arXiv subjects

E. J. Alfaro

Publications and source records attributed to E. J. Alfaro.

At least 55 records · Page 3Linked to original sources

arXiv2016

The young open cluster NGC 7067 using Strömgren photometry

NGC 7067 is a young open cluster located in the direction between the first and the second Galactic quadrants and close to the Perseus spiral arm. This makes it useful for studies of the nature of the Milky Way spiral arms. Stromgren photometry taken with the Wide Field Camera at the Isaac Newton Telescope allowed us to compute individual physical parameters for the observed stars and hence to derive cluster's physical parameters. Spectra from the 1.93-m telescope at the Observatoire de Haute-Provence helped to check and improve the results. We obtained photometry for 1233 stars, individual physical parameters for 515 and spectra for 9 of them. The 139 selected cluster members lead to a cluster distance of 4.4+/-0.4 kpc, with an age below log10(t(yr))=7.3 and a present Mass of 1260+/-160Msun. The morphology of the data reveals that the centre of the cluster is at (ra,dec)=(21:24:13.69,+48:00:39.2) J2000, with a radius of 6.1arcsec. Stromgren and spectroscopic data allowed us to improve the previous parameters available for the cluster in the literature.

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arXiv2016

Analysis of the Kinematic Structure of the Cygnus OB1 association

The main objective of this study is the characterization of the velocity field in the Cygnus OB1 association using the radial velocity data currently available in the literature. This association is part of a larger star-forming complex located in the direction of the Cygnus region, but whose main subsystems may be distributed at different distances from the sun. We have collected radial velocity data for more than 300 stars in the area of 5 x 5 square degrees centred on the Cygnus OB1 association. We present the results of a kinematic clustering analysis in the subspace of the phase space formed by angular coordinates and radial velocity using two independent methodologies. We have found evidence of structure in the phase space with the detection of two main groups, corresponding to different radial velocity and distance values, belonging to the association, and associated with two main shells defined by the Halpha emission. A third grouping well separated from the other two in velocity appears to occupy the whole region associated with what has been called "common shell".

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arXiv2016

The Gaia-ESO Survey: Lithium depletion in the Gamma Velorum cluster and inflated radii in low-mass pre-main-sequence stars

We show that non-magnetic models for the evolution of pre-main-sequence (PMS) stars *cannot* simultaneously describe the colour-magnitude diagram (CMD) and the pattern of lithium depletion seen in the cluster of young, low-mass stars surrounding $γ^2$ Velorum. The age of 7.5+/-1 Myr inferred from the CMD is much younger than that implied by the strong Li depletion seen in the cluster M-dwarfs and the Li depletion occurs at much redder colours than predicted. The epoch at which a star of a given mass depletes its Li and the surface temperature of that star are both dependent on its radius. We demonstrate that if the low-mass stars have radii ~10 per cent larger at a given mass and age, then both the CMD and Li depletion pattern of the Gamma Vel cluster are explained at a common age of 18-21 Myr. This radius inflation could be produced by some combination of magnetic suppression of convection and extensive cool starspots. Models that incorporate radius inflation suggest that PMS stars similar to those in the Gamma Vel cluster, in the range 0.2 30 per cent) than inferred from conventional, non-magnetic models in the Hertzsprung-Russell diagram. Systematic changes of this size may be of great importance in understanding the evolution of young stars, disc lifetimes and the formation of planetary systems.

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arXiv2016

The Gaia-ESO Survey: Probes of the inner disk abundance gradient

The nature of the metallicity gradient inside the solar circle (R_GC < 8 kpc) is poorly understood, but studies of Cepheids and a small sample of open clusters suggest that it steepens in the inner disk. We investigate the metallicity gradient of the inner disk using a sample of inner disk open clusters that is three times larger than has previously been studied in the literature to better characterize the gradient in this part of the disk. We used the Gaia-ESO Survey (GES) [Fe/H] values and stellar parameters for stars in 12 open clusters in the inner disk from GES-UVES data. Cluster mean [Fe/H] values were determined based on a membership analysis for each cluster. Where necessary, distances and ages to clusters were determined via comparison to theoretical isochrones. The GES open clusters exhibit a radial metallicity gradient of -0.10+-0.02 dex/kpc, consistent with the gradient measured by other literature studies of field red giant stars and open clusters in the range R_GC ~ 6-12 kpc. We also measure a trend of increasing [Fe/H] with increasing cluster age, as has also been found in the literature. We find no evidence for a steepening of the inner disk metallicity gradient inside the solar circle as earlier studies indicated. The age-metallicity relation shown by the clusters is consistent with that predicted by chemical evolution models that include the effects of radial migration, but a more detailed comparison between cluster observations and models would be premature.

