arXiv subjects

I. Hunter

Publications and source records attributed to I. Hunter.

At least 19 recordsLinked to original sources

arXiv2019

The NGC 346 massive star census: Nitrogen abundances for apparently single, narrow lined, hydrogen core burning B-type stars

Analyses of large spectroscopic surveys of early-type stars in the LMC have found an excess of nitrogen enriched B-type targets with a vsini<40 km/s compared with the predictions of single star evolutionary models incorporating rotational mixing. By contrast the number of such targets with 40<vsini<80 km/s were consistent with such models. We have undertaken a similar analysis for 61 B-type similar targets, towards the young cluster, NGC 346 in the Small Magellanic Cloud (SMC). Approximately 65% could have nitrogen enhancements of less than 0.3 dex, consistent with them having experienced only small amounts of mixing due to their low rotational velocities. However as with the previous LMC surveys, an excess of stars with low projected rotational velocities, vsini<40 km/s and significant nitrogen enrichments is found. This is estimated to be approximately 5% of the total population of apparently single B-type stars or 40% of all stars with current rotational velocities of less than 40 km/s. For all three surveys, the presence of undetected binaries and other uncertainties imply that these percentages might be underestimated and indeed it is possible that all single stars with current rotational velocities of less than 40 km/s are nitrogen enriched. Possible explanations incorporate the effects of magnetic fields, via either a stellar merger followed by magnetic breaking or the evolution of a single star with a large magnetic field. Both mechanisms are compatible with the observed frequency of nitrogen-enriched stars in the Magellanic Clouds. Differences in the properties of the nitrogen-enriched stars compared with the remainder of the sample would be consistent with the former mechanism. For the latter, a qualitative comparison with evolutionary models incorporating magnetic fields is encouraging in terms of the amount of nitrogen enrichment and its presence in stars near the ZAMS.

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arXiv2019

A census of massive stars in NGC 346. Stellar parameters and rotational velocities

Spectroscopy for 247 stars towards the young cluster NGC 346 in the Small Magellanic Cloud has been combined with that for 116 targets from the VLT-FLAMES Survey of Massive Stars. Spectral classification yields a sample of 47 O-type and 287 B-type spectra, while radial-velocity variations and/or spectral multiplicity have been used to identify 45 candidate single-lined systems, 17 double-lined systems, and one triple-lined system. Atmospheric parameters (T$_eff$ and log$g$) and projected rotational velocities ($v_e$sin$i$) have been estimated using TLUSTY model atmospheres; independent estimates of $v_e$sin$i$ were also obtained using a Fourier Transform method. Luminosities have been inferred from stellar apparent magnitudes and used in conjunction with the T$_eff$ and $v_e$sin$i$ estimates to constrain stellar masses and ages using the BONNSAI package. We find that targets towards the inner region of NGC 346 have higher median masses and projected rotational velocities, together with smaller median ages than the rest of the sample. There appears to be a population of very young targets with ages of less than 2 Myr, which have presumably all formed within the cluster. The more massive targets are found to have lower $v_e$sin$i$ consistent with previous studies. No significant evidence is found for differences with metallicity in the stellar rotational velocities of early-type stars, although the targets in the SMC may rotate faster than those in young Galactic clusters. The rotational velocity distribution for single non-supergiant B-type stars is inferred and implies that a significant number have low rotational velocity ($\simeq$10\% with $v_e$<40 km/s), together with a peak in the probability distribution at $v_e \simeq$300 km/s. Larger projected rotational velocity estimates have been found for our Be-type sample and imply that most have rotational velocities between 200-450 km/s.

