Found 132 talks archived in Stars

Over the past years observations of young and populous star clusters have shown that the stellar initial mass function (IMF) can be conveniently described by a two-part power-law with an exponent alpha2 = 2.3 for stars more massive than about 0.5 Msol and an exponent of alpha1 = 1.3 for less massive stars. A consensus has also emerged that most, if not all, stars form in stellar groups and star clusters, and that the mass function of these can be described as a power-law (the embedded cluster mass function, ECMF) with an exponent beta ~2. These two results imply that the integrated galactic IMF (IGIMF) for early-type stars cannot be a Salpeter power-law, but that they must have a steeper exponent. An application to star-burst galaxies shows that the IGIMF can become top-heavy. This has important consequences for the distribution of stellar remnants and for the chemo-dynamical and photometric evolution of galaxies.

We present the detailed Star Formation History of the nearby Sculptor and Fornax dwarf spheroidal galaxies, from wide-field photometry of resolved stars, going down to the oldest Main Sequence Turn-Off. The accurately flux calibrated, wide-field Colour-Magnitude Diagrams are used directly in combination with spectroscopic metallicities of individual RGB stars to constrain the ages of different stellar populations, and derive the Star Formation History with particular accuracy.
The detailed Star Formation History shows the star formation at different ages and metallicities, at different positions in the galaxy, and shows that the known metallicity gradients are well matched to an age gradient. The obtained SFH is used to determine accurate age estimates for individual RGB stars, for which spectroscopic abundances (alpha-elements, r- and s-process elements) are known. In this way, we obtain the accurate age-metallicity relation of each galaxy, as well as the temporal evolution of alpha-element abundances.
This allows us to study, for the first time, the timescale of chemical evolution in these two dwarf galaxies, and determine an accurate age of the "knee" in the alpha-element distribution. Finally, we compare the timescale of chemical evolution in both dwarf galaxies, and determine whether the chemical abundance patterns seen in galaxies with recent episodes of star formation are a direct continuation of those with only old populations.

I will review our current knowledge of soft gamma repeaters (SGR) and anomalous X-ray pulsars (AXP), two peculiar classes of pulsars believed to be 'magnetars', i.e. neutron stars powered by a huge magnetic field. Recent studies of transient events from SGRs and AXPs allowed a large jump in our understanding of these objects, although they also prompted new unanswered questions. In particular, the recent discovery of a low magnetic field magnetar is causing a re-think of some of the basic ingredients of the magnetar model.

Massive stars lose mass through powerful, radiatively driven stellar winds. Building on the original "CAK" model for steady, spherical winds driven by line-scattering, this talk will review recent research on the multi-faceted nature of such wind mass loss under varied conditions, for example due to rapid rotation, magnetic channeling, binary interaction, or a luminosity near the Eddington limit. An overall theme is that wind mass loss can in this way lead to a wide variety of astrophysical phenomena, including bipolar nebulae, massive star magnetospheres, colliding winds or compact companion accretion, and luminous blue variable eruption. The discussion here will summarize these with an emphasis on their varied observational signatures.

A serious limitation in the study of the Galactic inner halo and bulge globular clusters has been the existence of large and differential extinction by foreground dust. We have mapped the differential extinction and removed its effects, using a new dereddening technique, in a sample of 25 clusters in the direction of the inner Galaxy, observed in the optical using the Magellan 6.5m telescope and the Hubble Space Telescope. We have also observed a sample of 33 inner Galactic globular clusters in the framework of the VVV survey that is currently being conducted with the new Vista 4m telescope, in infrared bands where the extinction is highly reduced. Using these observations we have produced high quality color-magnitude diagrams of these poorly studied clusters that allow us to determine these clusters relative ages, distances and chemistry more accurately and to address important questions about the formation and the evolution of the inner Galaxy.