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arXiv2016

The Galactic O-Star Spectroscopic Survey (GOSSS). III. 142 additional O-type systems

This is the third installment of GOSSS, a massive spectroscopic survey of Galactic O stars, based on new homogeneous, high signal-to-noise ratio, R~2500 digital observations selected from the Galactic O-Star Catalog (GOSC). In this paper we present 142 additional stellar systems with O stars from both hemispheres, bringing the total of O-type systems published within the project to 590. Among the new objects there are 20 new O stars. We also identify 11 new double-lined spectroscopic binaries (SB2s), of which 6 are of O+O type and 5 of O+B type, and an additional new tripled-lined spectroscopic binary (SB3) of O+O+B type. We also revise some of the previous GOSSS classifications, present some egregious examples of stars erroneously classified as O-type in the past, introduce the use of luminosity class IV at spectral types O4-O5.5, and adapt the classification scheme to the work of Arias et al. (2016).

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arXiv2016

The Gaia-ESO Survey: membership and Initial Mass Function of the Gamma Velorum cluster

Understanding the properties of young open clusters, such as the Initial Mass Function (IMF), star formation history and dynamic evolution, is crucial to obtain reliable theoretical predictions of the mechanisms involved in the star formation process. We want to obtain a list, as complete as possible, of confirmed members of the young open cluster Gamma Velorum, with the aim of deriving general cluster properties such as the IMF. We used all available spectroscopic membership indicators within the Gaia-ESO public archive together with literature photometry and X-ray data and, for each method, we derived the most complete list of candidate cluster members. Then, we considered photometry, gravity and radial velocities as necessary conditions to select a subsample of candidates whose membership was confirmed by using the lithium and H$α$ lines and X-rays as youth indicators. We found 242 confirmed and 4 possible cluster members for which we derived masses using very recent stellar evolutionary models. The cluster IMF in the mass range investigated in this study shows a slope of $α=2.6\pm0.5$ for $0.5<M/M_\odot <1.3$ and $α=1.1\pm0.4$ for $0.16<M/M_\odot <0.5$ and is consistent with a standard IMF. The similarity of the IMF of the young population around $γ^2 $Vel to that in other star forming regions and the field suggests it may have formed through very similar processes.

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arXiv2016

The Gaia-ESO Survey: Dynamical Analysis of the L1688 region in Ophiuchus

The Gaia ESO Public Spectroscopic Survey (GES) is providing the astronomical community with high-precision measurements of many stellar parameters including radial velocities (RVs) of stars belonging to several young clusters and star-forming regions. One of the main goals of the young cluster observations is to study of their dynamical evolution and provide insight into their future, revealing if they will eventually disperse to populate the field, rather than evolve into bound open clusters. In this paper we report the analysis of the dynamical state of L1688 in the $ρ$~Ophiuchi molecular cloud using the dataset provided by the GES consortium. We performed the membership selection of the more than 300 objects observed. Using the presence of the lithium absorption and the location in the Hertzspung-Russell diagram, we identify 45 already known members and two new association members. We provide accurate RVs for all 47 confirmed members.A dynamical analysis, after accounting for unresolved binaries and errors, shows that the stellar surface population of L1688 has a velocity dispersion $σ\sim$1.14$\pm$0.35 km s$^{-1}$ that is consistent with being in virial equilibrium and is bound with a $\sim$80% probability. We also find a velocity gradient in the stellar surface population of $\sim$1.0 km s$^{-1}$pc$^{-1}$ in the northwest/southeast direction, which is consistent with that found for the pre-stellar dense cores, and we discuss the possibility of sequential and triggered star formation in L1688.

