Found 106 talks archived in Stars
Using the k-means cluster analysis algorithm, we carry out an unsupervised classification of all galaxy spectra in the seventh and final Sloan Digital Sky Survey data release (SDSS/DR7). Except for the shift to rest-frame wavelengths and the normalization to the g-band flux, no manipulation is applied to the original spectra. The algorithm guarantees that galaxies with similar spectra belong to the same class. We find that 99% of the galaxies can be assigned to only 17 major classes, with 11 additional minor classes including the remaining 1%. The classification is not unique since many galaxies appear in between classes; however, our rendering of the algorithm overcomes this weakness with a tool to identify borderline galaxies. Each class is characterized by a template spectrum, which is the average of all the spectra of the galaxies in the class. These low-noise template spectra vary smoothly and continuously along a sequence labeled from 0 to 27, from the reddest class to the bluest class. Our Automatic Spectroscopic K-means-based (ASK) classification separates galaxies in colors, with classes characteristic of the red sequence, the blue cloud, as well as the green valley. When red sequence galaxies and green valley galaxies present emission lines, they are characteristic of active galactic nucleus activity. Blue galaxy classes have emission lines corresponding to star formation regions. We find the expected correlation between spectroscopic class and Hubble type, but this relationship exhibits a high intrinsic scatter. Several potential uses of the ASK classification are identified and sketched, including fast determination of physical properties by interpolation, classes as templates in redshift determinations, and target selection in follow-up works (we find classes of Seyfert galaxies, green valley galaxies, as well as a significant number of outliers). The ASK classification is publicly accessible through various Web sites.
AbstractGamma Dor stars (M = 1.2-2.5 M⊙; spectral type A-F) are very interesting from an asteroseismic point of view. They show gravity modes, which are the only modes that provide information on the deep stellar interior. Observationally they are very challenging targets. Typical pulsation periods are of the order of a day and amplitudes are fairly small (below 0.05 mag; 2 km/s), making it extremely difficult to monitor the periodic variations from the ground. The asteroseismic space missions CoRoT and Kepler are providing uninterrupted time-series from space, with unprecedented accuracy, and hence promise a revolution in the study of gamma Dor stars. I will present results of the seismic analysis of CoRoT and Kepler gamma Dor targets, and the associated ground-based support observations, involving many telescopes at different observatories (including La Palma and Izaña).
We present the new stellar population synthesis models based on the empirical stellar spectral library MILES, which can be regarded nowadays as standard in the field of stellar population studies. The synthetic SEDs cover the whole optical range at resolution 2.3 Å (FWHM). The unprecedented stellar parameter coverage of MILES allowed us to extend our model predictions from intermediate- to very-old age regimes, and the metallicity coverage from super-solar to [M/H] = -2.3. Observed spectra can be studied by means of full spectrum fitting or line-strengths. For the latter we propose a new Line Index System (LIS) to avoid the intrinsic uncertainties associated with the popular Lick/IDS system and provide more appropriate, uniform, spectral resolution. We present a web-page with a suite of on-line tools to facilitate the handling and transformation of the spectra. Online examples with practical applications to work with stellar spectra for a variety of instrumental setups will be shown. Furthermore we will also show examples of how to compute spectra and colors with varying instrumental setup, redshift and velocity dispersion for a suite of Star Formation Histories.
Among the over 450 known exoplanets, the planets that transit their central star stand out, due to the wealth of information that can be gained about both planet and central star. The CoRoT mission has been designed to detect smaller and longer-periodic transiting exoplanets than can be found from ground observations. CoRoT-9b was detected by the satellite in summer 2008 and underwent follow-up observations from ground for another year. It stands out as having the largest periastron distance of all transiting planets, being expected to maintain permanently a moderate surface temperature, estimated between 250 and 430K. It is also the first exoplanet to which planet evolution models can be applied, without uncertain corrections that have been needed for 'hot' transiting planets. These models indicate it to be rather similar to Jupiter. Temperate gas-giant planets with low-to-moderate eccentric orbits constitute the largest group of currently known planets; they are probably similar to the gas giants of the solar system. With CoRoT-9b being this group’s first transiting planet, it may give rise to a much better understanding of these common planets. While CoRoT-9b itself is certainly not habitable, moons around it could be similar to Titan and provide some chance of habitability. Upcoming observations with the Spitzer space telescope are designed to improve on planet parameters and to perform a deeper search for the detection of its moons.
