arXiv subjects

D. Minniti

Publications and source records attributed to D. Minniti.

At least 145 records · Page 8Linked to original sources

arXiv2016

Near-infrared photometry of WISE J085510.74$-$071442.5

(Abridged) We aim at measuring the near-infrared photometry, and deriving the mass, age, temperature, and surface gravity of WISE J085510.74-071442.5 (J0855-0714), which is the coolest known object beyond the Solar System as of today. We use publicly available data from the archives of the HST and the VLT to determine the emission of this source at 1.153 micron (F110W) and 1.575 micron (CH_4). J0855-0714 is detected at both wavelengths with signal-to-noise ratio of ~10 (F110W) and ~4 (CH_4-off) at the peak of the corresponding PSFs. This is the first detection of J0855-0714 in the H-band. We measure 26.31 +/- 0.10 and 23.22 +/- 0.35 mag in F110W and CH_4 (Vega system). J0855-0714 remains unresolved in the HST images that have a spatial resolution of 0.22". Companions at separations of 0.5 AU (similar brightness) and at ~1 AU (~1 mag fainter in the F110W filter) are discarded. By combining the new data with published photometry, we build the spectral energy distribution of J0855-0714 from 0.89 to 22.09 micron, and contrast it against state-of-the-art solar-metallicity models of planetary atmospheres. We determine a temperature of 225-250 K, a bolometric luminosity of log L/Lsol = -8.57, and a high surface gravity of log g = 5.0 (cm/s2), which suggests an old age although such a high gravity is not fully compatible with evolutionary models. After comparison with the cooling theory for brown dwarfs and planets, we infer a mass in the interval 2-10 Mjup for ages of 1-12 Gyr and log g > 3.5 (cm/s2). At the age of the Sun, J0855-0714 would be a ~5-Mjup free-floating planetary-mass object. J0855-0714 may represent the old image of the free-floating planetary-mass objects of similar mass discovered in star-forming regions and young stellar clusters. As many J0855-0714-like objects as M5-L2 stars may be expected to populate the solar neighborhood.

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arXiv2016

High-resolution abundance analysis of red giants in the metal-poor bulge globular cluster HP~1

The globular cluster HP~1 is projected at only 3.33 degrees from the Galactic center. Together with its distance, this makes it one of the most central globular clusters in the Milky Way. It has a blue horizontal branch (BHB) and a metallicity of [Fe/H]~-1.0. This means that it probably is one of the oldest objects in the Galaxy. Abundance ratios can reveal the nucleosynthesis pattern of the first stars as well as the early chemical enrichment and early formation of stellar populations. High-resolution spectra obtained for six stars were analyzed to derive the abundances of the light elements C, N, O, Na, and Al, the alpha-elements Mg, Si, Ca, and Ti, and the heavy elements Sr, Y , Zr, Ba, La, and Eu.} High-resolution spectra of six red giants that are confirmed members of the bulge globular cluster HP~1 were obtained with the 8m VLT UT2-Kueyen telescope with the UVES spectrograph in FLAMES-UVES configuration. The spectroscopic parameter derivation was based on the excitation and ionization equilibrium of FeI and FeII. We confirm a mean metallicity of [Fe/H] = -1.06~0.10, by adding the two stars that were previously analyzed in HP~1. The alpha-elements O and Mg are enhanced by about +0.3<[O,Mg/Fe]<+0.5 dex, Si is moderately enhanced with +0.15<[Si/Fe]<+0.35dex, while Ca and Ti show lower values of -0.04<[Ca,Ti/Fe]<+0.28dex. The r-element Eu is also enhanced with [Eu/Fe]~+0.4, which together with O and Mg is indicative of early enrichment by type II supernovae. Na and Al are low, but it is unclear if Na-O are anticorrelated. The heavy elements are moderately enhanced, with -0.20<[La/Fe]<+0.43dex and 0.0<[Ba/Fe]<+0.75~dex, which is compatible with r-process formation. The spread in Y, Zr, Ba, and La abundances, on the other hand, appears to be compatible with the spinstar scenario or other additional mechanisms such as the weak r-process.

