$1032, 1038. Numerous interstellar lines are also identified.

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arXiv2002

Revised Stellar Temperatures for Magellanic Cloud O Supergiants from FUSE and VLT-UVES Spectroscopy

We have undertaken quantitative analysis of four LMC and SMC O4--9.7 extreme supergiants using far-ultraviolet FUSE, ultraviolet IUE/HSTand optical VLT UVES spectroscopy. Extended, non-LTE model atmospheres that allow for the consistent treatment of line blanketing are used to analyse wind & photospheric spectral features simultaneously. Using Halpha to constrain dM/dt, HeI-II photospheric lines reveal T_eff's which are systematically (5-7.5kK) & substantially (15-20%) lower than previously derived from unblanketed, plane-parallel, non-LTE photospheric studies. We have confidence in these revisions, since derived T_eff's generally yield consistent fits across the entire 912-7000Ang observed spectral range. In particular, we are able to resolve previous UV-optical T_eff discrepancies. `Of' classification criteria are directly linked to (strong) nitrogen enrichment (via NIII 4097) & (weak) carbon depletion (via CIII 4647-51), providing evidence for mixing of unprocessed and CNO processed material at their stellar surfaces. Oxygen abundances are more difficult to constrain, except via OII lines in the O9.7 supergiant for which it is also found to be somewhat depleted. The effect of wind clumping is also investigated, for which PV 1118-28 potentially provides a useful diagnostic in O-star winds. Revised stellar properties affect existing calibrations of (i) Lyman continuum photons - a factor of two lower for the O4 supergiant; and (ii) kinetic energy released into the ISM by O supergiants. Our results also have importance for the calibration of the wind momentum-luminosity relationship for OB stars.

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arXiv2001

SwSt 1: an O-rich planetary nebula around a C-rich central star

The hydrogen-deficient carbon-rich [WCL] type central star HD167362 and its oxygen-rich planetary nebula (PN) SwSt~1 are investigated. The nebular chemistry might indicate a recent origin for the carbon-rich stellar spectrum. Its stellar and nebular properties might therefore provide further understanding of the origin of the [WCL] central star class. The UV-IR stellar spectra are modelled with state of the codes and show ~40kK central star with a wind and a C/O~3, indicative of efficient third dredge-up. The synthetic stellar flux distribution is used to model the high density, compact PN, which has a solar C/O ratio, is still enshrouded by 1200K and 230K dust shells and, reported here for the first time, in molecular hydrogen. Although it appears that the change in C/O ratio has been recent, the published spectroscopy since 1895 has been re-examined and no clear spectral change is seen. If an event occurred that has turned it into a hydrogen-deficient central star, it did not happen in the last 100 years.

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arXiv2001

Chemical abundances and winds of massive stars in M31: a B-type supergiant and a WC star in OB10

We present high quality spectroscopic data for two massive stars in the OB10 association of M31, OB10-64 (B0Ia) and OB10-WR1 (WC6). Medium resolution spectra of both stars were obtained using the ISIS spectrograph on the William Hershel Telescope. This is supplemented with HST-STIS UV spectroscopy and KeckI HIRES data for OB10-64. A non-LTE model atmosphere and abundance analysis for OB10-64 is presented indicating that this star has similar photospheric CNO, Mg and Si abundances as solar neighbourhood massive stars. A wind analysis of this early B-type supergiant reveals a mass-loss rate of M_dot=1.6x10^-6 M_solar/yr,and v_infty=1650 km/s. The corresponding wind momentum is in good agreement with the wind momentum -- luminosity relationship found for Galactic early B supergiants. Observations of OB10W-R1 are analysed using a non-LTE, line-blanketed code, to reveal approximate stellar parameters of log L/L_solar \~ 5.7, T~75 kK, v_infty ~ 3000 km/s, M_dot ~ 10^-4.3 M_solar/yr, adopting a clumped wind with a filling factor of 10%. Quantitative comparisons are made with the Galactic WC6 star HD92809 (WR23) revealing that OB10-WR1 is 0.4 dex more luminous, though it has a much lower C/He ratio (~0.1 versus 0.3 for HD92809). Our study represents the first detailed, chemical model atmosphere analysis for either a B-type supergiant or a WR star in Andromeda, and shows the potential of how such studies can provide new information on the chemical evolution of galaxies and the evolution of massive stars in the local Universe.

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arXiv2000

Physical parameters of erupting Luminous Blue Variables: NGC 2363-V1 caught in the act

A quantitative study of the Luminous Blue Variable NGC 2363-V1 in the Magellanic galaxy NGC 2366 (D = 3.44 Mpc) is presented, based on ultraviolet and optical HST/STIS spectroscopy. Contemporary WFPC2 and WHT imaging reveals a modest V-band brightness increase of ~ 0.2 mag per year between 1996 January and 1997 November, reaching V=17.4 mag, corresponding to Mv=-10.4 mag. Subsequently, V1 underwent a similar decrease in V-band brightness, together with a UV brightening of 0.35 mag from 1997 November to 1999 November. The optical spectrum of V1 is dominated by H emission lines, with Fe II, He I and Na I also detected. In the ultraviolet, a forest of Fe absorption features and numerous absorption lines typical of mid-B supergiants are observed. From a spectral analysis with the non-LTE, line-blanketed code of Hillier & Miller (1998), we derive stellar parameters of T*=11kK, R*=420Ro, log(L/Lo)=6.35 during 1997 November, and T*=13kK, R*=315Ro, log(L/Lo)=6.4 for 1999 July. The wind properties of V1 are also exceptional, with Mdot ~ 4.4 x 10e-4 Mo/yr and $v_{\infty} ~ $300 km/s, allowing for a clumped wind (filling factor = 0.3), and assuming H/He ~ 4 by number. The presence of Fe lines in the UV and optical spectrum of V1 permits an estimate of the heavy elemental abundance of NGC 2363 from our spectral synthesis. Although some deficiencies remain, allowance for charge exchange reactions in our calculations supports a SMC-like metallicity, that has previously been determined for NGC 2363 from nebular oxygen diagnostics. Considering a variety of possible progenitor stars, V1 has definitely undergone a giant eruption, with a substantial increase in stellar luminosity, radius, and almost certainly mass-loss rate.

