arXiv subjects

Paul A. Crowther

Publications and source records attributed to Paul A. Crowther.

At least 55 records · Page 3Linked to original sources

arXiv2012

On the association between core-collapse supernovae and HII regions

Previous studies of the location of core-collapse supernovae (ccSNe) in their host galaxies have variously claimed an association with HII regions; no association; or an association only with hydrogen-deficient ccSNe. Here, we examine the immediate environments of 39 ccSNe whose positions are well known in nearby (<15 Mpc), low inclination (<65 degrees) hosts using mostly archival, continuum-subtracted H-alpha ground-based imaging. We find that 11 out of 29 hydrogen-rich ccSNe are spatially associated with HII regions (38 +/- 11%), versus 7 out of 10 hydrogen-poor ccSNe (70 +/- 26%). Similar results from Anderson et al. led to an interpretation that the progenitors of type Ib/c ccSNe are more massive than those of type II ccSNe. Here, we quantify the luminosities of HII region either coincident with, or nearby to the ccSNe. Characteristic nebulae are long-lived (~20 Myr) giant HII regions rather than short-lived (~4 Myr) isolated, compact HII regions. Therefore, the absence of a HII region from most type II ccSNe merely reflects the longer lifetime of stars with <12 Msun than giant HII regions. Conversely, the association of a HII region with most type Ib/c ccSNe is due to the shorter lifetime of stars with >12 Msun stars than the duty cycle of giant HII regions. Therefore, we conclude that the observed association between certain ccSNe and HII provides only weak constraints upon their progenitor masses. Nevertheless, we do favour lower mass progenitors for two type Ib/c ccSNe that lackassociated nebular emission, a host cluster or a nearby giant HII region. Finally, we also reconsider the association between long Gamma Ray Bursts and the peak continuum light from their (mostly) dwarf hosts, and conclude that this is suggestive of very high mass progenitors, in common with previous studies.

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arXiv2012

The VLT-FLAMES Tarantula Survey. IX. The interstellar medium seen through Diffuse Interstellar Bands and neutral sodium

The Tarantula Nebula (30 Dor) is a spectacular star-forming region in the Large Magellanic Cloud, seen through gas in the Galactic Disc and Halo. Diffuse Interstellar Bands offer a unique probe of the diffuse, cool-warm gas in these regions. The aim is to use DIBs as diagnostics of the local interstellar conditions, whilst at the same time deriving properties of the yet-unknown carriers. Spectra of over 800 early-type stars from the VLT Flames Tarantula Survey (VFTS) were analysed. Maps were created, separately, for the Galactic and LMC absorption in the DIBs at 4428 and 6614 Ang and - in a smaller region near the central cluster R136 - neutral sodium (Na I D); we also measured the DIBs at 5780 and 5797 Ang. The maps show strong 4428 and 6614 Ang DIBs in the quiescent cloud complex to the south of 30 Dor but weak absorption in the harsher environments to the north (bubbles) and near the OB associations. The Na maps show at least five kinematic components in the LMC and a shell-like structure surrounding R136, and small-scale structure in the Milky Way. The strengths of the 4428, 5780, 5797 and 6614 Ang DIBs are correlated, also with Na absorption and visual extinction. The strong 4428 Ang DIB is present already at low Na column density but the 6614, 5780 and 5797 Ang DIBs start to be detectable at subsequently larger Na column densities. The relative strength of the 5780 and 5797 Ang DIBs clearly confirm the Tarantula Nebula and Galactic high-latitude gas to represent a harsh radiation environment. The resilience of the 4428 Ang DIB suggests its carrier is large, compact and neutral. Structure is detected in the distribution of cool-warm gas on scales between one and >100 pc in the LMC and as little as 0.01 pc in the Sun's vicinity. Stellar winds from the central cluster R136 have created an expanding shell; some infalling gas is also detected, reminiscent of a galactic "fountain".

