arXiv subjects

D. A. García-Hernández

Publications and source records attributed to D. A. García-Hernández.

108 records · Page 6Linked to original sources

arXiv2015

Constraint on the time variation of the fine-structure constant with the SDSS-III/BOSS DR12 quasar sample

From the Sloan Digital Sky Survey (SDSS) Data Release 12, which covers the full Baryonic Oscillation Spectroscopic Survey (BOSS) footprint, we investigate the possible variation of the fine-structure constant over cosmological time-scales. We analyse the largest quasar sample considered so far in the literature, which contains 13175 spectra (10363 from SDSS-III/BOSS DR12 + 2812 from SDSS-II DR7) with redshift $z<\,$1. We apply the emission-line method on the [O III] doublet (4960, 5008 A) and obtain $Δα/α= \left(0.9 \pm 1.8\right)\times10^{-5}$ for the relative variation of the fine-structure constant. We also investigate the possible sources of systematics: misidentification of the lines, sky OH lines, H$\,β$ and broad line contamination, Gaussian and Voigt fitting profiles, optimal wavelength range for the Gaussian fits, chosen polynomial order for the continuum spectrum, signal-to-noise ratio and good quality of the fits. The uncertainty of the measurement is dominated by the sky subtraction. The results presented in this work, being systematics limited, have sufficient statistics to constrain robustly the variation of the fine-structure constant in redshift bins ($Δz\approx$ 0.06) over the last 7.9 Gyr. In addition, we study the [Ne III] doublet (3870, 3969 A) present in 462 quasar spectra and discuss the systematic effects on using these emission lines to constrain the fine-structure constant variation. Better constraints on $Δα/α $ ($<$10$^{-6}$) using the emission-line method would be possible with high-resolution spectroscopy and large galaxy/qso surveys.

astro-ph.CO↗

arXiv2015

A test for asymptotic giant branch evolution theories: Planetary Nebulae in the Large Magellanic Cloud

We used a new generation of asymptotic giant branch (AGB) stellar models that include dust formation in the stellar winds to find the links between evolutionary models and the observed properties of a homogeneous sample of Large Magellanic Cloud (LMC) planetary nebulae (PNe). Comparison between the evolutionary yields of elements such as CNO and the corresponding observed chemical abundances is a powerful tool to shed light on evolutionary processes such as hot bottom burning (HBB) and third dredge-up (TDU). We found that the occurrence of HBB is needed to interpret the nitrogen-enriched (log(N/H)+12>8) PNe. In particular, N-rich PNe with the lowest carbon content are nicely reproduced by AGB models of mass M >=6 Mo, whose surface chemistry reflects the pure effects of HBB. PNe with log(N/H)+12<7.5 correspond to ejecta of stars that have not experienced HBB, with initial mass below about 3 Mo. Some of these stars show very large carbon abundances, owing to the many TDU episodes experienced. We found from our LMC PN sample that there is a threshold to the amount of carbon accumulated at AGB surfaces, log(C/H)+12<9. Confirmation of this constraint would indicate that, after the C-star stage is reached,AGBs experience only a few thermal pulses, which suggests a rapid loss of the external mantle, probably owing to the effects of radiation pressure on carbonaceous dust particles present in the circumstellar envelope. The implications of these findings for AGB evolution theories and the need to extend the PN sample currently available are discussed.

astro-ph.SR↗

arXiv2015

Young alpha-enriched giant stars in the solar neighbourhood

We derive age constraints for 1639 red giants in the APOKASC sample for which seismic parameters from Kepler, as well as effective temperatures, metallicities and [alpha/Fe] values from APOGEE DR12 are available. We investigate the relation between age and chemical abundances for these stars, using a simple and robust approach to obtain ages. We first derive stellar masses using standard seismic scaling relations, then determine the maximum possible age for each star as function of its mass and metallicity, independently of its evolutionary stage. While the overall trend between maximum age and chemical abundances is a declining fraction of young stars with increasing [alpha/Fe], at least 14 out of 241 stars with [alpha/Fe]>0.13 are younger than 6 Gyr. Five stars with [alpha/Fe]>0.2 have ages below 4 Gyr. We examine the effect of modifications in the standard seismic scaling relations, as well as the effect of very low helium fractions, but these changes are not enough to make these stars as old as usually expected for alpha-rich stars (i.e., ages greater than 8-9 Gyr). Such unusual alpha-rich young stars have also been detected by other surveys, but defy simple explanations in a galaxy evolution context.