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arXiv2016

Mapping the outer bulge with RRab stars from the VVV Survey

The VISTA Variables in the Vía Láctea (VVV) is a near-IR time-domain survey of the Galactic bulge and southern plane. One of the main goals of this survey is to reveal the 3D structure of the Milky Way through their variable stars. Particularly the RR Lyrae stars have been massively discovered in the inner regions of the bulge ($-8^\circ \lesssim b \lesssim -1^\circ$) by optical surveys such as OGLE and MACHO but leaving an unexplored window of more than $\sim 47$ sq deg ($-10.0^\circ \lesssim \ell \lesssim +10.7^\circ$ and $-10.3^\circ \lesssim b \lesssim -8.0^\circ$) observed by the VVV Survey. Our goal is to characterize the RR Lyrae stars in the outer bulge in terms of their periods, amplitudes, Fourier coefficients, and distances, in order to evaluate the 3D structure of the bulge in this area. The distance distribution of RR Lyrae stars will be compared to the one of red clump stars that is known to trace a X-shaped structure in order to determine if these two different stellar populations share the same Galactic distribution. We report the detection of more than 1000 RR Lyrae ab-type stars in the VVV Survey located in the outskirts of the Galactic bulge. Some of them are possibly associated with the Sagittarius Dwarf Spheroidal Galaxy. We calculated colors, reddening, extinction, and distances of the detected RR Lyrae stars in order to determine the outer bulge 3D structure. Our main result is that, at the low galactic latitudes mapped here, the RR Lyrae stars trace a centrally concentrated spheroidal distribution. This is a noticeably different spatial distribution to the one traced by red clump stars known to follow a bar and X-shape structure. We estimate the completeness of our RRab sample in $80\%$ for $K_{\rm s}\lesssim15$ mag.

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arXiv2016

Detailed chemical abundances of distant RR Lyrae stars in the Virgo Stellar Stream

We present the first detailed chemical abundances for distant RR Lyrae stars members of the Virgo Stellar Stream (VSS), derived from X-Shooter medium-resolution spectra. Sixteen elements from carbon to barium have been measured in six VSS RR Lyrae stars, sampling all main nucleosynthetic channels. For the first time we will be able to compare in detail the chemical evolution of the VSS progenitor with those of Local Group dwarf spheroidal galaxies (LG dSph) as well as the one of the smooth halo.

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arXiv2016

Comparing the properties of the X-shaped bulges of NGC 4710 and the Milky Way with MUSE

We used the new ESO VLT instrument MUSE to obtain spectral and imaging coverage of NGC 4710. The wide area and excellent sampling of the MUSE integral field spectrograph allows us to investigate the dynamical properties of the X-shaped bulge of NGC 4710 and compare it with the properties of the Milky Way's own X-shaped bulge. We measured the radial velocities, velocity dispersion, and stellar populations using a penalized pixel full spectral fitting technique adopting simple stellar populations models, on a 1' x 1' area centred on the bulge of NGC 4710. We have constructed the velocity maps of the bulge of NGC 4710 and we investigated the presence of vertical metallicity gradients. These properties were compared to those of the Milky Way bulge and as well as to a simulated galaxy with boxy/peanut bulge. We find the line-of-sight velocity maps and 1D rotation curves of the bulge of NGC 4710 to be remarkably similar to those of the Milky Way bulge. Some specific differences that were identified are in good agreement with the expectations from variations in the bar orientation angle. The bulge of NGC 4710 has a boxy-peanut morphology with a pronounced X-shape, showing no indication of any additional spheroidally distributed bulge population, in which we measure a vertical metallicity gradient of 0.35 dex/kpc. The general properties of NGC 4710 are very similar to those observed in the MW bulge. However, it has been suggested that the MW bulge has an additional component that is comprised of the oldest, most metal-poor stars and which is not part of the boxy-peanut bulge structure. Such a population is not observed in NGC 4710, but could be hidden in the integrated light we observed.