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arXiv2012

IC4663: the first unambiguous [WN] Wolf-Rayet central star of a planetary nebula

Several [WC]-type central stars of planetary nebulae (PNe) are known to mimic the spectroscopic appearance of massive carbon-rich or WC-type Wolf-Rayet stars. In stark contrast, no [WN]-type central stars have yet been identified as clear-cut analogues of the common nitrogen-rich or WN-type Wolf-Rayet stars. We have identified the [WN3] central star of IC4663 to be the first unambiguous example in PNe. The low luminosity nucleus and an asymptotic giant branch (AGB) halo surrounding the main nebula prove the bona-fide PN nature of IC4663. Model atmosphere analysis reveals the [WN3] star to have an exotic chemical composition of helium (95%), hydrogen (<2%), nitrogen (0.8%), neon (0.2%) and oxygen (0.05%) by mass. Such an extreme helium-dominated composition cannot be predicted by current evolutionary scenarios for hydrogen deficient [WC]-type central stars. Only with the discovery of IC4663 and its unusual composition can we now connect [WN] central stars to the O(He) central stars in a second H-deficient and He-rich evolutionary sequence, [WN]->O(He), that exists in parallel to the carbon-rich [WC]->PG1159 sequence. This suggests a simpler mechanism, perhaps a binary merger, can better explain H-deficiency in PNe and potentially other H-deficient/He-rich stars. In this respect IC4663 is the best supported case for a possible merged binary central star of a PN.

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arXiv2012

Very Massive Stars and the Eddington Limit

We use contemporary evolutionary models for Very Massive Stars (VMS) to assess whether the Eddington limit constrains the upper stellar mass limit. We also consider the interplay between mass and age for the wind properties and spectral morphology of VMS, with reference to the recently modified classification scheme for O2-3.5If*/WN stars. Finally, the death of VMS in the local universe is considered in the context of pair instability supernovae.

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arXiv2012

Environments of massive stars and the upper mass limit

The locations of massive stars (> 8 Msun) within their host galaxies is reviewed. These range from distributed OB associations to dense star clusters within giant HII regions. A comparison between massive stars and the environments of core-collapse supernovae and long duration Gamma Ray Bursts is made, both at low and high redshift. We also address the question of the upper stellar mass limit, since very massive stars (VMS, Minit >> 100 Msun) may produce exceptionally bright core-collapse supernovae or pair instability supernovae.

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arXiv2011

The G305 star-forming complex: the central star clusters Danks 1 and Danks 2

The G305 HII complex (G305.4+0.1) is one of the most massive star forming structures yet identified within the Galaxy. It is host to many massive stars at all stages of formation and evolution, from embedded molecular cores to post main-sequence stars. Here, we present a detailed near-infrared analysis of the two central star clusters Danks 1 and Danks 2, using HST+NICMOS imaging and VLT+ISAAC spectroscopy. We find that the spectro-photometric distance to the clusters is consistent with the kinematic distance to the G305 complex, an average of all measurements giving a distance of 3.8\pm0.6kpc. From analysis of the stellar populations and the pre-main-sequence stars we find that Danks 2 is the elder of the two clusters, with an age of 3^{+3}_{-1}Myr. Danks 1 is clearly younger with an age of 1.5^{+1.5}_{-0.5}Myr, and is dominated by three very luminous H-rich Wolf-Rayet stars which may have masses \geq100\msun. The two clusters have mass functions consistent with the Salpeter slope, and total cluster masses of 8000\pm1500\msun\ and 3000\pm800\msun\ for Danks 1 and Danks 2 respectively. Danks 1 is significantly the more compact cluster of the two, and is one of the densest clusters in the Galaxy with $\log (ρ/M_{\odot}{\rm pc}^{-3}) = 5.5^{+0.5}_{-0.4}$. In addition to the clusters, there is a population of apparently isolated Wolf-Rayet stars within the molecular cloud's cavity. Our results suggest that the star-forming history of G305 began with the formation of Danks 2, and subsequently Danks 1, with the origin of the diffuse evolved population currently uncertain. Together, the massive stars at the centre of the G305 region appear to be clearing away what is left of the natal cloud, triggering a further generation of star formation at the cloud's periphery.