astro-ph.GA↗

arXiv2015

The Eleventh and Twelfth Data Releases of the Sloan Digital Sky Survey: Final Data from SDSS-III

The third generation of the Sloan Digital Sky Survey (SDSS-III) took data from 2008 to 2014 using the original SDSS wide-field imager, the original and an upgraded multi-object fiber-fed optical spectrograph, a new near-infrared high-resolution spectrograph, and a novel optical interferometer. All the data from SDSS-III are now made public. In particular, this paper describes Data Release 11 (DR11) including all data acquired through 2013 July, and Data Release 12 (DR12) adding data acquired through 2014 July (including all data included in previous data releases), marking the end of SDSS-III observing. Relative to our previous public release (DR10), DR12 adds one million new spectra of galaxies and quasars from the Baryon Oscillation Spectroscopic Survey (BOSS) over an additional 3000 sq. deg of sky, more than triples the number of H-band spectra of stars as part of the Apache Point Observatory (APO) Galactic Evolution Experiment (APOGEE), and includes repeated accurate radial velocity measurements of 5500 stars from the Multi-Object APO Radial Velocity Exoplanet Large-area Survey (MARVELS). The APOGEE outputs now include measured abundances of 15 different elements for each star. In total, SDSS-III added 2350 sq. deg of ugriz imaging; 155,520 spectra of 138,099 stars as part of the Sloan Exploration of Galactic Understanding and Evolution 2 (SEGUE-2) survey; 2,497,484 BOSS spectra of 1,372,737 galaxies, 294,512 quasars, and 247,216 stars over 9376 sq. deg; 618,080 APOGEE spectra of 156,593 stars; and 197,040 MARVELS spectra of 5,513 stars. Since its first light in 1998, SDSS has imaged over 1/3 of the Celestial sphere in five bands and obtained over five million astronomical spectra.

astro-ph.IM↗

arXiv2015

Rapid Rotation of Low-Mass Red Giants Using APOKASC: A Measure of Interaction Rates on the Post-main-sequence

We investigate the occurrence rate of rapidly rotating ($v\sin i$$>$10 km s$^{-1}$), low-mass giant stars in the APOGEE-Kepler (APOKASC) fields with asteroseismic mass and surface gravity measurements. Such stars are likely merger products and their frequency places interesting constraints on stellar population models. We also identify anomalous rotators, i.e. stars with 5 km s$^{-1}$$<$$v\sin i$$<$10 km s$^{-1}$ that are rotating significantly faster than both angular momentum evolution predictions and the measured rates of similar stars. Our data set contains fewer rapid rotators than one would expect given measurements of the Galactic field star population, which likely indicates that asteroseismic detections are less common in rapidly rotating red giants. The number of low-mass moderate (5-10 km s$^{-1}$) rotators in our sample gives a lower limit of 7% for the rate at which low-mass stars interact on the upper red giant branch because single stars in this mass range are expected to rotate slowly. Finally, we classify the likely origin of the rapid or anomalous rotation where possible. KIC 10293335 is identified as a merger product and KIC 6501237 is a possible binary system of two oscillating red giants.

astro-ph.SR↗

arXiv2015

The Large Magellanic Cloud as a laboratory for Hot Bottom Burning in massive Asymptotic Giant Branch stars

We use Spitzer observations of the rich population of Asymptotic Giant Branch stars in the Large Magellanic Cloud (LMC) to test models describing the internal structure and nucleosynthesis of the most massive of these stars, i.e. those with initial mass above $\sim 4M_{\odot}$. To this aim, we compare Spitzer observations of LMC stars with the theoretical tracks of Asymptotic Giant Branch models, calculated with two of the most popular evolution codes, that are known to differ in particular for the treatment of convection. Although the physical evolution of the two models are significantly different, the properties of dust formed in their winds are surprisingly similar, as is their position in the colour-colour (CCD) and colour-magnitude (CMD) diagrams obtained with the Spitzer bands. This model independent result allows us to select a well defined region in the ($[3.6]-[4.5], [5.8]-[8.0]$) plane, populated by AGB stars experiencing Hot Bottom Burning, the progeny of stars with mass $M\sim 5.5M_{\odot}$. This result opens up an important test of the strength hot bottom burning using detailed near-IR (H and K bands) spectroscopic analysis of the oxygen-rich, high luminosity candidates found in the well defined region of the colour-colour plane. This test is possible because the two stellar evolution codes we use predict very different results for the surface chemistry, and the C/O ratio in particular, owing to their treatment of convection in the envelope and of convective boundaries during third dredge-up. The differences in surface chemistry are most apparent when the model stars reach the phase with the largest infrared emission.