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arXiv2011

The VLT-FLAMES survey of massive stars: NGC346-013 as a test case for massive close binary evolution

NGC346-013 is a peculiar double-lined eclipsing binary in the Small Magellanic Cloud discovered by the VLT-FLAMES survey of massive stars. Spectra obtained with VLT-FLAMES are used to construct a radial velocity curve and photometry obtained with the Faulkes Telescope South is then used to derive orbital parameters, while spectra of the secondary are compared with synthetic spectra from TLUSTY model atmospheres. The orbital period is found to be 4.20381(12) days, with masses of 19.1+/-1.0 and 11.9+/-0.6 Msun. The primary is a rapidly rotating late-O dwarf while the secondary, an early-B giant, displays near-synchronous rotation and has filled its Roche lobe, implying that it was originally the more massive component with recent mass transfer `spinning up' the primary to near-critical rotation. Comparison with synthetic spectra finds temperatures of 34.5kK and 24.5kK for the primary and secondary respectively, with the nitrogen abundance of the secondary enhanced compared to baseline values for the SMC, consistent with the predictions of models of interacting binaries. NGC346-013 likely evolved via non-conservative mass transfer in a system with initial masses ~22+15Msun, with the well-constrained orbital solution and atmospheric parameters making it an excellent candidate for tailored modelling with binary evolution codes. This system will form a cornerstone in constraining the physics of thermal timescale mass transfer, and the associated mass transfer efficiency, in massive close binary systems.

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arXiv2011

The VLT-FLAMES survey of massive stars: Nitrogen abundances for Be-type stars in the Magellanic Clouds

Aims. We compare the predictions of evolutionary models for early-type stars with atmospheric parameters, projected rotational velocities and nitrogen abundances estimated for a sample of Be-type stars. Our targets are located in 4 fields centred on the Large Magellanic Cloud cluster: NGC 2004 and the N 11 region as well as the Small Magellanic Cloud clusters: NGC 330 and NGC 346. Methods. Atmospheric parameters and photospheric abundances have been determined using the non-LTE atmosphere code tlusty. Effective temperature estimates were deduced using three different methodologies depending on the spectral features observed; in general they were found to yield consistent estimates. Gravities were deduced from Balmer line profiles and microturbulences from the Si iii spectrum. Additionally the contributions of continuum emission from circumstellar discs were estimated. Given its importance in constraining stellar evolutionary models, nitrogen abundances (or upper limits) were deduced for all the stars analysed. Results. Our nitrogen abundances are inconsistent with those predicted for targets spending most of their main sequence life rotating near to the critical velocity. This is consistent with the results we obtain from modelling the inferred rotational velocity distribution of our sample and of other investigators. We consider a number of possibilities to explain the nitrogen abundances and rotational velocities of our Be-type sample.

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arXiv2010

Atmospheric parameters and rotational velocities for a sample of Galactic B-type supergiants

High resolution optical spectra of 57 Galactic B-type supergiant stars have been analyzed to determine their rotational and macroturbulent velocities. In addition, their atmospheric parameters (effective temperature, surface gravity and microturbulent velocity) and surface nitrogen abundances have been estimated using a non-LTE grid of model atmospheres. Comparisons of the projected rotational velocities have been made with the predictions of stellar evolutionary models and in general good agreement was found. However for a small number of targets, their observed rotational velocities were significantly larger than predicted, although their nitrogen abundances were consistent with the rest of the sample. We conclude that binarity may have played a role in generating their large rotational velocities. No correlation was found between nitrogen abundances and the current projected rotational velocities. However a correlation was found with the inferred projected rotational velocities of the main sequence precursors of our supergiant sample. This correlation is again in agreement with the predictions of single star evolutionary models that incorporate rotational mixing. The origin of the macroturbulent and microturbulent velocity fields is discussed and our results support previous theoretical studies that link the former to sub-photospheric convection and the latter to non-radial gravity mode oscillations. In addition, we have attempted to identify differential rotation in our most rapidly rotating targets.