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arXiv2015

Heavy elements Ba, La, Ce, Nd, and Eu in 56 Galactic bulge red giants

Aims. The aim of this work is the study of abundances of the heavy elements Ba, La, Ce, Nd, and Eu in 56 bulge giants (red giant branch and red clump) with metallicities ranging from -1.3 dex to 0.5 dex. Methods. We obtained high-resolution spectra of our giant stars using the FLAMES-UVES spectrograph on the Very Large Telescope. We inspected four bulge fields along the minor axis. Results. We measure the chemical evolution of heavy elements, as a function of metallicity, in the Galactic bulge. Conclusions. The [Ba, La, Ce, Nd/Fe] vs. [Fe/H] ratios decrease with increasing metallicity, in which aspect they differ from disc stars. In our metal-poor bulge stars, La and Ba are enhanced relative to their thick disc counterpart, while in our metal-rich bulge stars La and Ba are underabundant relative to their disc counterpart. Therefore, this contrast between bulge and discs trends indicates that bulge and (solar neighbourhood) thick disc stars could behave differently. An increase in [La, Nd/Eu] with increasing metallicity, for metal-rich stars with [Fe/H] > 0 dex, may indicate that the s-process from AGB stars starts to operate at a metallicity around solar. Finally, [Eu/Fe] follows the [α/Fe] behaviour, as expected, since these elements are produced by SNe type II.

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arXiv2015

Stellar density profile and mass of the Milky Way Bulge from VVV data

We present the first stellar density profile of the Milky Way bulge reaching latitude $b=0^\circ$. It is derived by counting red clump stars within the colour\--magnitude diagram constructed with the new PSF-fitting photometry from VISTA Variables in the V\'ıa Láctea (VVV) survey data. The new stellar density map covers the area between $|l|\leq 10^\circ$ and $|b|\leq 4.5^\circ$ with unprecedented accuracy, allowing to establish a direct link between the stellar kinematics from the Giraffe Inner Bulge Spectroscopic Survey (GIBS) and the stellar mass density distribution. In particular, the location of the central velocity dispersion peak from GIBS matches a high overdensity in the VVV star count map. By scaling the total luminosity function (LF) obtained from all VVV fields to the LF from Zoccali et al.(2003), we obtain the first fully empirical estimate of the mass in stars and remnants of the Galactic bulge. The Milky Way bulge stellar mass within ($|b|<9.5^\circ$, $|l|<10^\circ$) is $2.0\pm0.3\times 10^{10}M_{\odot}$.

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arXiv2015

Reinforcing the link between the double red clump and the X-shaped bulge of the Milky Way

The finding of a double red clump in the luminosity function of the Milky Way bulge has been interpreted as evidence for an X-shaped structure. Recently, an alternative explanation has been suggested, where the double red clump is an effect of multiple stellar populations in a classical spheroid. In this letter we provide an observational assessment of this scenario and show that it is not consistent with the behaviour of the red clump across different lines of sight, particularly at high distances from the Galactic plane. Instead, we confirm that the shape of the red clump magnitude distribution closely follows the distance distribution expected for an X-shaped bulge at critical Galactic latitudes. We also emphasize some key observational properties of the bulge red clump that should not be neglected in the search for alternative scenarios.