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arXiv2010

The Onfp Class in the Magellanic Clouds

The Onfp class of rotationally broadened, hot spectra was defined some time ago in the Galaxy, where its membership to date numbers only eight. The principal defining characteristic is a broad, centrally reversed He II $λ$4686 emission profile; other emission and absorption lines are also rotationally broadened. Recent surveys in the Magellanic Clouds (MCs) have brought the class membership there, including some related spectra, to 28. We present a survey of the spectral morphology and rotational velocities, as a first step toward elucidating the nature of this class. Evolved, rapidly rotating hot stars are not expected theoretically, because the stellar winds should brake the rotation. Luminosity classification of these spectra is not possible, because the principal criterion (He II $λ$4686) is peculiar; however, the MCs provide reliable absolute magnitudes, which show that they span the entire range from dwarfs to supergiants. The Onfp line-broadening distribution is distinct and shifted toward larger values from those of normal O dwarfs and supergiants with >99.99% confidence. All cases with multiple observations show line-profile variations, which even remove some objects from the class temporarily. Some of them are spectroscopic binaries; it is possible that the peculiar profiles may have multiple causes among different objects. The origin and future of these stars are intriguing; for instance, they could be stellar mergers and/or gamma-ray-burst progenitors.

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arXiv2010

Interstellar Ti II in the Milky Way and Magellanic Clouds

We discuss several sets of Ti II absorption-line data, which probe a variety of interstellar environments in our Galaxy and in the Magellanic Clouds. Comparisons of high-resolution (FWHM ~ 1.3-1.5 km/s) Ti II spectra of Galactic targets with corresponding high-resolution spectra of Na I, K I, and Ca II reveal both similarities and differences in the detailed structure of the absorption-line profiles -- reflecting component-to-component differences in the ionization and depletion behaviour of those species. Moderate-resolution (FWHM ~ 3.4-4.5 km/s) spectra of more heavily reddened Galactic stars provide more extensive information on the titanium depletion in colder, denser clouds -- where more than 99.9 per cent of the Ti may be in the dust phase. Moderate-resolution (FWHM ~ 4.5-8.7 km/s) spectra of stars in the Magellanic Clouds suggest that the titanium depletion is generally much less severe in the LMC and SMC than in our Galaxy [for a given N(H_tot), E(B-V), or molecular fraction f(H_2)] -- providing additional evidence for differences in depletion patterns in those two lower-metallicity galaxies. We briefly discuss possible implications of these results for the interpretation of gas-phase abundances in QSO absorption-line systems and of variations in the D/H ratio in the local Galactic ISM.

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arXiv2008

Westerlund 1 as a Template for Massive Star Evolution

With a dynamical mass M_dyn ~ 1.3x10e5 M_sun and a lower limit M_cl>5x10e4 M_sun from star counts, Westerlund 1 is the most massive young open cluster known in the Galaxy and thus the perfect laboratory to study massive star evolution. We have developed a comprehensive spectral classification scheme for supergiants based on features in the 6000-9000A range, which allows us to identify >30 very luminous supergiants in Westerlund 1 and ~100 other less evolved massive stars, which join the large population of Wolf-Rayet stars already known. Though detailed studies of these stars are still pending, preliminary rough estimates suggest that the stars we see are evolving to the red part of the HR diagram at approximately constant luminosity.

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arXiv2007

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.

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arXiv2006

A census of the Wolf-Rayet content in Westerlund 1 from near-infrared imaging and spectroscopy

New NTT/SOFI imaging and spectroscopy of the Wolf-Rayet population in Westerlund 1 are presented. Narrow-band near-IR imaging together with follow up spectroscopy reveals four new WR stars, of which three were independently identified recently by Groh et al., bringing the confirmed WR content to 24 (23 excluding source S) [..] A quantitative near-IR spectral classification scheme for WR stars is presented and applied to members of Westerlund 1. Late subtypes are dominant, with no subtypes earlier than WN5 or WC8 for the nitrogen and carbon sequences, respectively. A qualitative inspection of the WN stars suggests that most (75%) are highly H-deficient. The WR binary fraction is high (>62%), on the basis of dust emission from WC stars, in addition to a significant WN binary fraction from hard X-ray detections according to Clark et al. We exploit the large WN population of Westerlund 1 to reassess its distance (~5.0kpc) and extinction (A_Ks ~ 0.96 mag), such that it is located at the edge of the Galactic bar, [..]. The observed ratio of WR stars to red and yellow hypergiants, N(WR)/N(RSG+YHG)~3, favours an age of 4.5-5.0 Myr, with individual WR stars descended from progenitors of initial mass ~ 40-55 Msun. Qualitative estimates of current masses for non-dusty, H-free WR stars are presented, revealing 10-18 Msun, such that ~75% of the initial stellar mass has been removed via stellar winds or close binary evolution. We present a revision to the cluster turn-off mass for other Milky Way clusters in which WR stars are known, based upon the latest temperature calibration for OB stars. Finally, comparisons between the observed WR population and subtype distribution in Westerlund 1 and instantaneous burst evolutionary synthesis models are presented.