astro-ph.SR↗

arXiv2014

AGB stars in the LMC: evolution of dust in circumstellar envelopes

We calculated theoretical evolutionary sequences of asymptotic giant branch (AGB) stars, including formation and evolution of dust grains in their circumstellar envelope. By considering stellar populations of the Large Magellanic Cloud (LMC), we calculate synthetic colour-colour and colour-magnitude diagrams, which are compared with those obtained by the Spitzer Space Telescope. The comparison between observations and theoretical predictions outlines that extremely obscured carbon-stars and oxygen-rich sources experiencing hot bottom burning (HBB) occupy well defined, distinct regions in the colour-colour ($[3.6]-[4.5]$, $[5.8]-[8.0]$) diagram. The C-rich stars are distributed along a diagonal strip that we interpret as an evolutionary sequence, becoming progressively more obscured as the stellar surface layers enrich in carbon. Their circumstellar envelopes host solid carbon dust grains with size in the range $0.05 < a < 0.2 μm$. The presence of SiC particles is expected only in the more metal-rich stars. The reddest sources, with $[3.6]-[4.5] > 2$, are the descendants of stars with initial mass $M_{in} \sim 2.5 - 3 M_{\odot}$ in the very latest phases of the AGB life. The oxygen-rich stars with the reddest colours ($[5.8]-[8.0] > 0.6$) are those experiencing HBB, the descendants of $\sim 5 M_{\odot}$ objects formed $10^{8}$ yr ago; alumina and silicates dust start forming at different distances from the central star. The overall dust production rate in the LMC is $\sim 4.5 \times 10^{-5} M_{\odot}/yr$, the relative percentages due to C- and M- star being respectively 85$%$ and 15 $%$.

astro-ph.SR↗

arXiv2014

Tracking Advanced Planetary Systems with HARPS-N (TAPAS). I. A multiple planetary system around the red giant star TYC 1422-614-1

Context. Stars that have evolved-off the Main Sequence are crucial in expanding the frontiers of knowledge on exoplanets toward higher stellar masses, and to constrain star-planet interaction mechanisms. These stars, however suffer from intrinsic activity that complicates the interpretation of precise radial velocity measurement and are often avoided in planet searches. We have, over the last 10 years, monitored about 1000 evolved stars for radial velocity variations in search for low-mass companions under the Penn State - Toruń Centre for Astronomy Planet Search with the Hobby-Eberly Telescope. Selected prospective candidates that required higher RV precision meassurements have been followed with HARPS-N at the 3.6 m Telescopio Nazionale Galileo. Aims. To detect planetary systems around evolved stars, to be able to build sound statistics on the frequency and intrinsic nature of these systems, and to deliver in-depth studies of selected planetary systems with evidences of star-planet interaction processes. Methods. We have obtained for TYC 1422-614-1 69 epochs of precise radial velocity measurements collected over 3651 days with the Hobby-Eberly Telescope, and 17 epochs of ultra precise HARPS-N data collected over 408 days. We have complemented these RV data with photometric time-series from the All Sky Automatic Survey archive. Results. We report the discovery of a multiple planetary system around the evolved K2 giant star TYC 1422-614-1. The system orbiting the 1.15 M$_\odot$ star is composed of a planet with mass m$sin i$=2.5 M$_J$ in a 0.69 AU orbit, and a planet/brown dwarf with m$sin i$=10 M$_J$ in a 1.37 AU orbit. The multiple planetary system orbiting TYC 1422-614-1 is the first finding of the TAPAS project, a HARPS-N monitoring of evolved planetary systems identified with the Hobby-Eberly Telescope.

astro-ph.EP↗

arXiv2014

On the alumina dust production in the winds of O-rich Asymptotic Giant Branch stars