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arXiv2016

The evolution of Balmer jump selected galaxies in the ALHAMBRA survey

We present a new color-selection technique, based on the Bruzual & Charlot models convolved with the bands of the ALHAMBRA survey, and the redshifted position of the Balmer jump to select star-forming galaxies in the redshift range 0.5 < z < 1.5. These galaxies are dubbed Balmer jump Galaxies BJGs. We apply the iSEDfit Bayesian approach to fit each detailed SED and determine star-formation rate (SFR), stellar mass, age and absolute magnitudes. The mass of the haloes where these samples reside are found via a clustering analysis. Five volume-limited BJG sub-samples with different mean redshifts are found to reside in haloes of median masses $\sim 10^{12.5 \pm 0.2} M_\odot$ slightly increasing toward z=0.5. This increment is similar to numerical simulations results which suggests that we are tracing the evolution of an evolving population of haloes as they grow to reach a mass of $\sim 10^{12.7 \pm 0.1} M_\odot$ at z=0.5. The likely progenitors of our samples at z$\sim$3 are Lyman Break Galaxies, which at z$\sim$2 would evolve into star-forming BzK galaxies, and their descendants in the local Universe are elliptical galaxies.Hence, this allows us to follow the putative evolution of the SFR, stellar mass and age of these galaxies. From z$\sim$1.0 to z$\sim$0.5, the stellar mass of the volume limited BJG samples nearly does not change with redshift, suggesting that major mergers play a minor role on the evolution of these galaxies. The SFR evolution accounts for the small variations of stellar mass, suggesting that star formation and possible minor mergers are the main channels of mass assembly.

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arXiv2015

The Gaia-ESO Survey: Separating disk chemical substructures with cluster models

(Abridged) Recent spectroscopic surveys have begun to explore the Galactic disk system outside the solar neighborhood on the basis of large data samples. In this way, they provide valuable information for testing spatial and temporal variations of disk structure kinematics and chemical evolution. We used a Gaussian mixture model algorithm, as a rigurous mathematical approach, to separate in the [Mg/Fe] vs. [Fe/H] plane a clean disk star subsample from the Gaia-ESO survey internal data release 2. We find that the sample is separated into five groups associated with major Galactic components; the metal-rich end of the halo, the thick disk, and three subgroups for the thin disk sequence. This is confirmed with a sample of red clump stars from the Apache Point Observatory Galactic Evolution Experiment (APOGEE) survey. The two metal-intermediate and metal-rich groups of the thin disk decomposition ([Fe/H]>-0.25 dex) highlight a change in the slope at solar metallicity. This holds true at different radial regions. The distribution of Galactocentric radial distances of the metal-poor part of the thin disk ([Fe/H]<-0.25 dex) is shifted to larger distances than those of the more metal-rich parts. Moreover, the metal-poor part of the thin disk presents indications of a scale height intermediate between those of the thick and the rest of the thin disk, and it displays higher azimuthal velocities than the latter. These stars might have formed and evolved in parallel and/or dissociated from the inside-out formation taking place in the internal thin disk. Their enhancement levels might be due to their origin from gas pre-enriched by outflows from the thick disk or the inner halo. The smooth trends of their properties (their spatial distribution with respect to the plane, in particular) with [Fe/H] and [Mg/Fe] suggested by the data indicates a quiet dynamical evolution, with no relevant merger events.

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arXiv2015

Spectroscopic and photometric analysis of the early-type spectroscopic binary HD 161853 in the centre of an H II region

We study the O-type star HD 161853, which has been noted as a probable double-lined spectroscopic binary system. We secured high-resolution spectra of HD 161853 during the past nine years. We separated the two components in the system and measured their respective radial velocities for the first time. We confirm that HD 161853 is an $\sim$1 Ma old binary system consisting of an O8 V star ($M_{\rm A,RV} \geq 22$ M$_\odot$) and a B1--3 V star ($M_{\rm B,RV} \geq 7.2$ M$_\odot$) at about 1.3 kpc. From the radial velocity curve, we measure an orbital period $P$ = 2.66765$\pm$0.00001 d and an eccentricity $e$ = 0.121$\pm$0.007. Its $V$-band light curve is constant within 0.014 mag and does not display eclipses, from which we impose a maximum orbital inclination $i=54$ deg. HD 161853 is probably associated with an H II region and a poorly investigated very young open cluster. In addition, we detect a compact emission region at 50 arcsec to HD 161853 in 22$μ$m-WISE and 24$μ$m-Spitzer images, which may be identified as a dust wave piled up by the radiation pressure of the massive binary system.