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arXiv2006

Metallicity Dependent Wolf-Rayet Winds

Observational and theoretical evidence in support of metallicity dependent winds for Wolf-Rayet stars is considered. Well known differences in Wolf-Rayet subtype distributions in the Milky Way, LMC and SMC may be attributed to the sensitivity of subtypes to wind density. Implications for Wolf-Rayet stars at low metallicity include a hardening of ionizing flux distributions, an increased WR population due to reduced optical line fluxes, plus support for the role of single WR stars as Gamma Ray Burst progenitors.

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arXiv2004

Properties of Hot, Massive Stars: The Impact of FUSE

The impact of FUSE upon the fundamental parameters of OB stars and Wolf-Rayet stars is reviewed. The stellar wind signatures available in the far-UV provide us with important additional diagnostics of effective temperature. Together with improved non-LTE stellar atmosphere models allowing for line blanketing and stellar winds, this has led to a downward revision in the spectral type-temperature calibration for O stars versus Vacca et al. (1996) In addition, the Lyman continuum ionizing fluxes from O dwarfs are compared with previous calibrations of Panagia (1973) and Vacca et al. We also discuss mass-loss rates in OB stars, such that agreement between recent theoretical predictions (Vink et al. 2000, 2001) and observations of O supergiants is possible, solely if winds are clumped in the far-UV and H-alpha line forming regions, as favoured by line profile comparisons for PV 1118-28 (early to mid O) or SIV 1062-1073 (late O to early B) in FUSE datasets. In contrast, B supergiant wind strengths are predicted to be much higher than observations indicates, especially if their winds are also clumped. Finally, significant upward revisions in wind velocities of very late WN stars are indicated by NII 1085 resonance line observations, plus elemental abundances in OB and WR stars are briefly discussed.

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arXiv2004

MSX Mid infrared imaging of massive star birth environments. II: Giant HII regions

We conduct a Galactic census of Giant HII regions, based on the all sky 6cm dataset of Kuchar & Clark, plus the kinematic distances obtained by Russeil. From an inspection of mid-IR Mid-course Space Experiment (MSX) and far-IR IRAS Sky Survey Atlas images we identify a total of 56 GHII regions in the Milky Way, of which 15% (65%) can be seen at optical (near-IR) wavelengths.The mid to far-IR fluxes from each GH {\sc ii} region are measured, and sample the thermal emission from the ubiquitous dust present within the exciting clusters of OB stars, arising from the integrated luminosity of the hot stars heating the cluster dust, for which we obtain log L(IR)=5.5-7.3 L_sun. The mid-IR 21micron spatial morphology is presented for each GHII region, and often indicates multiple emission sources, suggesting complicated cluster formation. IR colour-colour diagrams are presented, providing information about the temperature distribution and optical depth of the dust. For the clusters of our study, the dust is not optically thick to all the stellar radiation, thus the measured infra-red luminosity is lower than the L(bol). As the dust environment of a cluster begins to dissipate, the thermal emission and its optical depth ought to decrease even before the stars appreciably evolve. We see evidence of this in our empirical relationship between the integrated IR and Lyman continuum luminosities.