The O-rich Asymptotic Giant Branch (AGB) stars experience strong mass loss with efficient dust condensation and they are major sources of dust in the interstellar medium. Alumina dust (Al$_2$O$_3$) is an important dust component in O-rich circumstellar shells and it is expected to be fairly abundant in the winds of the more massive and O-rich AGB stars. By coupling AGB stellar nucleosynthesis and dust formation, we present a self-consistent exploration on the Al$_2$O$_3$ production in the winds of AGB stars with progenitor masses between $\sim$3 and 7 M$_{\odot}$ and metallicities in the range 0.0003 $\le$ Z $\le$ 0.018. We find that Al$_2$O$_3$ particles form at radial distances from the centre between $\sim2$ and 4 R$_*$ (depending on metallicity), which is in agreement with recent interferometric observations of Galactic O-rich AGB stars. The mass of Al$_2$O$_3$ dust is found to scale almost linearly with metallicity, with solar metallicity AGBs producing the highest amount (about 10$^{-3}$ M$_{\odot}$) of alumina dust. The Al$_2$O$_3$ grain size decreases with decreasing metallicity (and initial stellar mass) and the maximum size of the Al$_2$O$_3$ grains is $\sim$0.075 $μm$ for the solar metallicity models. Interestingly, the strong depletion of gaseous Al observed in the low-metallicity HBB AGB star HV 2576 seems to be consistent with the formation of Al$_2$O$_3$ dust as predicted by our models. We suggest that the content of Al may be used as a mass (and evolutionary stage) indicator in AGB stars experiencing HBB.

astro-ph.SR↗

arXiv2014

The Tenth Data Release of the Sloan Digital Sky Survey: First Spectroscopic Data from the SDSS-III Apache Point Observatory Galactic Evolution Experiment

The Sloan Digital Sky Survey (SDSS) has been in operation since 2000 April. This paper presents the tenth public data release (DR10) from its current incarnation, SDSS-III. This data release includes the first spectroscopic data from the Apache Point Observatory Galaxy Evolution Experiment (APOGEE), along with spectroscopic data from the Baryon Oscillation Spectroscopic Survey (BOSS) taken through 2012 July. The APOGEE instrument is a near-infrared R~22,500 300-fiber spectrograph covering 1.514--1.696 microns. The APOGEE survey is studying the chemical abundances and radial velocities of roughly 100,000 red giant star candidates in the bulge, bar, disk, and halo of the Milky Way. DR10 includes 178,397 spectra of 57,454 stars, each typically observed three or more times, from APOGEE. Derived quantities from these spectra (radial velocities, effective temperatures, surface gravities, and metallicities) are also included.DR10 also roughly doubles the number of BOSS spectra over those included in the ninth data release. DR10 includes a total of 1,507,954 BOSS spectra, comprising 927,844 galaxy spectra; 182,009 quasar spectra; and 159,327 stellar spectra, selected over 6373.2 square degrees.

astro-ph.IM↗

arXiv2012

The second release of the Toruń catalogue of Galactic post-AGB objects: new classification scheme

The investigation of post-AGB objects (proto-planetary nebulae) is very important from the standpoint of physical and chemical changes occurring during the late stages of stellar evolution. The Toruń catalogue of Galactic post-AGB and related objects is an evolutive catalogue containing astrometric, photometric and spectroscopic data as well as HST images for all known post-AGB objects and candidates in our Galaxy. This free-access catalogue can serve as an ideal tool to study different groups of post-AGB objects, especially due to the fact that all information is gathered in one place. The second release of our catalogue introduces a simple classification scheme of post-AGB objects and includes a significant number of new objects, photometric data, spectra and images. Here, using objects from the catalogue we consider the problem of the termination of the AGB phase.

astro-ph.SR↗

arXiv2012

Short-lived radioactivity in the early Solar System: the Super-AGB star hypothesis

