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Register a new userUsage of Red supergiant spectral features as age indicators in starburst regions
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Y. D. Mayya
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We investigate techniques that can be used to determine ages of starburst regions containing populations beyond their early nebular phase. In particular, we study the strength of the CaII triplet (lambda 8498, 8542, 8662 Ang) and the CO index (2.31-2.40 micron band) using synthetic models as the starburst evolves. For an instantaneous burst of star formation both of these absorption features remain strongest between 7-14 Myr corresponding to the red supergiant population. The detailed evolutionary behavior of the starburst is strongly metallicity dependent. Low metallicity starburst models successfully reproduce the distribution of equivalent widths of CaII triplet with age in Large Magellanic Cloud clusters. The clusters in the red supergiant phase strongly favor the stellar evolutionary models incorporating mass-loss rates higher than the standard values. We suggest usage of diagrams involving CaII triplet equivalent width, CO index and nebular recombination lines to infer the history as well as age of starburst regions.
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We investigate the uncertainties in the synthetic integrated colors of simple stellar populations. Three types of uncertainties are from the stellar models, the population synthesis techniques, and from the spectral libraries. Despite some skepticism, synthetic colors appear to be reliable age indicators when used for select age ranges. Rest-frame optical colors are good age indicators at ages 2 -- 7Gyr. At ages sufficiently large to produce hot HB stars, the UV-to-optical colors provide an alternative means for measuring ages. This UV technique may break the age-metallicity degeneracy because it separates old populations from young ones even in the lack of metallicity information. One can use such techniques on extragalactic globular clusters and perhaps even for high redshift galaxies that are passively evolving to study galaxy evolution history.
We present H-band interferometric observations of the red supergiant (RSG) AZ Cyg made with the Michigan Infra-Red Combiner (MIRC) at the six-telescope Center for High Angular Resolution Astronomy (CHARA) Array. The observations span 5 years (2011-2016), offering insight into the short and long-term evolution of surface features on RSGs. Using a spectrum of AZ Cyg obtained with SpeX on the NASA InfraRed Telescope Facility (IRTF) and synthetic spectra calculated from spherical MARCS, spherical PHOENIX, and SAtlas model atmospheres, we derive $T_{text{eff}}$ is between $3972 K$ and $4000 K$ and $log~g$ between $-0.50$ and $0.00$, depending on the stellar model used. Using fits to the squared visibility and Gaia parallaxes we measure its average radius $R=911^{+57}_{-50}~R_{odot}$. Reconstructions of the stellar surface using our model-independent imaging codes SQUEEZE and OITOOLS.jl show a complex surface with small bright features that appear to vary on a timescale of less than one year and larger features that persist for more than one year. 1D power spectra of these images suggest a characteristic size of $0.52-0.69~R_{star}$ for the larger, long lived features. This is close to the values of $0.51-0.53~R_{star}$ derived from 3D RHD models of stellar surfaces. We conclude that interferometric imaging of this star is in line with predictions of 3D RHD models but that short-term imaging is needed to more stringently test predictions of convection in RSGs.
We review the main stellar features observed in starburst spectra from the UV to the near-IR and their use as fundamental tools to determine the properties of stellar populations from integrated spectra. The origin and dependence of the features on stellar properties are discussed, and we summarise existing modeling techniques used for quantitative analysis. Recent results from studies based on UV, optical and near-IR observations of starbursts and active galaxies are summarised. Finally, we briefly discuss combined starburst + photoionisation models including also observations from nebular emission lines. The present review is complementary to the recent summary by Schaerer (2000) (http://xxx.lpthe.jussieu.fr/abs/astro-ph/0007307) discussing more extensively nebular analysis of starbursts and related objects.
We present near-IR spectroscopy of red supergiant (RSG) stars in NGC 6822, obtained with the new VLT-KMOS instrument. From comparisons with model spectra in the J-band we determine the metallicity of 11 RSGs, finding a mean value of [Z] = -0.52 $pm$ 0.21 which agrees well with previous abundance studies of young stars and HII regions. We also find an indication for a low-significance abundance gradient within the central 1 kpc. We compare our results with those derived from older stellar populations and investigate the difference using a simple chemical evolution model. By comparing the physical properties determined for RSGs in NGC 6822 with those derived using the same technique in the Galaxy and the Magellanic Clouds, we show that there appears to be no significant temperature variation of RSGs with respect to metallicity, in contrast with recent evolutionary models.
A significant percentage of OB stars are runaways, so we should expect a similar percentage of their evolved descendants to also be runaways. However, recognizing such stars presents its own set of challenges, as these older, more evolved stars will have drifted further from their birthplace, and thus their velocities might not be obviously peculiar. Several Galactic red supergiants (RSGs) have been described as likely runaways, based upon the existence of bow shocks, including Betelgeuse. Here we announce the discovery of a runaway RSG in M31, based upon a 300 km s$^{-1}$ discrepancy with M31s kinematics. The star is found about 21 (4.6 kpc) from the plane of the disk, but this separation is consistent with its velocity and likely age ($sim$10 Myr). The star, J004330.06+405258.4, is an M2 I, with $M_V=-5.7$, $log L/L_odot$=4.76, an effective temperature of 3700 K, and an inferred mass of 12-15$M_odot$. The star may be a high-mass analog of the hypervelocity stars, given that its peculiar space velocity is probably 400-450 km s$^{-1}$, comparable to the escape speed from M31s disk.
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