arXiv subjects

D. Minniti

Publications and source records attributed to D. Minniti.

At least 127 records · Page 7Linked to original sources

arXiv2017

The Gaia-ESO Survey: Exploring the complex nature and origins of the Galactic bulge populations

Abridged: We used the fourth internal data release of the Gaia-ESO survey to characterize the bulge chemistry, spatial distribution, kinematics, and to compare it chemically with the thin and thick disks. The sample consist on ~2500 red clump stars in 11 bulge fields ($-10^\circ\leq l\leq+8^\circ$ and $-10^\circ\leq b\leq-4^\circ$), and a set of ~6300 disk stars selected for comparison. The bulge MDF is confirmed to be bimodal across the whole sampled area, with metal-poor stars dominating at high latitudes. The metal-rich stars exhibit bar-like kinematics and display a bimodality in their magnitude distribution, a feature which is tightly associated with the X-shape bulge. They overlap with the metal-rich end of the thin disk sequence in the [Mg/Fe] vs. [Fe/H] plane. Metal-poor bulge stars have a more isotropic hot kinematics and do not participate in the X-shape bulge. With similar Mg-enhancement levels, the position of the metal-poor bulge sequence "knee" is observed at [Fe/H]$_{knee}=-0.37\pm0.09$, being 0.06 dex higher than that of the thick disk. It suggests a higher SFR for the bulge than for the thick disk. Finally, we present a chemical evolution model that suitably fits the whole bulge sequence by assuming a fast ($<1$ Gyr) intense burst of stellar formation at early epochs. We associate metal-rich stars with the B/P bulge formed from the secular evolution of the early thin disk. On the other hand, the metal-poor subpopulation might be the product of an early prompt dissipative collapse dominated by massive stars. Nevertheless, our results do not allow us to firmly rule out the possibility that these stars come from the secular evolution of the early thick disk. This is the first time that an analysis of the bulge MDF and $α$-abundances has been performed in a large area on the basis of a homogeneous, fully spectroscopic analysis of high-resolution, high S/N data.

astro-ph.GA↗

arXiv2017

Near-IR period-luminosity relations for pulsating stars in $ω$ Centauri (NGC 5139)

$ω$ Centauri (NGC 5139) hosts hundreds of pulsating variable stars of different types, thus representing a treasure trove for studies of their corresponding period-luminosity (PL) relations. Our goal in this study is to obtain the PL relations for RR Lyrae, and SX Phoenicis stars in the field of the cluster, based on high-quality, well-sampled light curves in the near-infrared (IR). $ω$ Centauri was observed using VIRCAM mounted on VISTA. A total of 42 epochs in $J$ and 100 epochs in $K_{\rm S}$ were obtained, spanning 352 days. Point-spread function photometry was performed using DoPhot and DAOPHOT in the outer and inner regions of the cluster, respectively. Based on the comprehensive catalogue of near-IR light curves thus secured, PL relations were obtained for the different types of pulsators in the cluster, both in the $J$ and $K_{\rm S}$ bands. This includes the first PL relations in the near-IR for fundamental-mode SX Phoenicis stars. The near-IR magnitudes and periods of Type II Cepheids and RR Lyrae stars were used to derive an updated true distance modulus to the cluster, with a resulting value of $(m-M)_0 = 13.708 \pm 0.035 \pm 0.10$ mag, where the error bars correspond to the adopted statistical and systematic errors, respectively. Adding the errors in quadrature, this is equivalent to a heliocentric distance of $5.52\pm 0.27$ kpc.

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arXiv2017

Near-IR trigonometric parallaxes of nearby stars in the Galactic plane using the VVV

We use the multi-epoch K$\rm _S$ band observations, covering a $\sim$5 years baseline to obtain milli and sub-milli arcsec precision astrometry for a sample of eighteen previously known high proper motion sources, including precise parallaxes for these sources for the first time. In this study we show the capability of the VVV project to measure high precision trigonometric parallaxes for very low mass stars (VLMS) up to distances of $\sim$250\,pc reaching farther than most other ground based surveys or space missions for these types of stars. Additionally, we used spectral energy distribution to search for evidence of unresolved binary systems and cool sub-dwarfs. We detected five systems that are most likely VLMS belonging to the Galactic halo based on their tangential velocities, and four objects within 60 pc that are likely members of the thick disk. A more comprehensive study of high proper motion sources and parallaxes of VLMS and brown dwarfs with the VVV is ongoing , including thousands of newly discovered objects.