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arXiv2015

TraMoS IV: Discarding the Quick Orbital Decay Hypothesis for OGLE-TR-113b

In the context of the TraMoS project we present nine new transit observations of the exoplanet OGLE-TR-113b observed with the Gemini South, Magellan Baade, Danish-1.54m and SOAR telescopes. We perform a homogeneous analysis of these new transits together with ten literature transits to probe into the potential detection of an orbital decay for this planet reported by \citet{adams2010}. Our new observations extend the transit monitoring baseline for this system by 6 years, to a total of more than 13 years. With our timing analysis we obtained a $\dot{P}=-1.0 \pm 6.0$ ms~yr$^{-1}$, which rejects previous hints of a larger orbital decay for OGLE-TR-113b. With our updated value of $\dot{P}$ we can discard tidal quality factors of $Q_{\star} < 10^{5}$ for its host star. Additionally, we calculate a 1$σ$ dispersion of the Transit Timing Variations (TTVs) of 42 seconds over the 13 years baseline, which discards additional planets in the system more massive than $0.5-3.0~M_{\oplus}$ in 1:2, 5:3, 2:1 and 3:1 Mean Motion Resonances with OGLE-TR-113b. Finally, with the joint analysis of the 19 light curves we update transit parameters, such as the relative semi-major axis $a / R_s = 6.44^{+0.04}_{-0.05}$, the planet-to-star radius ratio $R_p / R_s =0.14436^{+0.00096}_{-0.00088}$, and constrains its orbital inclination to $i =89.27^{+0.51}_{-0.68}$~degrees.

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arXiv2015

Massive open star clusters using the VVV survey IV. WR 62-2, a new very massive star in the core of the VVV CL041 cluster

Context The ESO Public Survey VISTA Variables in the Vía Láctea (VVV) provides deep multi-epoch infrared observations for an unprecedented 562 sq. degrees of the Galactic bulge and adjacent regions of the disk. Nearly 150 new open clusters and cluster candidates have been discovered in this survey. Aims We present the fourth article in a series of papers focussed on young and massive clusters discovered in the VVV survey. This article is dedicated to the cluster VVV CL041, which contains a new very massive star candidate, WR 62-2. Methods Following the methodology presented in the first paper of the series, wide-field, deep JHKs VVV observations, combined with new infrared spectroscopy, are employed to constrain fundamental parameters (distance, reddening, mass, age) of VVV CL041. Results We confirm that the cluster VVV CL041 is a young (less than 4 Myrs) and massive (3 +/- 2 x 10^3 Msol) cluster, and not a simple asterism. It is located at a distance of 4.2 +/- 0.9 kpc, and its reddening is A_V = 8.0 +/- 0.2 mag, which is slightly lower than the average for the young clusters towards the centre of the Galaxy. Spectral analysis shows that the most luminous star of the cluster, of the WN8h spectral type, is a candidate to have an initial mass larger than 100 Msol.

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arXiv2015

Discovery of a brown dwarf companion to the A3V star β Circini

We report the discovery of an L dwarf companion to the A3V star β Circini. VVV J151721.49-585131.5, or β Cir B, was identified in a proper motion and parallax catalogue of the Vista Variables in the Vía Láctea survey as having near infrared luminosity and colour indicative of an early L dwarf, and a proper motion and parallax consistent with that of β Cir. The projected separation of $\sim$3.6' corresponds to $6656$ au, which is unusually wide. The most recent published estimate of the age of the primary combined with our own estimate based on newer isochrones yields an age of $370-500$ Myr. The system therefore serves as a useful benchmark at an age greater than that of the Pleiades brown dwarfs and most other young L dwarf benchmarks. We have obtained a medium resolution echelle spectrum of the companion which indicates a spectral type of L1.0$\pm$0.5 and lacks the typical signatures of low surface gravity seen in younger brown dwarfs. This suggests that signs of low surface gravity disappear from the spectra of early L dwarfs by an age of $\sim370-500$ Myr, as expected from theoretical isochrones. The mass of β Cir B is estimated from the BHAC15 isochrones as $0.056\pm0.007$ M$_{\odot}$.

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arXiv2015

The VVV Survey reveals classical Cepheids tracing a young and thin stellar disk across the Galaxy's bulge

Solid insight into the physics of the inner Milky Way is key to understanding our Galaxy's evolution, but extreme dust obscuration has historically hindered efforts to map the area along the Galactic mid-plane. New comprehensive near-infrared time-series photometry from the VVV Survey has revealed 35 classical Cepheids, tracing a previously unobserved component of the inner Galaxy, namely a ubiquitous inner thin disk of young stars along the Galactic mid-plane, traversing across the bulge. The discovered period (age) spread of these classical Cepheids implies a continuous supply of newly formed stars in the central region of the Galaxy over the last 100 million years.