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arXiv2015

The eccentric short-period orbit of the supergiant fast X-ray transient HD 74194 (=LM Vel)

Aims. We present the first orbital solution for the O-type supergiant star HD 74194, which is the optical counterpart of the supergiant fast X-ray transient IGR J08408-4503. Methods. We measured the radial velocities in the optical spectrum of HD 74194, and we determined the orbital solution for the first time. We also analysed the complex Hα profile. Results. HD 74194 is a binary system composed of an O-type supergiant and a compact object in a short-period ($P=9.5436\pm0.0002$ d) and high-eccentricity ($e=0.63\pm0.03$) orbit. The equivalent width of the Hα line is not modulated entirely with the orbital period, but seems to vary in a superorbital period ($P=285\pm10$ d) nearly 30 times longer than the orbital one.

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arXiv2015

The Gaia-ESO Survey: chemical signatures of rocky accretion in a young solar-type star

It is well known that newly formed planetary systems undergo processes of orbital reconfiguration and planetary migration. As a result, planets or protoplanetary objects may accrete onto the central star, being fused and mixed into its external layers. If the accreted mass is sufficiently high and the star has a sufficiently thin convective envelope, such events may result in a modification of the chemical composition of the stellar photosphere in an observable way, enhancing it with elements that were abundant in the accreted mass. The recent Gaia-ESO Survey observations of the 10-20 Myr old Gamma Velorum cluster have enabled identifying a star that is significantly enriched in iron with respect to other cluster members. In this Letter we further investigate the abundance pattern of this star, showing that its abundance anomaly is not limited to iron, but is also present in the refractory elements, whose overabundances are correlated with the condensation temperature. This finding strongly supports the hypothesis of a recent accretion of rocky material.

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arXiv2015

The Spanish Square Kilometre Array White Book

The Square Kilometre Array (SKA) is called to revolutionise essentially all areas of Astrophysics. With a collecting area of about a square kilometre, the SKA will be a transformational instrument, and its scientific potential will go beyond the interests of astronomers. Its technological challenges and huge cost requires a multinational effort, and Europe has recognised this by putting the SKA on the roadmap of the European Strategy Forum for Research Infrastructures (ESFRI). The Spanish SKA White Book is the result of the coordinated effort of 120 astronomers from 40 different research centers. The book shows the enormous scientific interest of the Spanish astronomical community in the SKA and warrants an optimum scientific exploitation of the SKA by Spanish researchers, if Spain enters the SKA project.

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arXiv2015

The Gaia-ESO Survey: Empirical determination of the precision of stellar radial velocities and projected rotation velocities

The Gaia-ESO Survey (GES) is a large public spectroscopic survey at the European Southern Observatory Very Large Telescope. A key aim is to provide precise radial velocities (RVs) and projected equatorial velocities (v sin i) for representative samples of Galactic stars, that will complement information obtained by the Gaia astrometry satellite. We present an analysis to empirically quantify the size and distribution of uncertainties in RV and v sin i using spectra from repeated exposures of the same stars. We show that the uncertainties vary as simple scaling functions of signal-to-noise ratio (S/N) and v sin i, that the uncertainties become larger with increasing photospheric temperature, but that the dependence on stellar gravity, metallicity and age is weak. The underlying uncertainty distributions have extended tails that are better represented by Student's t-distributions than by normal distributions. Parametrised results are provided, that enable estimates of the RV precision for almost all GES measurements, and estimates of the v sin i precision for stars in young clusters, as a function of S/N, v sin i and stellar temperature. The precision of individual high S/N GES RV measurements is 0.22-0.26 km/s, dependent on instrumental configuration.

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arXiv2015

The Gaia-ESO Survey: N-body modelling of the Gamma Velorum cluster

The Gaia-ESO Survey has recently unveiled the complex kinematic signature of the Gamma Velorum cluster: this cluster is composed of two kinematically distinct populations (hereafter, population A and B), showing two different velocity dispersions and a relative ~2 km s^-1 radial velocity (RV) shift. In this paper, we propose that the two populations of the Gamma Velorum cluster originate from two different sub-clusters, born from the same parent molecular cloud. We investigate this possibility by means of direct-summation N-body simulations. Our scenario is able to reproduce not only the RV shift and the different velocity dispersions, but also the different centroid (~0.5 pc), the different spatial concentration and the different line-of-sight distance (~5 pc) of the two populations. The observed 1-2 Myr age difference between the two populations is also naturally explained by our scenario, in which the two sub-clusters formed in two slightly different star formation episodes. Our simulations suggest that population B is strongly supervirial, while population A is close to virial equilibrium. We discuss the implications of our models for the formation of young star clusters and OB associations in the Milky Way.