astro-ph.SR↗

arXiv2017

The nature of fifty Palermo Swift-BAT hard X-ray objects through optical spectroscopy

We present the nature of 50 hard X-ray emitting objects unveiled through an optical spectroscopy campaign performed at seven telescopes in the northern and southern hemispheres. These objects were detected with Swift-BAT and listed as of unidentified nature in the 54-month Palermo BAT catalogue. In detail, 45 sources in our sample are identified as active galactic nuclei of which, 27 are classified as type 1 (with broad and narrow emission lines) and 18 are classified as type 2 (with only narrow emission lines). Among the broad-line emission objects, one is a type 1 high-redshift quasi-stellar object, and among the narrow-line emission objects, one is a starburst galaxy, one is a X-ray bright optically normal galaxy, and one is a low ionization nuclear emission line region. We report 30 new redshift measurements, 13 confirmations and 2 more accurate redshift values. The remaining five objects are galactic sources: three are Cataclismic Variables, one is a X-ray Binary probably with a low mass secondary star, and one is an active star.

astro-ph.HE↗

arXiv2017

Baade's window with APOGEE: Metallicities, ages and chemical abundances

Baade's window (BW) is one of the most observed Galactic bulge fields in terms of chemical abundances. Due to its low and homogeneous interstellar absorption it is considered as a calibration field for Galactic bulge studies. In the era of large spectroscopic surveys, calibration fields such as BW are necessary to cross calibrate the stellar parameters and individual abundances of the APOGEE survey. We use the APOGEE BW stars to derive their metallicity distribution function (MDF) and individual abundances, for $α$- and iron-peak elements of the APOGEE ASPCAP pipeline (DR13), as well as the age distribution for stars in BW. We determine the MDF of APOGEE stars in BW and find a remarkable agreement with that of the Gaia-ESO survey (GES). Both exhibit a clear bimodal distribution. We also find that the Mg-metallicity planes of both surveys agree well, except for the metal-rich part ([Fe/H] >0.1), where APOGEE finds systematically higher Mg abundances with respect to the GES. The ages based on the [C/N] ratio reveal a bimodal age distribution, with a major old population at 10 Gyr, with a decreasing tail towards younger stars. A comparison between APOGEE estimates and stellar parameters, and those determined by other sources reveals detectable systematic offsets, in particular for spectroscopic surface gravity estimates. In general, we find a good agreement between individual abundances of O, Na, Mg, Al, Si, K, Ca, Cr, Mn, Co, and Ni from APOGEE with that of literature values. We have shown that in general APOGEE data show a good agreement in terms of MDF and individual chemical abundances with respect to literature works. Using the [C/N] ration we found a significant fraction of young stars in BW which is in agreement with the model of Haywood et al. (2016).

astro-ph.GA↗

arXiv2017

Galactic Doppelganger: The chemical similarity among field stars and among stars with a common birth origin

We explore to which extent stars within Galactic disk open clusters resemble each other in the high-dimensional space of their photospheric element abundances, and contrast this with pairs of field stars. Our analysis is based on abundances for 20 elements, homogeneously derived from APOGEE spectra (with carefully quantified uncertainties, with a median value of $\sim 0.03$ dex). We consider 90 red giant stars in seven open clusters and find that most stars within a cluster have abundances in most elements that are indistinguishable (in a $χ^2$-sense) from those of the other members, as expected for stellar birth siblings. An analogous analysis among pairs of $>1000$ field stars shows that highly significant abundance differences in the 20-dimensional space can be established for the vast majority of these pairs, and that the APOGEE-based abundance measurements have high discriminating power. However, pairs of field stars whose abundances are indistinguishable even at 0.03~dex precision exist: $\sim 0.3$ percent of all field star pairs, and $\sim 1.0$ percent of field star pairs at the same (solar) metallicity [Fe/H] = $0 \pm 0.02$. Most of these pairs are presumably not birth siblings from the same cluster, but rather doppelganger. Our analysis implies that 'chemical tagging' in the strict sense, identifying birth siblings for typical disk stars through their abundance similarity alone, will not work with such data. However, our approach shows that abundances have extremely valuable information for probabilistic chemo-orbital modeling and combined with velocities, we have identified new cluster members from the field.