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arXiv2010

Two Type Ic supernovae in low-metallicity, dwarf galaxies: diversity of explosions

We present BVRI photometry and optical spectroscopy of two Type Ic supernovae SN 2007bg and SN 2007bi discovered in wide-field, non-targeted surveys and associated with sub-luminous blue dwarf galaxies. Neither SNe 2007bg nor 2007bi were found in association with an observed GRB, but are found to inhabit similar low-metallicity environments as GRB associated supernovae. The radio-bright SN 2007bg is hosted by an extremely sub-luminous galaxy of magnitude MB = -12.4+/-0.6 mag with an estimated oxygen abundance of 12+log(O/H) = 8.18+/-0.17. The lightcurve of SN 2007bg displays one of the fastest post-maximum decline rates of all broad-lined Type Ic supernovae known to date and, when combined with its high expansion velocities, a high kinetic energy to ejected mass ratio (E_K/Mej ~ 2.7). We show that SN 2007bi is possibly the most luminous Type Ic known, reaching a peak magnitude of MR ~ 21.3 mag and displays a remarkably slow decline, following the radioactive decay rate of 56Co to 56Fe throughout the course of its observed lifetime. From a simple model of the bolometric light curve of SN 2007bi we estimate a total ejected 56Ni mass of M_Ni = 3.5 - 4.5 solar masses, the largest 56Ni mass measured in the ejecta of a supernova to date. There are two models that could explain the high luminosity and large ejected 56Ni mass. One is a pair-instability supernova (PISN) which has been predicted to occur for massive stars at low metallicities. We measure the host galaxy metallicity of SN 2007bi to be 12 + log(O/H) = 8.15+/-0.15 which is somewhat high to be consistent with the PISN model. An alternative is the core-collapse of a C+O star of 20 - 40 solar masses which is the core of a star of originally 50 - 100 solar masses. (Abridged)

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arXiv2009

The VLT-FLAMES survey of massive stars: constraints on stellar evolution from the chemical compositions of rapidly rotating Galactic and Magellanic Cloud B-type stars

We have previously analysed the spectra of 135 early B-type stars in the LMC and found several groups of stars that have chemical compositions that conflict with the theory of rotational mixing. Here we extend this study to Galactic and SMC metallicities with the analysis of ~50 Galactic and ~100 SMC early B-type stars with rotational velocities up to ~300km/s. The surface nitrogen abundances are utilised as a probe of the mixing process. In the SMC, we find a population of slowly rotating nitrogen-rich stars amongst the early B type core-hydrogen burning stars, similar to the LMC. In the Galactic sample we find no significant enrichment amongst the core hydrogen-burning stars, which appears to be in contrast with the expectation from both rotating single-star and close binary evolution models. However, only a small number of the rapidly rotating stars have evolved enough to produce a significant nitrogen enrichment, and these may be analogous to the non-enriched rapid rotators previously found in the LMC sample. Finally, in each metallicity regime, a population of highly enriched supergiants is observed, which cannot be the immediate descendants of core-hydrogen burning stars. Their abundances are, however, compatible with them having gone through a previous red supergiant phase. Together, these observations paint a complex picture of the nitrogen enrichment in massive main sequence and supergiant stellar atmospheres, where age and binarity cause crucial effects. Whether rotational mixing is required to understand our results remains an open question at this time, but could be answered by identifying the true binary fraction in those groups of stars that do not agree with single-star evolutionary models (abridged).

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arXiv2009

Rotational Mixing in Magellanic Clouds B Stars - Theory versus Observation

We have used VLT FLAMES data to constrain the uncertain physics of rotational mixing in stellar evolution models. We have simulated a population of single stars and find two groups of observed stars that cannot be explained: (1) a group of fast rotating stars which do not show evidence for rotational mixing and (2) a group of slow rotators with strong N enrichment. Binary effects and fossil magnetic fields may be considered to explain those two groups. We suggest that the element boron could be used to distinguish between rotational mixing and the binary scenario. Our single star population simulations quantify the expected amount of boron in fast and slow rotators and allow a comparison with measured nitrogen and boron abundances in B-stars.

