arXiv subjects

Ben Davies

Publications and source records attributed to Ben Davies.

66 records · Page 4Linked to original sources

arXiv2009

Young Massive Clusters as probes of stellar evolution

Young Massive Clusters (YMCs) represent ideal testbeds in which to study massive stellar evolution as they contain large, coeval, chemically homogeneous, samples of massive stars. By studying YMCs with a range of ages (and hence turn-off masses), we can investigate the post main-sequence evolution of massive stars as a function of initial mass. Recent discoveries of YMCs over a range of ages within our own Galaxy - where we can successfully resolve individual stars - offers the unprecedented opportunity to test our ideas of massive stellar evolution. Here, I review some of the recent works in this field, and describe how we can use YMCs to investigate several topics, including (a) the evolutionary state of H-rich Wolf-Rayet stars; (b) the influence of binarity on stellar evolution in dense clusters; and (c) Red Supergiants and the post-supernova remnants they leave behind.

astro-ph.SR↗

arXiv2009

Chemical abundance patterns in the inner Galaxy: the Scutum Red Supergiant Clusters

The location of the Scutum Red-Supergiant (RSG) clusters at the end of the Galactic Bar makes them an excellent probe of the Galaxy's secular evolution; while the clusters themselves are ideal testbeds in which to study the predictions of stellar evolutionary theory. To this end, we present a study of the RSGs' surface abundances using a combination of high-resolution H-band spectroscopy and spectral synthesis analysis. We provide abundance measurements for elements C, O, Si, Mg, Ti, and Fe. We find that the surface abundances of the stars studied are consistent with CNO burning and deep, rotationally enhanced mixing. The average a/Fe ratios of the clusters are solar, consistent with a thin-disk population. However, we find significantly sub-solar Fe/H ratios for each cluster, a result which strongly contradicts a simple extrapolation of the Galactic metallicity gradient to lower Galacto-centric distances. We suggest that a simple one-dimensional parameterization of the Galaxy's abundance patterns is insufficient at low Galactocentric distances, as large azimuthal variations may be present. Indeed, we show that the abundances of O, Si and Mg are consistent with independent measurements of objects in similar locations in the Galaxy. In combining our results with other data in the literature, we present evidence for large-scale (~kpc) azimuthal variations in abundances at Galacto-centric distances of 3-5kpc. While we cannot rule-out that this observed behaviour is due to systematic offsets between different measurement techniques, we do find evidence for similar behaviour in a study of the barred-spiral galaxy NGC4736 which uses homogeneous methodology. We suggest that these azimuthal abundance variations could result from the intense but patchy star formation driven by the potential of the central bar.

astro-ph.GA↗

arXiv2008

The chemical abundances in the Galactic Centre from the atmospheres of Red Supergiants

The Galactic Centre (GC) has experienced a high degree of recent star-forming activity, as evidenced by the large number of massive stars currently residing there. The relative abundances of chemical elements in the GC may provide insights into the origins of this activity. Here, we present high-resolution $H$-band spectra of two Red Supergiants in the GC (IRS~7 and VR~5-7), and in combination with spectral synthesis we derive abundances for Fe and C, as well as other $α$-elements Ca, Si, Mg Ti and O. We find that the C-depletion in VR~5-7 is consistent with the predictions of evolutionary models of RSGs, while the heavy depletion of C and O in IRS~7's atmosphere is indicative of deep mixing, possibly due to fast initial rotation and/or enhanced mass-loss. Our results indicate that the {\it current} surface Fe/H content of each star is slightly above Solar. However, comparisons to evolutionary models indicate that the {\it initial} Fe/H ratio was likely closer to Solar, and has been driven higher by H-depletion at the stars' surface. Overall, we find $α$/Fe ratios for both stars which are consistent with the thin Galactic disk. These results are consistent with other chemical studies of the GC, given the precision to which abundances can currently be determined. We argue that the GC abundances are consistent with a scenario in which the recent star-forming activity in the GC was fuelled by either material travelling down the Bar from the inner disk, or from the winds of stars in the inner Bulge -- with no need to invoke top-heavy stellar Initial Mass Functions to explain anomalous abundance ratios.