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arXiv2016

New Planetary Systems from the Calan-Hertfordshire Extrasolar Planet Search

We report the discovery of eight new giant planets, and updated orbits for four known planets, orbiting dwarf and subgiant stars using the CORALIE, HARPS, and MIKE instruments as part of the Calan-Hertfordshire Extrasolar Planet Search. The planets have masses in the range 1.1-5.4MJs, orbital periods from 40-2900 days, and eccentricities from 0.0-0.6. They include a double-planet system orbiting the most massive star in our sample (HD147873), two eccentric giant planets (HD128356b and HD154672b), and a rare 14 Herculis analogue (HD224538b). We highlight some population correlations from the sample of radial velocity detected planets orbiting nearby stars, including the mass function exponential distribution, confirmation of the growing body of evidence that low-mass planets tend to be found orbiting more metal-poor stars than giant planets, and a possible period-metallicity correlation for planets with masses >0.1MJ, based on a metallicity difference of 0.16 dex between the population of planets with orbital periods less than 100 days and those with orbital periods greater than 100 days.

astro-ph.EP↗

arXiv2016

A population of eruptive variable protostars in VVV

We present the discovery of 816 high amplitude infrared variable stars ($ΔK_{\rm s} >$ 1 mag) in 119 deg$^2$ of the Galactic midplane covered by the Vista Variables in the Via Lactea (VVV) survey. Almost all are new discoveries and about 50$\%$ are YSOs. This provides further evidence that YSOs are the commonest high amplitude infrared variable stars in the Galactic plane. In the 2010-2014 time series of likely YSOs we find that the amplitude of variability increases towards younger evolutionary classes (class I and flat-spectrum sources) except on short timescales ($<$25 days) where this trend is reversed. Dividing the likely YSOs by light curve morphology, we find 106 with eruptive light curves, 45 dippers, 39 faders, 24 eclipsing binaries, 65 long-term periodic variables (P$>$100 days) and 162 short-term variables. Eruptive YSOs and faders tend to have the highest amplitudes and eruptive systems have the reddest SEDs. Follow up spectroscopy in a companion paper verifies high accretion rates in the eruptive systems. Variable extinction is disfavoured by the 2 epochs of colour data. These discoveries increase the number of eruptive variable YSOs by a factor of at least 5, most being at earlier stages of evolution than the known FUor and EXor types. We find that eruptive variability is at least an order of magnitude more common in class I YSOs than class II YSOs. Typical outburst durations are 1 to 4 years, between those of EXors and FUors. They occur in 3 to 6\% of class I YSOs over a 4 year time span.

astro-ph.SR↗

arXiv2016

Infrared spectroscopy of eruptive variable protostars from VVV

In a companion work (Paper I) we detected a large population of highly variable Young Stellar Objects (YSOs) in the Vista Variables in the Via Lactea (VVV) survey, typically with class I or flat spectrum spectral energy distributions and diverse light curve types. Here we present infrared spectra (0.9--2.5 $μ$m) of 37 of these variables, many of them observed in a bright state. The spectra confirm that 15/18 sources with eruptive light curves have signatures of a high accretion rate, either showing EXor-like emission features ($Δ$v=2 CO, Br$γ$) and/or FUor-like features ($Δ$v=2 CO and H$_{2}$O strongly in absorption). Similar features were seen in some long term periodic YSOs and faders but not in dippers or short-term variables. The sample includes some dusty Mira variables (typically distinguished by smooth Mira-like light curves), 2 cataclysmic variables and a carbon star. In total we have added 19 new objects to the broad class of eruptive variable YSOs with episodic accretion. Eruptive variable YSOs in our sample that were observed at bright states show higher accretion luminosities than the rest of the sample. Most of the eruptive variables differ from the established FUor and EXor subclasses, showing intermediate outburst durations and a mixture of their spectroscopic characteristics. This is in line with a small number of other recent discoveries. Since these previously atypical objects are now the majority amongst embedded members of the class, we propose a new classification for them as MNors. This term (pronounced emnor) follows V1647 Ori, the illuminating star of McNeil's Nebula.