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arXiv2004

A CNO Dichotomy among O2 Giant Spectra in the Magellanic Clouds

From a survey of the 3400 A region in the earliest O-type spectra, we have found that two of the four O2 giants observed in the LMC have the OIV lines there stronger than the NIV, while the other two have the opposite. A SMC counterpart also has the NIV stronger than OIV. Inspection of the blue spectra of these stars shows that the former pair have weaker N lines in all ionization states (III, IV, V) present as well as lines of CIV lambda 4658, while the latter three have stronger N lines and greater He/H. Space ultraviolet observations of two of the N-strong stars show NV wind profiles substantially stronger than those of CIV, while in the N-weak stars the CIV features are equal to or stronger than the NV. The N-strong stars are now reclassified as ON2 III(f*), newly defining that category. These characteristics strongly suggest a larger fraction of processed material in the atmospheres of the ON2 stars, which we confirm by modeling the optical spectra. In the context of current models, it is in turn implied that the ON2 stars are in a more advanced evolutionary state than the others, and/or that they had higher initial rotational velocities. The recent formulation of the effects of rotation on massive stellar evolution introduces an additional fundamental parameter, which the CNO abundances are in principle able to constrain. We present some illustrative comparisons with current Geneva evolutionary models for rotating massive stars. It is possible that these very hot, nitrogen-rich objects are products of homogeneous evolution. Our results will provide motivation for further physical modeling of the atmospheres and evolutionary histories of the most massive hot stars.

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arXiv2003

Massive Stars in Transition

We discuss the various post-main sequence phases of massive stars, focusing on Wolf-Rayet stars, Luminous Blue Variables, plus connections with other early-type and late-type supergiants. End states for massive stars are also investigated, emphasising connections between Supernovae originating from core-collapse massive stars and Gamma Ray Bursts.

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arXiv2003

MSX Mid Infrared Imaging of Massive Star Birth Environments. I: Ultracompact HII Regions

We present mid-IR 21micron images of a sample of radio selected Ultracompact HII (UCHII) regions, obtained with the Midcourse Space Experiment (MSX). All, with one possible exception, are detected at mid-IR wavelengths, sampling the warm dust emission of the cocoons of the OB star central exciting sources. Many of the UCHII regions have nearby (up to a few pc distant) companion dust emission sources, which represent other potential star birth sites. In some objects the companion dominates the IRAS point source catalogue entry for the UCHII region. We compare the mid- and far-IR dust emission, measuring the embedded hot star luminosity, with published UCHII radio emission, measuring the Lyman continuum luminosity. We find a spectral type dependence, as predicted by the standard model of an ultracompact ionized hydrogen region, surrounded by a natal dust shell, with some scatter, which can be understood by consideration of: 1) dust absorption of some fraction of the emitted Lyman continuum photons; 2) fainter companion stars within the UCHII region; 3) the structure of the UCHII regions differing from star to star. Overall, the higher spatial resolution offered by MSX alleviates difficulties often encountered by comparison of IRAS far-IR fluxes with radio derived ionizing fluxes for UCHII regions.

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arXiv2002

Realistic Ionizing Fluxes for Young Stellar Populations from 0.05 to twice solar metallicity

We present a new grid of ionizing fluxes for O and Wolf-Rayet stars for use with evolutionary synthesis codes and single star H II region analyses. A total of 230 expanding, non-LTE, line-blanketed model atmospheres have been calculated for five metallicities (0.05, 0.2, 0.4, 1 and 2 solar) using the WM-basic code of Pauldrach et al. (2001) and the CMFGEN code of Hillier & Miller (1998). The stellar wind parameters are scaled with metallicity for both O and W-R stars. We incorporate the new models into Starburst99 (Leitherer et al. 1999) and compare the ionizing outputs with Schaerer & Vacca (1998) and Leitherer et al. (1999). The changes in the output ionizing fluxes are dramatic, particularly below 228 A. We also find lower fluxes in the He I continuum for Z > 0.4 solar and ages < 7 Myr because of the increased line blanketing. We test the accuracy of the new models by constructing photoionization models. We show that for the dwarf O star grid, He I 5876/H beta decreases between Z = 1 and twice solar in a similar manner to observations (e.g. Bresolin et al. 1999) due to the increased effect of line blanketing. We therefore suggest that a lowering of the upper mass limit at high abundances is not required to explain the observations. For the case of an instantaneous burst, we plot the softness parameter "eta prime" against the abundance indicator R_23. The new models are coincident with the data of Bresolin et al. (1999), particularly during the W-R phase, unlike previous models which over-predict the hardness of the ionizing radiation.

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