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arXiv2008

Massive stars exploding in a He-rich circumstellar medium. I. Type Ibn (SN 2006jc-like) events

We present new spectroscopic and photometric data of the type Ibn supernovae 2006jc, 2000er and 2002ao. We discuss the general properties of this recently proposed supernova family, which also includes SN 1999cq. The early-time monitoring of SN 2000er traces the evolution of this class of objects during the first few days after the shock breakout. An overall similarity in the photometric and spectroscopic evolution is found among the members of this group, which would be unexpected if the energy in these core-collapse events was dominated by the interaction between supernova ejecta and circumstellar medium. Type Ibn supernovae appear to be rather normal type Ib/c supernova explosions which occur within a He-rich circumstellar environment. SNe Ibn are therefore likely produced by the explosion of Wolf-Rayet progenitors still embedded in the He-rich material lost by the star in recent mass-loss episodes, which resemble known luminous blue variable eruptions. The evolved Wolf-Rayet star could either result from the evolution of a very massive star or be the more evolved member of a massive binary system. We also suggest that there are a number of arguments in favour of a type Ibn classification for the historical SN 1885A (S-Andromedae), previously considered as an anomalous type Ia event with some resemblance to SN 1991bg.

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arXiv2008

The blue supergiant Sher 25 and its intriguing hourglass nebula

The blue supergiant Sher 25 is surrounded by an asymmetric, hourglass-shaped circumstellar nebula. Its structure and dynamics have been studied previously through high-resolution imaging and spectroscopy, and it appears dynamically similar to the ring structure around SN 1987A. Here we present long-slit spectroscopy of the circumstellar nebula around Sher 25, and of the background nebula of the host cluster NGC 3603. We perform a detailed nebular abundance analysis to measure the gas-phase abundances of oxygen, nitrogen, sulphur, neon and argon. The oxygen abundance in the circumstellar nebula (12 + log[O/H] = 8.61 +/- 0.13 dex) is similar to that in the background nebula (8.56 +/- 0.07), suggesting the composition of the host cluster is around solar. However, we confirm that the circumstellar nebula is very rich in nitrogen, with an abundance of 8.91 +/- 0.15, compared to the background value of 7.47 +/- 0.18. A new analysis of the stellar spectrum with the FASTWIND model atmosphere code suggests that the photospheric nitrogen and oxygen abundances in Sher 25 are consistent with the nebular results. While the nitrogen abundances are high, when compared to stellar evolutionary models they do not unambiguously confirm that the star has undergone convective dredge-up during a previous red supergiant phase. We suggest that the more likely scenario is that the nebula was ejected from the star while it was in the blue supergiant phase. The star's initial mass was around 50 M_sun, which is rather too high for it to have had a convective envelope stage as a red supergiant. Rotating stellar models that lead to mixing of core-processed material to the stellar surface during core H-burning can quantitatively match the stellar results with the nebula abundances.

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arXiv2008

The VLT-FLAMES Survey of Massive Stars

The VLT-FLAMES Survey of Massive Stars was an ESO Large Programme to understand rotational mixing and stellar mass-loss in different metallicity environments, in order to better constrain massive star evolution. We gathered high-quality spectra of over 800 stars in the Galaxy and in the Magellanic Clouds. A sample of this size is unprecedented, enabled by the first high-resolution, wide-field, multi-object spectrograph on an 8-m telescope. We developed spectral analysis techniques that, in combination with non-LTE, line-blanketed model atmospheres, were used to quantitatively characterise every star. The large sample, combined with the theoretical developments, has produced exciting new insights into the evolution of the most massive stars.

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arXiv2008

Rotation and massive close binary evolution

We review the role of rotation in massive close binary systems. Rotation has been advocated as an essential ingredient in massive single star models. However, rotation clearly is most important in massive binaries where one star accretes matter from a close companion, as the resulting spin-up drives the accretor towards critical rotation. Here, we explore our understanding of this process, and its observable consequences. When accounting for these consequences, the question remains whether rotational effects in massive single stars are still needed to explain the observations.