astro-ph.SR↗

arXiv2016

The GIRAFFE Inner Bulge Survey (GIBS) III. Metallicity distributions and kinematics of 26 Galactic bulge fields

Several recent studies have demonstrated that the Galactic bulge hosts two components with different mean metallicities, and possibly different spatial distribution and kinematics. As a consequence, both the metallicity distribution and the radial velocity of bulge stars vary across different line of sights. We present here the metallicity distribution function of red clump stars in 26 fields spread across a wide area of the bulge, with special emphasis on fields close to Galactic plane, at latitudes b=-2 and b=-1, that were not explored before. This paper includes new metallicities from a sample of about 5000 K giant stars, observed at spectral resolution R=6500, in the Calcium II Triplet region. They are the main dataset of the GIRAFFE Inner Bulge Survey. As part of the same survey we have previously published results for a sample of about 600 K giant stars, at latitude b=-4 , derived from higher resolution spectra (R=22,500). Results. The combined sample allows us to trace and characterize the metal poor and metal rich bulge populations down to the inner bulge. We present a density map for each of the two components. Contrary to the expectations from previous works, we found the metal poor population to be more centrally concentrated than the metal rich one, and with a more axisymmetric spatial distribution. The metal rich population, on the other hand, is arranged in a boxy distribution, consistent with an edge-on bar. By coupling metallicities and radial velocities we show that the metal poor population has a velocity dispersion that varies rather mildly with latitude. On the contrary, the metal rich population has a low velocity dispersion far from the plane (b=-8.5), but it has a steeper gradient with latitude, becoming higher than the metal poor one in the innermost field (b=-1). [abridged]

astro-ph.GA↗

arXiv2016

The VLT LBG Redshift Survey - VI. Mapping HI in the proximity of $z\sim3$ LBGs with X-Shooter

We present an analysis of the spatial distribution of gas and galaxies using new X-Shooter observations of $z\sim3-4$ quasars. Adding the X-Shooter data to an existing dataset of high resolution quasar spectroscopy, we use a total sample of 29 quasars alongside $\sim1700$ Lyman Break Galaxies in the redshift range $2<z<3.5$. Analysing the Ly$α$ forest auto-correlation function using the full quasar sample, we find $s_0=0.081\pm0.006h^{-1}$Mpc. We then investigate the clustering and dynamics of Ly$α$ forest absorbers around $z\sim3$ LBGs. From the redshift-space cross-correlation, we find $s_0=0.27\pm0.14h^{-1}$Mpc, with power-law slope $γ=1.1\pm0.2$. We make a first analysis of the dependence of this clustering length on absorber strength based on cuts in the sightline transmitted flux, finding a clear preference for stronger absorption features to be more strongly clustered around the galaxy population than weaker absorption features. Further, we calculate the projected correlation function, finding $r_0=0.24\pm0.04h^{-1}$Mpc (assuming a fixed slope $γ=1.1$). Taking this as the underlying real-space clustering, we fit the 2D cross-correlation function with a dynamical model incorporating the infall parameter and the peculiar velocity, finding $β_{\rm F}=1.02\pm0.22$ and $240\pm60$ km s$^{-1}$ respectively. This result shows a significant detection of gas infall relative to the galaxy population, whilst the measured velocity dispersion is consistent with the velocity uncertainties on the galaxy redshifts. We evaluate the Cauchy-Schwarz inequality between the galaxy-galaxy, absorber-absorber, and galaxy-absorber correlation functions, finding a result significantly less than unity: $ξ_{\rm ag}^2/(ξ_{\rm gg}ξ_{\rm aa})=0.25\pm0.14$, implying that galaxies and Ly$α$ absorbers do not linearly trace the underlying dark matter distribution in the same way.

astro-ph.GA↗

arXiv2016

Young Stellar Clusters Containing Massive Young Stellar Objects in the VVV Survey