astro-ph↗

arXiv2008

Discovery of a young massive stellar cluster near HESS J1813-178

We present the serendipitous discovery of a young stellar cluster in the Galactic disk at l=12deg. Using Keck/NIRSPEC, we obtained high- and low-resolution spectroscopy of several stars in the cluster, and we identified one red supergiant and two blue supergiants. The radial velocity of the red supergiant provides a kinematic cluster distance of 4.7pm0.4 kpc, implying luminosities of the stars consistent with their spectral types. Together with the known Wolf-Rayet star located 2.4' from the cluster center, the presence of the red supergiant and the blue supergiants suggests a cluster age of 6-8 Myr, and an initial mass of 2000 Msun. Several stars in the cluster are coincident with X-ray sources, including the blue supergiants and the Wolf-Rayet star. This is indicative of a high binary fraction, and is reminiscent of the massive young cluster Westerlund 1. The cluster is coincident with two supernova remnants, SNR G12.72-0.0 and G12.82-0.02, and the highly magnetized pulsar associated with the TeV gamma-ray source HESS J1813-178. The mixture of spectral types suggests that the progenitors of these objects had initial masses of 20 - 30 Msun.

astro-ph↗

arXiv2008

Post-Red Supergiants

The yellow hypergiants are found in a stage between the massive Red Supergiants and the Wolf-Rayet stars. This review addresses current issues concerning the evolution of massive stars, concentrating on the transitional post-Red Supergiant phase. Few yellow hypergiants are known and even fewer show direct evidence for having evolved off the Red Supergiant branch. Indeed, only two such rare objects with clear evidence for having gone through of a previous mass losing phase are known, IRC +10420 and HD 179821. We will review their properties and present recent results employing near-infrared interferometry, integral field spectroscopy and polarimetry. Finally, their real-time evolution is discussed.

astro-ph↗

arXiv2007

The cool supergiant population of the massive young star cluster RSGC1

We present new high-resolution near-IR spectroscopy and OH maser observations to investigate the population of cool luminous stars of the young massive Galactic cluster RSGC1. Using the 2.293\micron CO-bandhead feature, we make high-precision radial velocity measurements of 16 of the 17 candidate Red Supergiants (RSGs) identified by Figer et al. We show that F16 and F17 are foreground stars, while we confirm that the rest are indeed physically-associated RSGs. We determine that Star F15, also associated with the cluster, is a Yellow Hypergiant based on its luminosity and spectroscopic similarity to $ρ$ Cas. Using the cluster's radial velocity, we have derived the kinematic distance to the cluster and revisited the stars' temperatures and luminosities. We find a larger spread of luminosities than in the discovery paper, consistent with a cluster age 30% older than previously thought (12$\pm$2Myr), and a total initial mass of $(3\pm1) \times 10^{4}$\msun. The spatial coincidence of the OH maser with F13, combined with similar radial velocities, is compelling evidence that the two are related. Combining our results with recent SiO and H$_2$O maser observations, we find that those stars with maser emission are the most luminous in the cluster. From this we suggest that the maser-active phase is associated with the end of the RSG stage, when the luminosity-mass ratios are at their highest.

astro-ph↗

arXiv2007

A massive cluster of Red Supergiants at the base of the Scutum-Crux arm

We report on the unprecedented Red Supergiant (RSG) population of a massive young cluster, located at the base of the Scutum-Crux Galactic arm. We identify candidate cluster RSGs based on {\it 2MASS} photometry and medium resolution spectroscopy. With follow-up high-resolution spectroscopy, we use CO-bandhead equivalent width and high-precision radial velocity measurements to identify a core grouping of 26 physically-associated RSGs -- the largest such cluster known to-date. Using the stars' velocity dispersion, and their inferred luminosities in conjuction with evolutionary models, we argue that the cluster has an initial mass of $\sim$40,000\msun, and is therefore among the most massive in the galaxy. Further, the cluster is only a few hundred parsecs away from the cluster of 14 RSGs recently reported by Figer et al (2006). These two RSG clusters represent 20% of all known RSGs in the Galaxy, and now offer the unique opportunity to study the pre-supernova evolution of massive stars, and the Blue- to Red-Supergiant ratio at uniform metallicity. We use GLIMPSE, MIPSGAL and MAGPIS survey data to identify several objects in the field of the larger cluster which seem to be indicative of recent region-wide starburst activity at the point where the Scutum-Crux arm intercepts the Galactic bulge. Future abundance studies of these clusters will therefore permit the study of the chemical evolution and metallicity gradient of the Galaxy in the region where the disk meets the bulge.

astro-ph↗

arXiv2007

Integral-Field Spectroscopy of the Post Red Supergiant IRC +10420: evidence for an axi-symmetric wind