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arXiv2008

The VLT-FLAMES survey of massive stars: rotation and nitrogen enrichment as the key to understanding massive star evolution

Rotation has become an important element in evolutionary models of massive stars, specifically via the prediction of rotational mixing. Here, we study a sample of stars, including rapid rotators, to constrain such models and use nitrogen enrichments as a probe of the mixing process. Chemical compositions (C, N, O, Mg and Si) have been estimated for 135 early B-type stars in the Large Magellanic Cloud with projected rotational velocities up to ~300km/s using a non-LTE TLUSTY model atmosphere grid. Evolutionary models, including rotational mixing, have been generated attempting to reproduce these observations by adjusting the overshooting and rotational mixing parameters and produce reasonable agreement with 60% of our core hydrogen burning sample. We find (excluding known binaries) a significant population of highly nitrogen enriched intrinsic slow rotators vsini less than 50km/s incompatible with our models ~20% of the sample). Furthermore, while we find fast rotators with enrichments in agreement with the models, the observation of evolved (log g less than 3.7dex) fast rotators that are relatively unenriched (a further ~20% of the sample) challenges the concept of rotational mixing. We also find that 70% of our blue supergiant sample cannot have evolved directly from the hydrogen burning main-sequence. We are left with a picture where invoking binarity and perhaps fossil magnetic fields are required to understand the surface properties of a population of massive main sequence stars.

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arXiv2008

Detection of diffuse interstellar bands in M31

We investigate the diffuse interstellar band (DIB) spectrum in the interstellar medium of M31. The DEIMOS spectrograph of the W. M. Keck observatory was used to make optical spectroscopic observations of two supergiant stars, MAG 63885 and MAG 70817, in the vicinity of the OB78 association in M31 where the metallicity is approximately equal to solar. The 5780, 5797, 6203, 6283 and 6613 DIBs are detected in both sightlines at velocities matching the M31 interstellar Na I absorption. The spectra are classified and interstellar reddenings are derived for both stars. Diffuse interstellar band (DIB) equivalent widths and radial velocities are presented. The spectrum of DIBs observed in M31 towards MAG 63885 is found to be similar to that observed in the Milky Way. Towards MAG 70817 the DIB equivalent widths per unit reddening are about three times the Galactic average. Compared to observations elsewhere in the Universe, relative to reddening the M31 ISM in the vicinity of OB78 is apparently a highly favourable environment for the formation of DIB carriers.

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arXiv2007

The VLT-FLAMES survey of massive stars: atmospheric parameters and rotational velocity distributions for B-type stars in the Magellanic Clouds

We provide atmospheric parameters and rotational velocities of a large sample (~400) of O- and early B-type stars, analysed in a homogeneous and consistent manner, for use in constraining theoretical models. Comparison of the rotational velocities with evolutionary tracks suggest that the end of core hydrogen burning occurs later than currently predicted. We also show that the large number of the luminous blue supergiants observed in the fields are unlikely to have directly evolved from main-sequence massive O-type stars as neither their low rotational velocities or position on the H-R diagram are predicted. We suggest that blue-loops or mass-transfer binary systems may populate the blue supergiant regime. By comparing the rotational velocity distributions of the Magellanic Cloud stars to a similar Galactic sample we find that (at 3sigma confidence level) massive stars (above 8Msun) in the SMC rotate faster than those in the solar neighbourhood. However there appears to be no significant difference between the rotational velocity distributions in the Galaxy and the LMC. We find that the vsini distributions in the SMC and LMC can modelled with an intrinsic rotational velocity distribution which is a Gaussian peaking at 175km/s (SMC) and 100km/s (LMC). We find that in NGC346 in the SMC, the 10-25Msun main-sequence stars appear to rotate faster than their higher mass counterparts. Recently Yoon et al. (2006) have determined rates of GRBs by modelling rapidly rotating massive star progenitors. Our measured rotational velocity distribution for the 10-25Msun stars is peaked at slightly higher velocities than they assume, supporting the idea that GRBs could come from rapid rotators with initial masses as low as 14Msun at low metallicities. (abridged).

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arXiv2007

How Efficient is Rotational Mixing in Massive Stars ?