The purpose of this research is to study the connection of global properties of eight young stellar clusters projected in the Vista Variables in the Via Lactea (VVV) ESO Large Public Survey disk area and their young stellar object population. The analysis in based on the combination of spectroscopic parallax-based reddening and distance determinations with main sequence and pre-main sequence ishochrone fitting to determine the basic parameters (reddening, age, distance) of the sample clusters. The lower mass limit estimations show that all clusters are low or intermediate mass (between 110 and 1800 Mo), the slope Gamma of the obtained present-day mass functions of the clusters is close to the Kroupa initial mass function. On the other hand, the young stellar objects in the surrounding cluster's fields are classified by low resolution spectra, spectral energy distribution fit with theoretical predictions, and variability, taking advantage of multi-epoch VVV observations. All spectroscopically confirmed young stellar objects (except one) are found to be massive (more than 8 Mo). Using VVV and GLIMPSE color-color cuts we have selected a large number of new young stellar object candidates, which are checked for variability and 57% are found to show at least low-amplitude variations. In few cases it was possible to distinguish between YSO and AGB classification on the basis of the light curves.

astro-ph.GA↗

arXiv2016

Discovery of RR Lyrae Stars in the Nuclear Bulge of the Milky Way

Galactic nuclei, like the one of the Milky Way, are extreme places with high stellar densities and, in most cases, hosting a supermassive black hole. One of the scenarios proposed for the formation of the Galactic nucleus is by merging of primordial globular clusters (Capuzzo-Dolcetta 1993). An implication of this model is that this region should host stars characteristically found in old Milky Way globular clusters. RR Lyrae stars are primary distance indicators, well known representatives of old and metal-poor stellar populations, and therefore regularly found in globular clusters. Here we report the discovery of a dozen RR Lyrae ab-type stars in the vicinity of the Galactic center, i.e. in the so-called nuclear stellar bulge of the Milky Way. This discovery provides the first direct observational evidence that the Galactic nuclear stellar bulge contains ancient stars (>10 Gyr old). Based on this we conclude that merging globular clusters likely contributed to building-up the high stellar density in the nuclear stellar bulge of the Milky Way.

astro-ph.GA↗

arXiv2016

High proper motion objects towards the inner Milky Way: characterisation of newly identified nearby stars from the VISTA Variables in the Via Lactea Survey

The census of the Solar neighbourhood is still incomplete, as demonstrated by recent discoveries of many objects within 5-10 pc from the Sun. The area around the mid-plane and bulge of the Milky Way presents the most difficulties in searches for such nearby objects, and is therefore deficient in the known population. This is largely due to high stellar densities encountered. Spectroscopic, photometric and kinematic characterization of these objects allows better understand the local mass function, the binary fraction, and provides new interesting targets for more detailed studies. We report the spectroscopic follow-up and characterisation of 12 bright high PM objects, identified from the VISTA Variables in Via Lactea survey (VVV). We used the 1.9-m telescope of the South African Astronomical Observatory (SAAO) for low-resolution optical spectroscopy and spectral classification, and the MPG/ESP 2.2m telescope Fiber-fed Extended Range Optical Spectrograph (FEROS) high-resolution optical spectroscopy to obtain the radial and space velocities for three of them. Six of our objects have co-moving companions. We derived optical spectral types and photometric distances, and classified all of them as K and M dwarfs within 27-264 pc of the Sun. Finally, we found that one of the sources, VVV J141421.23-602326.1 (a co-moving companion of VVV J141420.55-602337.1), appears to be a rare massive white dwarf that maybe close to the ZZ Ceti instability strip. Many of the objects in our list are interesting targets for exoplanet searches.