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arXiv2015

The Gaia-ESO Survey: Detailed Abundances in the Metal-poor Globular Cluster NGC 4372

We present the abundance analysis for a sample of 7 red giant branch stars in the metal-poor globular cluster NGC 4372 based on UVES spectra acquired as part of the Gaia-ESO Survey. This is the first extensive study of this cluster from high resolution spectroscopy. We derive abundances of O, Na, Mg, Al, Si, Ca, Sc, Ti, Fe, Cr, Ni, Y, Ba, and La. We find a metallicity of [Fe/H] = -2.19 $\pm$ 0.03 and find no evidence for a metallicity spread. This metallicity makes NGC 4372 one of the most metal-poor galactic globular clusters. We also find an α-enhancement typical of halo globular clusters at this metallicity. Significant spreads are observed in the abundances of light elements. In particular we find a Na-O anti-correlation. Abundances of O are relatively high compared with other globular clusters. This could indicate that NGC 4372 was formed in an environment with high O for its metallicity. A Mg-Al spread is also present which spans a range of more than 0.5 dex in Al abundances. Na is correlated with Al and Mg abundances at a lower significance level. This pattern suggests that the Mg-Al burning cycle is active. This behavior can also be seen in giant stars of other massive, metal-poor clusters. A relation between light and heavy s-process elements has been identified.

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arXiv2015

Rotation, spectral variability, magnetic geometry and magnetosphere of the Of?p star CPD -28 2561

We report magnetic and spectroscopic observations and modeling of the Of?p star CPD -28 2561. Using more than 75 new spectra, we have measured the equivalent width variations and examined the dynamic spectra of photospheric and wind-sensitive spectral lines. A period search results in an unambiguous 73.41 d variability period. High resolution spectropolarimetric data analyzed using Least-Squares Deconvolution yield a Zeeman signature detected in the mean Stokes V profile corresponding to phase 0.5 of the spectral ephemeris. Interpreting the 73.41 d period as the stellar rotational period, we have phased the equivalent widths and inferred longitudinal field measurements. The phased magnetic data exhibit a weak sinusoidal variation, with maximum of about 565 G at phase 0.5, and a minimum of about -335 G at phase 0.0, with extrema approximately in phase with the (double-wave) Halpha equivalent width variation. Modeling of the Halpha equivalent width variation assuming a quasi-3D magnetospheric model produces a unique solution for the ambiguous couplet of inclination and magnetic obliquity angles: (i, beta) or (beta, i)=(35 deg,90 deg). Adopting either geometry, the longitudinal field variation yields a dipole polar intensity Bd=2.6\pm 0.9~kG, consistent with that obtained from direct modelling of the Stokes V profiles. We derive a wind magnetic confinement parameter eta*\simeq 100, leading to an Alfvén radius RA\simeq 3-5~R*, and a Kepler radius RK\simeq 20~R*. This supports a physical scenario in which the Halpha emission and other line variability have their origin in an oblique, co-rotating 'dynamical magnetosphere' structure resulting from a magnetically channeled wind. Nevertheless, the details of the formation of spectral lines and their variability within this framework remain generally poorly understood.

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arXiv2015

The Gaia-ESO Survey: Tracing interstellar extinction

Large spectroscopic surveys have enabled in the recent years the computation of three-dimensional interstellar extinction maps thanks to accurate stellar atmospheric parameters and line-of-sight distances. Such maps are complementary to 3D maps extracted from photometry, allowing a more thorough study of the dust properties. Our goal is to use the high-resolution spectroscopic survey Gaia-ESO in order to obtain with a good distance resolution the interstellar extinction and its dependency as a function of the environment and the Galactocentric position. We use the stellar atmospheric parameters of more than 5000 stars, obtained from the Gaia-ESO survey second internal data release, and combine them with optical (SDSS) and near-infrared (VISTA) photometry as well as different sets of theoretical stellar isochrones, in order to calculate line-of-sight extinction and distances. The extinction coefficients are then compared with the literature to discuss their dependancy on the stellar parameters and position in the Galaxy. Within the errors of our method, our work does not show that there is any dependence of the interstellar extinction coefficient on the atmospheric parameters of the stars. We do not find any evidence of the variation of E(J-H)/E(J-K) with the angle from the Galactic centre nor with Galactocentric distance. This suggests that we are dealing with a uniform extinction law in the SDSS ugriz bands and the near-IR JHKs bands. Therefore, extinction maps using mean colour-excesses and assuming a constant extinction coefficient can be used without introducing any systematic errors.

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