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arXiv2015

Spectro-photometric characterization of high proper motion sources from WISE

The census of the solar neighborhood is almost complete for stars and becoming more complete in the brown dwarf regime. Spectroscopic, photometric and kinematic characterization of nearby objects helps us to understand the local mass function, the binary fraction, and provides new targets for sensitive planet searches. We aim to derive spectral types and spectro-photometric distances of a sample of new high proper motion sources found with the WISE satellite, and obtain parallaxes for those objects that fall within the area observed by the Vista Variables in the Vía Láctea survey (VVV). We used low resolution spectroscopy and template fitting to derive spectral types, multiwavelength photometry to characterize the companion candidates and obtain photometric distances. Multi-epoch imaging from the VVV survey was used to measure the parallaxes and proper motions for three sources. We confirm a new T2 brown dwarf within $\sim$15 pc. We derived optical spectral types for twenty four sources, mostly M dwarfs within 50 pc. We addressed the wide binary nature of sixteen objects found by the WISE mission and previously known high proper motion sources. Six of these are probably members of wide binaries, two of those are new, and present evidence against the physical binary nature of two candidate binary stars found in the literature, and eight that we selected as possible binary systems. We discuss a likely microlensing event produced by a nearby low mass star and a galaxy, that is to occur in the following five years.

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arXiv2015

VVV Survey Observations of a Microlensing Stellar Mass Black Hole Candidate in the Field of the Globular Cluster NGC 6553

We report the discovery of a large timescale candidate microlensing event of a bulge stellar source based on near-infrared observations with the VISTA Variables in the Via Lactea Survey (VVV). The new microlensing event is projected only 3.5 arcmin away from the center of the globular cluster NGC 6553. The source appears to be a bulge giant star with magnitude Ks = 13.52, based on the position in the color-magnitude diagram. The foreground lens may be located in the globular cluster, which has well-known parameters such as distance and proper motions. If the lens is a cluster member, we can directly estimate its mass simply following Paczynski et al. (1996) which is a modified version of the more general case due to Refsdal. In that case, the lens would be a massive stellar remnant, with M = 1.5-3.5 Msun. If the blending fraction of the microlensing event appears to be small, and this lens would represent a good isolated black hole (BH) candidate, that would be the oldest BH known. Alternative explanations (with a larger blending fraction) also point to a massive stellar remnant if the lens is located in the Galactic disk and does not belong to the globular cluster.

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arXiv2015

The Giraffe Inner Bulge Survey (GIBS) II. Metallicity distributions and alpha element abundances at fixed Galactic latitude

High resolution (R$\sim$22,500) spectra for 400 red clump giants, in four fields within $\rm -4.8^{\circ} \lesssim b \lesssim -3.4^{\circ}$ and $\rm -10^{\circ} \lesssim l \lesssim +10^{\circ}$, were obtained within the GIRAFFE Inner Bulge Survey (GIBS) project. To this sample we added another $\sim$ 400 stars in Baade's Window, observed with the identical instrumental configuration. We constructed the metallicity distributions for the entire sample, as well as for each field individually, in order to investigate the presence of gradients or field-to-field variations in the shape of the distributions. The metallicity distributions in the five fields are consistent with being drawn from a single parent population, indicating the absence of a gradient along the major axis of the Galactic bar. The global metallicity distribution is well fitted by two Gaussians. The metal poor component is rather broad, with a mean at $\rm <[Fe/H]>=-0.31$ dex and $σ=0.31$ dex. The metal-rich one is narrower, with mean $\rm <[Fe/H]>=+0.26$ and $σ=0.2$ dex. The [Mg/Fe] ratio follows a tight trend with [Fe/H], with enhancement with respect to solar in the metal-poor regime, similar to the one observed for giant stars in the local thick disc. [Ca/Fe] abundances follow a similar trend, but with a considerably larger scatter than [Mg/Fe]. A decrease in [Mg/Fe] is observed at $\rm [Fe/H]=-0.44$ dex. This \textit{knee} is in agreement with our previous bulge study of K-giants along the minor axis, but is 0.1 dex lower in metallicity than the one reported for the Bulge micro lensed dwarf and sub-giant stars. We found no variation in $α$-element abundance distributions between different fields.