Supernovae are at the heart of some of the most important problems of modern astronomy. To fully understand their importance and to enable their use as probes of stellar evolution throughout cosmic time, it is
absolutely essential to determine their stellar origins, i.e., their progenitors or progenitor systems. Even with over 5600 known SNe, we have only direct information about the progenitor star for a handful of explosions. Based on the statistics of 20 SNe II-P for which progenitors have been isolated or upper mass limits established, it has been derived a
more limited range of 8-17 solar masses for these stars, and it appears that all of these progenitors exploded in the RSG phase, as we would theoretically expect. However there has been no detection of a higher mass stars in the range 20-40 solar masses, which should be the most luminous and brightest stars in these galaxies. Therefore, I will present here the
results of our group in the analysis of Hubble Space Telescope (HST) and deep ground-based images, isolating the massive progenitor stars of several recent core-collapse supernovae.

Long Gamma-Ray Bursts (GRBs) are the most dramatic examples of massive stellar deaths, usually associated with supernovae (Woosley et al. 2006). They release ultra-relativistic jets producing non-thermal emission through synchrotron radiation as they interact with the surrounding medium (Zhang et al. 2004). Here we report observations of the peculiar GRB 101225A (the "Christmas burst"). Its gamma-ray emission was exceptionally long and followed by a bright X-ray transient with a hot thermal component and an unusual optical counterpart. During the first 10 days, the optical emission evolved as an expanding, cooling blackbody after which an additional component, consistent with a faint supernova, emerged. We determine its distance to 1.6 Gpc by fitting the spectral-energy distribution and light curve of the optical emission with a GRB-supernova template. Deep optical observations may have revealed a faint, unresolved host galaxy. Our proposed progenitor is a helium star-neutron star merger that underwent a common envelope phase expelling its hydrogen envelope. The resulting explosion created a GRB-like jet which gets thermalized by interacting with the dense, previously ejected material and thus creating the observed black-body, until finally the emission from the supernova dominated. An alternative explanation is a minor body falling onto a neutron star in the Galaxy (Campana et al. 2011).

To understand the formation and evolution of galaxies, it is important to have a full comprehension of the role played by Metallicity, Star Formation Rate (SFR), and stellar mass of galaxies. The interplay of these parameters at different redshifts will substantially affect the evolution of galaxies and, as a consequence, the evolution of these parameters provides important constraints for the galaxy evolution models. We studied the relationships and dependencies between the SFR, stellar mass, and gas metallicity of star forming galaxies from the Sloan Digital Sky Survey-Data Release 7 (SDSS DR7) and Galaxy and Mass Assembly (GAMA) surveys. We have combined both surveys finding evidence of SFR and metallicity evolution for galaxies down to redshift ~0.2. Also, we have proved the existence of a Fundamental Plane in the 3D space formed by the SFR, mass and metallicity for the SDSS and GAMA samples.

The vision for the use of the WHT in the coming decade is taking shape.   A key element is the construction and deployment of WEAVE, a wide-field massive-multiplex spectrograph.  With 1000 fibres and spectral resolutions of 5000 and 20000, the opportunities for discovery are tremendous.  Three key fields will be: Milky-Way and Local Group archaeology linked to the   Gaia mission; cosmology redshift surveys; and galaxy evolution studies linked to photometric surveys such as VISTA, UKIDSS, LOFAR, EUCLID, and  others. IAC has the opportunity to get involved in this important instrument for ORM from the beginning.

The Kepler spacecraft is providing photometric time series with micromagnitude precision for thousands of variable stars. The continuous time-series of unprecedented timespan open up the opportunity to study the pulsational variability in much more detail than was previously possible from the ground. We present a first general characterization of the variability of A-F type stars as observed in the Kepler light curves of a sample of 750 candidate variable A-F type stars, and investigate the relation between gamma Doradus, delta Scuti, and hybrid stars. Our results suggest a revision of the current observational instability strips, and imply an investigation of pulsation mechanisms to drive hybrid pulsations.