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arXiv2000

The Detection of Wind Variability in Magellanic Cloud O Stars

We present Far Ultraviolet Explorer (FUSE) spectra for three Magellanic Cloud O stars (Sk 80, Sk -67 05 and Sk -67 111) with repeated observations. The data demonstrate the capabilities of FUSE to perform time-resolved spectroscopy on extragalactic stars. The wavelength coverage of FUSE provides access to resonance lines due to less abundant species, such as sulfur, which are unsaturated in O supergiants. This allows us to examine wind variability at all velocities in resonance lines for stars with higher mass loss rates than can be studied at longer (lambda > 1150 A) wavelengths. The FUSE wavelength range also includes resonance lines from ions which bracket the expected dominant ionization stage of the wind. Our observations span 1-4 months with several densely sampled intervals of 10 hours or more. These observations reveal wind variability in all of the program stars and distinctive differences in the ionization structure and time scales of the variability. Sk -67 111 demonstrates significant wind variability on a time scale less than 10 hours and the coolest O star (Sk -67 05) exhibits the largest variations in O VI.

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arXiv2000

FUSE Observations of the Stellar Winds of Two O7 Supergiants in the Magellanic Clouds

We compare the stellar wind features in far-UV spectra of Sk -67 111, an O7 Ib(f) star in the LMC, with Sk 80, an O7 Iaf+ star in the SMC. The most striking differences are that Sk 80 has a substantially lower terminal velocity, much weaker O VI absorption, and stronger S IV emission. We have used line-blanketed, hydrodynamic, non-LTE atmospheric models to explore the origin of these differences. The far-UV spectra require systematically lower stellar temperatures than previous determinations for O7 supergiants derived from plane-parallel, hydrostatic models of photospheric line profiles. At these temperatures, the O VI in Sk -67 111 must be due primarily to shocks in the wind.

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arXiv2000

The gamma Velorum binary system. II. WR stellar parameters and the photon loss mechanism

In this paper we derive stellar parameters for the Wolf-Rayet star in the gamma Velorum binary system (WR11), from a detailed non-LTE model of its optical and infrared spectra. Compared to the study of Schaerer et al., the parameters of the WC8 star are revised to a hotter effective temperature (Teff~57kK),a higher luminosity (log(L/Lsun = 5.00), and a lower mass loss rate (log(Mdot / Msun/yr) = -5.0 - using a 10% clumping filling factor). These changes lead to a significant decrease in wind efficiency number, from 144 to 7, so that the driving mechanism of the wind of this WR star may be simply radiation pressure on lines. The derived spectroscopic luminosity is found to be 40% lower than that derived by De Marco & Schmutz through the mass-luminosity relationship for WR stars (log(L/Lsun = 5.2). The paper furthermore presents a comparison of the independently-developed modelling programs, CMFGEN and ISA-Wind . Overall, there seems to be very reasonable agreement between the derived parameters for WR11, except for the carbon content, which is 2 times higher for CMFGEN (C/He=0.15 vs. 0.06, by number).The comparison also confirms a disparity in the predicted flux at lambda<$400A, found by Crowther et al., which will have effects on several nebular line strengths. The paper also presents the first independent check of the photon loss mechanism proposed by Schmutz. We conclude that, not only is it important to include very many lines to realistically model line blanketing, but in particular those ones that critically interact with strong resonance lines (e.g. HeII lambda303.78). The inclusion of these latter lines may significantly alter the wind ionization structure.

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arXiv1999

Wolf-Rayet nebulae as tracers of stellar ionizing fluxes: I. M1-67

We use WR124 (WN8h) and its associated nebula M1-67, to test theoretical non-LTE models for Wolf-Rayet (WR) stars. Lyman continuum ionizing flux distributions derived from a stellar analysis of WR124, are compared with nebular properties via photo-ionization modelling. Our study demonstrates the significant role that line blanketing plays in affecting the Lyman ionizing energy distribution of WR stars, of particular relevance to the study of HII regions containing young stellar populations. We confirm previous results that non-line blanketed WR energy distributions fail to explain the observed nebular properties of M1-67, such that the predicted ionizing spectrum is too hard. A line blanketed analysis of WR124 is carried out using the method of Hillier & Miller (1998), with stellar properties in accord with previous results, except that the inclusion of clumping in the stellar wind reduces its wind performance factor to only approx2. The ionizing spectrum of the line blanketed model is much softer than for a comparable temperature unblanketed case, such that negligible flux is emitted with energy above the HeI 504 edge. Photo-ionization modelling, incorporating the observed radial density distribution for M1-67 reveals excellent agreement with the observed nebular electron temperature, ionization balance and line strengths. An alternative stellar model of WR124 is calculated, following the technique of de Koter et al. (1997), augmented to include line blanketing following Schmutz et al. (1991). Good consistency is reached regarding the stellar properties of WR124, but agreement with the nebular properties of M1-67 is somewhat poorer than for the Hillier & Miller code.

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