AbstractThe current databases of empirical star spectra for modelling single-aged stellar populations (SSPs) generally do not chemically characterize their stars completely. Spectral properties of stars and their populations may change considerably if the elemental abundance ratios E/Fe differ from the solar-scaled values. We intend to build up robust integrated spectral energy distribution of SSPs older than 1 Gyr by adopting the MILES database (Medium-resolution Isaac Newton Telescope Library of Empirical Spectra) and taking into account the Mg/Fe ratio of its stars. Magnesium is a proxy of the alpha-capture elements and the alpha/Fe ratio has been widely used as an indicator of the star formation time scale. In this talk, I present how accurate and extensive our compilation and determination of [Mg/Fe] were obtained around MILES to compute state-of-the-art SSP models. Published high resolution measurements were adopted to define a uniform scale of [Mg/Fe] and calibrate our results at medium resolution that were based on the spectral synthesis of two strong Mg features.
Up to now more than 400 extrasolar planets have been discovered, about 60 of them are transiting. Transiting extra-solar planets are particularly interesting, because their masses, diameters, densities and orientations of their orbits can be determined. Observations with the CoRoT Satellite have now turned up 10 transiting extrasolar planets. Although most of them are gas giants, it turns out that each of them is very special, and many of them have surprising properties. An unexpected discovery was for example the detection of emission lines from CoRoT 1b. Other interesting discoveries are CoRoT 2b, a planet orbiting a young star, and CoRoT 3b the first transiting brown dwarf orbiting a main sequence star. While the detection of transiting gas giants is interesting, the ultimate goal of CoRoT clearly was the detection of rocky planets. CoRoT has detected a solar-like star which shows transits that are only 0.03% deep. In this talk it it is demonstrated that this planet is in fact the first planet found outside our solar system from which we can firmly say that it is a rocky planet. New observations of this interesting object even constrain the properties of its exosphere.
AbstractIn this Breaking News seminar, I will describe our project dedicated to the search for ultracool low-metallicity dwarfs (or subdwarfs) in the large-scale databases. The highlight of the seminar is the discovery of a mid-L subdwarf, the fifth known to date, and the first one identified in the UKIRT Infrared Deep Sky Survey (UKIDSS). The spectroscopic nature of this subdwarf was confirmed with data obtained with GTC/OSIRIS in April 2009.
The formation of massive stars is not fully understood. The high luminosity and temperature of massive protostars complicate the accretion mechanism at work in intermediate and low mass young stellar objects. Nonetheless, several scenarios exist to explain the formation of massive stars. In this talk, we will focus on the process of triggered star formation on the borders of H II regions. Due to the feedback effects of OB stars, a layer of molecular material is collected during the expansion of the H II region. Instabilities develop in this layer and give birth to new stars. We will present a detailed study of three Galactic H II regions (RCW79, RCW82 and RCW120). Near-infrared integral field observations have been carried out with SINFONI on the VLT. We will see how they reveal the nature of both the ionizing stars and of the YSOs in the collected layer and how they support the scenario of 'triggered star formation'.
AbstractThe surface abundance of lithium on the Sun is 140 times less than protosolar, yet the temperature at the base of the surface convective zone is not hot enough to burn Li. A large range of Li abundances in solar type stars of the same age, mass and metallicity is observed, but theoretically difficult to understand. An earlier suggestion that Li is more depleted in stars with planets was weakened by the lack of a proper comparison sample of stars without detected planets. Here we report Li abundances for an unbiased sample of solar-analogue stars with and without detected planets. We find that the planet-bearing stars have less than 1 per cent of the primordial Li abundance, while about 50 per cent of the solar analogues without detected planets have on average 10 times more Li. The presence of planets may increase the amount of mixing and deepen the convective zone to such an extent that the Li can be burned. We also present Be abundances for a sample of stars with and without known planets and discuss the possible relation of these light element with the presence of planetary systems.
I present some recent results from our Optical and NIR studies of five short period low-mass X-ray binaries (LMXB's; X1822-371, X1957+115, UW CrB, X1916-05 and X0614+091). Optical photometry and spectroscopy reveal some surprising results on the geometry and evolution of accretions discs in LMXB's. Based on our data, it is increasingly clear that accretion discs in these systems are far from being stable and must undergo substantial precession and/or warping behaviour on timescales less than a day in case of the shortest period systems.
- The participation of the IAC Solar System group in the OSIRIS-REx mission: date to be confirmed!Dr. Javier LicandroThursday April 12, 2018 - 10:30
- Finding the double sunsets: close binary stars, large spectroscopic surveysDr. Carles BadenesThursday May 3, 2018 - 10:30