The composition of the most primitive Solar System condensates, such as calcium-aluminum-rich inclusions (CAI) and micron-sized corundum grains, show that short-lived radionuclides (SLR), e.g., 26Al, were present in the early Solar System. Their abundances require a local origin, which however is far from being understood. We present for the first time the abundances of several SLR up to 60Fe predicted from stars with initial mass in the range roughly 7-11 Msun. These stars evolve through core H, He, and C burning. After core C burning they go through a "Super"-asymptotic giant branch (Super-AGB) phase, with the H and He shells activated alternately, episodic thermal pulses in the He shell, a very hot temperature at the base of the convective envelope (~ 10^8 K), and strong stellar winds driving the H-rich envelope into the surrounding interstellar medium. The final remnants of the evolution of Super-AGB stars are mostly O-Ne white dwarfs. Our Super-AGB models produce 26Al/27Al yield ratios ~ 0.02 - 0.26. These models can account for the canonical value of the 26Al/27Al ratio using dilutions with the Solar Nebula of the order of 1 part of Super-AGB mass per several 10^2 to several 10^3 of Solar Nebula mass, resulting in associated changes in the O composition in the range 3 permil to 20 permil. This is in agreement with observations of the O isotopic ratios in primitive Solar System condensated, which do not carry the signature of a stellar pollutor. The radionuclides 41Ca and 60Fe are produced by neutron captures in Super-AGB stars and their meteoritic abundances are also matched by some of our models, depending on the nuclear and stellar physics uncertainties as well as the meteoritic experimental data. We also expect and are currently investigating Super-AGB production of SLR heavier than iron, such as 107Pd.

astro-ph.SR↗

arXiv2012

The Ninth Data Release of the Sloan Digital Sky Survey: First Spectroscopic Data from the SDSS-III Baryon Oscillation Spectroscopic Survey

The Sloan Digital Sky Survey III (SDSS-III) presents the first spectroscopic data from the Baryon Oscillation Spectroscopic Survey (BOSS). This ninth data release (DR9) of the SDSS project includes 535,995 new galaxy spectra (median z=0.52), 102,100 new quasar spectra (median z=2.32), and 90,897 new stellar spectra, along with the data presented in previous data releases. These spectra were obtained with the new BOSS spectrograph and were taken between 2009 December and 2011 July. In addition, the stellar parameters pipeline, which determines radial velocities, surface temperatures, surface gravities, and metallicities of stars, has been updated and refined with improvements in temperature estimates for stars with T_eff<5000 K and in metallicity estimates for stars with [Fe/H]>-0.5. DR9 includes new stellar parameters for all stars presented in DR8, including stars from SDSS-I and II, as well as those observed as part of the SDSS-III Sloan Extension for Galactic Understanding and Exploration-2 (SEGUE-2). The astrometry error introduced in the DR8 imaging catalogs has been corrected in the DR9 data products. The next data release for SDSS-III will be in Summer 2013, which will present the first data from the Apache Point Observatory Galactic Evolution Experiment (APOGEE) along with another year of data from BOSS, followed by the final SDSS-III data release in December 2014.

astro-ph.IM↗

arXiv2012

Heavy Element Nucleosynthesis in the Brightest Galactic Asymptotic Giant Branch stars

We present updated calculations of stellar evolutionary sequences and detailed nucleosynthesis predictions for the brightest asymptotic giant branch (AGB) stars in the Galaxy with masses between 5Msun to 9Msun, with an initial metallicity of Z =0.02 ([Fe/H] = 0.14). In our previous studies we used the Vassiliadis & Wood mass-loss rate, which stays low until the pulsation period reaches 500 days after which point a superwind begins. Vassiliadis & Wood noted that for stars over 2.5Msun the superwind should be delayed until P ~ 750 days at 5Msun. We calculate evolutionary sequences where we delay the onset of the superwind to pulsation periods of P ~ 700-800 days in models of M = 5, 6, and 7Msun. Post-processing nucleosynthesis calculations show that the 6 and 7Msun models produce the most Rb, with [Rb/Fe] ~ 1 dex, close to the average of most of the Galactic Rb-rich stars ([Rb/Fe] ~ 1.4 plus or minus 0.8 dex). Changing the rate of the 22Ne + alpha reactions results in variations of [Rb/Fe] as large as 0.5 dex in models with a delayed superwind. The largest enrichment in heavy elements is found for models that adopt the NACRE rate of the 22Ne(a,n)25Mg reaction. Using this rate allows us to best match the composition of most of the Rb-rich stars. A synthetic evolution algorithm is then used to remove the remaining envelope resulting in final [Rb/Fe] of ~ 1.4 dex although with C/O ratios > 1. We conclude that delaying the superwind may account for the large Rb overabundances observed in the brightest metal-rich AGB stars.