The VLT-Flames Survey for Massive Stars (Evans05,Evans06) provides recise measurements of rotational velocities and nitrogen surface abundances of massive stars in the Magellanic Clouds. Specifically, for the first time, such abundances have been estimated for stars with significant rotational velocities. This extraordinary data set gives us the unique possibility to calibrate rotationally and magnetically induced mixing processes. Therefore, we have computed a grid of stellar evolution models varying in mass, initial rotational velocity and chemical composition. In our models we find that although magnetic fields generated by the Spruit-Taylor dynamo are essential to understand the internal angular momentum transport (and hence the rotational behavior), the corresponding chemical mixing must be neglected to reproduce the observations. Further we show that for low metallicities detailed initial abundances are of prime importance, as solar-scaled abundances may result in significant calibration errors.

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arXiv2007

The VLT-FLAMES survey of massive stars: Evolution of surface N abundances and effective temperature scales in the Galaxy and Magellanic Clouds

We present an analysis of high resolution VLT-FLAMES spectra of 61 B-type stars with relatively narrow-lined spectra located in 4 fields centered on the Milky Way clusters; NGC3293 & NGC4755 and the Large and Small Magellanic cloud clusters; NGC2004 and NGC330. For each object a quantitative analysis was carried out using the non-LTE model atmosphere code TLUSTY; resulting in the determination of their atmospheric parameters and photospheric abundances of the dominant metal species (C, N, O, Mg, Si, Fe). The results are discussed in relation to our earlier work on 3 younger clusters in these galaxies; NGC6611, N11 and NGC346 paying particular attention to the nitrogen abundances which are an important probe of the role of rotation in the evolution of stars. This work along with that of the younger clusters provides a consistent dataset of abundances and atmospheric parameters for over 100 B-type stars in the three galaxies. We provide effective temperature scales for B-type dwarfs in all three galaxies and for giants and supergiants in the SMC and LMC. In each galaxy a dependence on luminosity is found between the three classes with the unevolved dwarf objects having significantly higher effective temperatures. A metallicity dependence is present between the SMC and Galactic dwarf objects, and whilst the LMC stars are only slightly cooler than the SMC stars, they are significantly hotter than their Galactic counterparts.

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arXiv2007

Observations towards early-type stars in the ESO-POP survey: II -- searches for intermediate and high velocity clouds

We present CaII K and TiII optical spectra of early-type stars taken mainly from the UVES Paranal Observatory Project, plus HI 21-cm spectra from the Vila-Elisa and Leiden-Dwingeloo surveys, which are employed to obtain distances to intermediate and high velocity clouds. HI emission at a velocity of -117 km/s towards the sightline HD 30677 with NHI=1.7x10^19 cm-2 has no corresponding CaII K absorption, which has a signal-to-noise (S/N) ratio of 610. The star has a spectroscopically determined distance of 2.7-kpc, and hence sets this as a firm lower distance limit towards Anti-Centre cloud ACII. Towards another sightline (HD 46185), HI at +122 km/s and NHI=1.2x10^19 cm-2 is seen. The CaII K spectrum has a S/N = 780, although no absorption is observed at the cloud velocity. This similarly places a firm lower distance limit of 2.9-kpc towards this parcel of gas that may be an intermediate velocity cloud. The lack of intermediate velocity (IV) CaII K absorption towards HD 196426 at a S/N = 500 reinforces a lower distance limit of 700-pc towards this part of Complex gp, where NHI is 1.1x10^19 cm-2 and velocity is +78 km/s. Additionally, no IV CaII K is seen in absorption in the spectrum of HD 19445, which is strong in HI with NHI=8x10^19 cm-2 at -42 km/s, placing a firm although uninteresting lower distance limit of 39-pc to this part of IV South. Finally, no HV CaII K absorption is seen towards HD 115363 at a S/N = 410, placing a lower distance of 3.2-kpc towards the HVC gas at velocity of +224 km/s and NHI=5.2x10^19 cm-2. This gas is in the same region of the sky as complex WE (Wakker 2001), but at higher velocities. The non-detection of CaII K absorption sets a lower distance of 3.2-kpc towards the HVC, which is unsurprising if this feature is indeed related to the Magellanic System.

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