astro-ph.SR↗

arXiv2016

VVV High proper motion stars I. The catalogue of bright Ks < 13.5 stars

Knowledge of the stellar content near the Sun is important for a broad range of topics ranging from the search for planets to the study of Milky Way structure. The most powerful method for identifying potentially nearby stars is proper motion (PM) surveys. All old optical surveys avoid, or are at least substantially incomplete, near the Galactic plane. The depth and breadth of the "Vista Variables in Via Lactea" (VVV) near-IR survey significantly improves this situation. Taking advantage of the VVV survey database, we have measured PMs in the densest regions of the MW bulge and southern plane in order to complete the census of nearby objects. We have developed a custom PM pipeline based on VVV catalogues from the Cambridge Astronomy Survey Unit (CASU), by comparing the first epoch of JHKs with the multi-epoch Ks-bands acquired later. Taking advantage of the large time baseline between the 2MASS and the VVV observations, we also obtained 2MASS-VVV PMs. We present a near-IR proper motion catalogue for the whole area of the VVV survey, which includes 3003 moving stellar sources. All of these have been visually inspected and are real PM objects. Our catalogue is in very good agreement with the proper motion data supplied in IR catalogues outside the densest zone of the MW. The majority of the PM objects in our catalogue are nearby M-dwarfs, as expected. This new database allow us to identify 57 common proper motion binary candidates, among which are two new systems within ~30pc of the Sun.

astro-ph.SR↗

arXiv2016

Constraining Dust Extinction Properties via the VVV Survey

Near-infrared color-excess and extinction ratios are essential for establishing the cosmic distance scale and probing the Galaxy, particularly when analyzing targets attenuated by significant dust. A robust determination of those ratios followed from leveraging new infrared observations from the VVV survey, wherein numerous bulge RR Lyrae and Type II Cepheids were discovered, in addition to $BVJHK_{s}(3.4\rightarrow22)μm$ data for classical Cepheids and O-stars occupying the broader Galaxy. The apparent optical color-excess ratios vary significantly with Galactic longitude ($\ell$), whereas the near-infrared results are comparatively constant with $\ell$ and Galactocentric distance ($\langle E(J-\overline{3.5μm})/E(J-K_s) \rangle =1.28\pm0.03$). The results derived imply that classical Cepheids and O-stars display separate optical trends ($R_{V,BV}$) with $\ell$, which appear to disfavor theories advocating a strict and marked decrease in dust size with increasing Galactocentric distance. The classical Cepheid, Type II Cepheid, and RR Lyrae variables are characterized by $\langle A_{J}/E(J-K_s) \rangle = \langle R_{J,JK_s} \rangle =1.49\pm0.05$ ($\langle A_{K_s}/A_J \rangle =0.33\pm0.02$), whereas the O-stars are expectedly impacted by emission beyond $3.6 μm$. The mean optical ratios characterizing classical Cepheids and O-stars are approximately $\langle R_{V,BV} \rangle \sim3.1$ and $\langle R_{V,BV} \rangle \sim3.3$, respectively.

astro-ph.GA↗

arXiv2016

Variable Stars in the Quintuplet stellar cluster with the VVV Survey

The Quintuplet cluster is one of the most massive star clusters in the Milky Way, situated very close to the Galactic center. We present a new search for variable stars in the vicinity of the cluster, using the five-year database of the Vista Variables in the Via Lactea (VVV) ESO Public Survey in the near-infrared. A total of 7586 objects were identified in the zone around $2'$ from the cluster center, using 55 $K_S$-band epochs. Thirty-three stars show $K_S$-band variability, 24 of them being previously undiscovered. Most of the variable stars found are slow/semiregular variables, long-period variables of the Mira type, and OH/IR stars. In addition, a good number of our candidates show variations in a rather short timescale. We also propose four Young Stellar Object (YSO) candidates, which could be cluster members.

astro-ph.SR↗

arXiv2016

Near-infrared photometry and spectroscopy of the low Galactic latitude globular cluster 2MASS-GC03

We present deep near-infrared photometry and spectroscopy of the globular cluster 2MASS-GC03 projected in the Galactic disk using MMIRS on the Clay telescope (Las Campanas Observatory) and VISTA Variables in the Via Lactea survey (VVV) data. Most probable cluster member candidates were identified from near-infrared photometry. Out of ten candidates that were followed-up spectroscopically, five have properties of cluster members, from which we calculate <[Fe/H]> = -0.9 +- 0.2 and a radial velocity of v_r > = -78 +- 12km/s. A distance of 10.8kpc is estimated from 3 likely RRLyrae members. Given that the cluster is currently at a distance of 4.2kpc from the Galactic center, the cluster's long survival time of an estimated 11.3 +- 1.2Gyr strengthens the case for its globular-cluster nature. The cluster has a hint of elongation in the direction of the Galactic center.

astro-ph.SR↗