astro-ph.SR↗

arXiv2009

AKARI/IRC observations of heavily obscured oxygen-rich AGB and post-AGB stars

We present AKARI/IRC observations of a sample of six extremely red IRAS sources, of which three are variable OH/IR stars and the rest are early post-AGB stars. The OH/IR stars show a red continuum with the expected strong 10 micron silicate absorption feature, while the post-AGB stars show an even redder continuum accompanied with a comparably weak silicate absorption. We modelled the spectral energy distributions with DUSTY. While for the OH/IR stars a reasonable fit can be obtained with almost pure silicate dust, the post-AGB stars require a mixture of silicate and carbon-rich dust. We assume that in the latter objects the inner dust shell is carbon-rich, while the outer shells are still oxygen-rich.

astro-ph.SR↗

arXiv2009

AKARI and Spitzer observations of heavily obscured C-rich AGB/post-AGB stars

We present AKARI/IRC and Spitzer/IRS observations of a selected sample of galactic IRAS sources considered to be heavily obscured AGB/post-AGB stars based on their characteristic IRAS colours. All of them are completely invisible in the optical range but extremely bright in the infrared. Based on AKARI and Spitzer spectroscopy and using DUSTY we are able to determine the dominant chemistry of their circumstellar shells as well as the properties of the dust grains contained in these shells. Most of the sources are found to be C-rich (being the reddest C-rich stars observed so far). We find only molecular absorptions (and no PAH features) such as acetylene (C2H2) at 13.7 micron, indicative of an early post-AGB stage. We shortly discuss our findings in the context of stellar evolution during the hidden "transition phase" from AGB stars to Planetary Nebulae.

astro-ph.SR↗

arXiv2009

The transition from AGB to post-AGB evolution as observed by AKARI and Spitzer

The AKARI and Spitzer satellites provided an unique opportunity to observe a variety of stars, which are considered as departing from the Asymptotic Giant Branch (AGB) and have started their post-AGB evolution recently. Most of these stars are absent optically and are bright in the mid-IR wavelength range. Spectra of close to 200 objects have been obtained. For all of them the 1-60 micron spectral energy distribution has been constructed using photometric data from various surveys. We report here on the results of Spitzer observations of 88 IRAS selected post-AGB candidates and discuss them in comparison to the results of the AKARI observations of post-AGB candidates reported elsewhere in these proceedings. The dust compositions can be divided broadly in oxygen- and carbon-rich types, but a variety of intermediate types have been found. Among the oxygen-rich stars amorphous dust prevails, but a few sources show emission features from crystalline dust. The spectra from carbon-rich shells may be completely featureless, may show emission features from PAHs or a molecular absorption line from C2H2. We found also sources with a neon emission line at 12.8 micron. More than a third of all sources show a near-infrared excess at < 5 micron and almost all of them show evidence of C-rich dust in their shells. We postulate that the emerging post-AGB wind after the end of AGB evolution contains always carbon-rich dust irrespective of the chemistry of the former AGB star.

astro-ph.SR↗

arXiv2008

Evidence for the naphthalene cation in a region of the interstellar medium with anomalous microwave emission

We report high resolution spectroscopy of the moderately reddened (A$_V$=3) early type star Cernis 52 located in a region of the Perseus molecular cloud complex with anomalous microwave emission. In addition to the presence of the most common diffuse interstellar bands (DIBs) we detect two new interstellar or circumstellar bands coincident to within 0.01% in wavelength with the two strongest bands of the naphthalene cation (C$_{10}$H$_{8}^+$) as measured in gas-phase laboratory spectroscopy at low temperatures and find marginal evidence for the third strongest band. Assuming these features are caused by the naphthalene cation, from the measured intensity and available oscillator strengths we find that 0.008 % of the carbon in the cloud could be in the form of this molecule. We expect hydrogen additions to cause hydronaphthalene cations to be abundant in the cloud and to contribute via electric dipole radiation to the anomalous microwave emission. The identification of new interstellar features consistent with transitions of the simplest polycyclic aromatic hydrocarbon adds support to the hypothesis that this type of molecules are the carriers of both diffuse interstellar bands and anomalous microwave emission.

astro-ph↗