We present NAOMI/OASIS adaptive-optics assisted integral-field spectroscopy of the transitional massive hypergiant IRC +10420, an extreme mass-losing star apparently in the process of evolving from a Red Supergiant toward the Wolf-Rayet phase. To investigate the present-day mass-loss geometry of the star, we study the appearance of the line-emission from the inner wind as viewed when reflected off the surrounding nebula. We find that, contrary to previous work, there is strong evidence for wind axi-symmetry, based on the equivalent-width and velocity variations of H$α$ and Fe {\sc ii} $λ$6516. We attribute this behaviour to the appearance of the complex line-profiles when viewed from different angles. We also speculate that the Ti {\sc ii} emission originates in the outer nebula in a region analogous to the Strontium Filament of $η$ Carinae, based on the morphology of the line-emission. Finally, we suggest that the present-day axisymmetric wind of IRC +10420, combined with its continued blueward evolution, is evidence that the star is evolving toward the B[e] supergiant phase.

astro-ph↗

arXiv2007

Modelling the clumping-induced polarimetric variability of hot star winds

Clumping in the winds of massive stars may significantly reduce empirical mass-loss rates, and which in turn may have a large impact on our understanding of massive star evolution. Here, we investigate wind-clumping through the linear polarization induced by light scattering off the clumps. Through the use of an analytic wind clumping model, we predict the time evolution of the linear polarimetry over a large parameter space. We concentrate on the Luminous Blue Variables, which display the greatest amount of polarimetric variability and for which we recently conducted a spectropolarimetric survey. Our model results indicate that the observed level of polarimetric variability can be reproduced for two regimes of parameter space: one of a small number of massive, optically-thick clumps; and one of a very large number of low-mass clumps. Although a systematic time-resolved monitoring campaign is required to distinguish between the two scenarios, we currently favour the latter, given the short timescale of the observed polarization variability. As the polarization is predicted to scale linearly with mass-loss rate, we anticipate that all hot stars with very large mass-loss rates should display polarimetric variability. This is consistent with recent findings that intrinsic polarization is more common in stars with strong H$α$ emission.

astro-ph↗

arXiv2006

Using polarization to study the winds of massive stars

The topic of wind-clumping has been the subject of much activity in recent years, due to the impact that it can have on derived mass-loss rates. Here we present an alternative method of investigating wind-clumping, that of polarimetry. We present simulations of the polarization produced by a clumpy wind, and argue that the observations may be reproduced just by statistical deviations from spherical symmetry when the outflow is only slightly fragmented. Here, the polarization scales with $\dot{M}$, which is consistent with observations of LBVs, WRs and O supergiants. Finally, we find clumping factors in the inner 2$R_{\star}$ of $\sim 2-3$, and speculate as to the clumping stratification of hot stars.

astro-ph↗

arXiv2005

The clumpiness of Luminous Blue Variable winds

We present the first systematic spectropolarimetric study of Luminous Blue Variables (LBVs), and find that at least half those objects studied display evidence for intrinsic polarization -- a signature of significant inhomogeneity at the base of the wind. Furthermore, multi-epoch observations reveal that the polarization is variable in both strength and position angle. This evidence points away from a simple axi-symmetric wind structure à la the B[e] supergiants, and instead suggests a wind consisting of localised density enhancements, or `clumps'. We show with an analytical model that, in order to produce the observed variability, the clumps must be large, produced at or below the photosphere, and ejected on timescales of days. More details of LBV wind-clumping will be determined through further analysis of the model and a polarimetric monitoring campaign.

astro-ph↗

arXiv2005

Asphericity and clumpiness in the winds of Luminous Blue Variables

We present the first systematic spectropolarimetric study of Luminous Blue Variables (LBVs) in the Galaxy and the Magellanic Clouds, in order to investigate the geometries of their winds. We find that at least half of our sample show changes in polarization across the strong H$α$ emission line, indicating that the light from the stars is intrinsically polarized and therefore that asphericity already exists at the base of the wind. Multi-epoch spectropolarimetry on four targets reveals variability in their intrinsic polarization. Three of these, AG Car, HR Car and P Cyg, show a position angle (PA) of polarization which appears random with time. Such behaviour can be explained by the presence of strong wind-inhomogeneities, or `clumps' within the wind. Only one star, R 127, shows variability at a constant PA, and hence evidence for axi-symmetry as well as clumpiness. However, if viewed at low inclination, and at limited temporal sampling, such a wind would produce a seemingly random polarization of the type observed in the other three stars. Time-resolved spectropolarimetric monitoring of LBVs is therefore required to determine if LBV winds are axi-symmetric in general. The high fraction of LBVs ($>$ 50%) showing intrinsic polarization is to be compared with the lower $\sim$ 20-25 % for similar studies of their evolutionary neighbours, O supergiants and Wolf-Rayet stars. We anticipate that this higher incidence is due to the lower effective gravities of the LBVs, coupled with their variable temperatures within the bi-stability jump regime. This is also consistent with the higher incidence of wind asphericity that we find in LBVs with strong H$α$ emission and recent (last $\sim$ 10 years) strong variability.

astro-ph↗