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arXiv2015

Zinc abundances in Galactic bulge field red giants: implications for DLA systems

Zinc in stars is an important reference element because it is a proxy to Fe in studies of damped Lyman-alpha systems, permitting a comparison of chemical evolution histories of bulge stellar populations and DLAs. In terms of nucleosynthesis, it behaves as an alpha element because it is enhanced in metal-poor stars. The aim of this work is to derive the iron-peak element Zn abundances in 56 bulge giants from high resolution spectra. These results are compared with data from other bulge samples, as well as from disk and halo stars, and damped Lyman-alpha systems, in order to better understand the chemical evolution in these environments. High-resolution spectra were obtained using FLAMES+UVES on the Very Large Telescope. We find [Zn/Fe]=+0.24+-0.02 in the range -1.3 < [Fe/H] < -0.5 and [Zn/Fe]=+0.06+-0.02 in the range -0.5 < [Fe/H] < -0.1, whereas for [Fe/H] > -0.1, it shows a spread of -0.60 < [Zn/Fe] < +0.15, with most of these stars having low [Zn/Fe]<0.0. These low zinc abundances at the high metallicity end of the bulge define a decreasing trend in [Zn/Fe] with increasing metallicities. A comparison with Zn abundances in DLA systems is presented, where a dust-depletion correction was applied for both Zn and Fe. Finally, we present a chemical evolution model of Zn enrichment in massive spheroids, representing a typical classical bulge.

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arXiv2015

WISE J061213.85-303612.5: a new T-dwarf binary candidate

T and Y-dwarfs are among the coolest and least luminous objects detected, and they can help to understand the properties of giant planets. Their multiplicity properties can shed light on the formation process. We observed a sample six T dwarfs and one L9 dwarf with the Laser Guide Star (LGS) and NAOS-CONICA, the adaptive optics (AO) facility, and the near infrared camera at the ESO Very Large Telescope. From the seven observed objects, we have identified a subarcsecond binary system, WISE J0612-3036, composed of two similar components with spectral types of T6. We measure a separation of $ρ$ = 350$\pm$5 mas and a position angle of $PA = 235\pm1^{\circ}$. Using the mean absolute magnitudes of T6 dwarfs in the 2MASS $JHK_s$ bands, we estimate a distance of $d$=31$\pm$6 pc and derive a projected separation of $ρ$ = 11$\pm$2 au. Another target, WISE J2255-3118, shows a very faint object at 1.3 arcsec in the $K_{\rm s}$ image. The object is marginally detected in $H$, and we derive a near infrared color of $H-K_{\rm s}$$>$ 0.1\,mag. $HST/WFC3$ public archival data reveals that the companion candidate is an extended source.Together with the derived color, this suggests that the source is most probably a background galaxy. The five other sources are apparently single, with 3-$σ$ sensitivity limits between $H$=19-21 for companions at separations $\geq$ 0.5 arcsec. WISE 0612-3036 is probably a new T-dwarf binary composed of two T6 dwarfs. As in the case of other late T-dwarf binaries, it shows a mass ratio close to 1, although its projected separation, $\sim$11 au, is larger than the average ($\sim$ 5 au). Additional observations are needed to confirm that the system is bound.

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arXiv2015

Hundreds of new cluster candidates in the VISTA variables in the Via Lactea survey DR1

VISTA variables in the Via Lactea is an ESO Public survey dedicated to scan the bulge and an adjacent portion of the Galactic disk in the fourth quadrant using the VISTA telescope and the near-infrared camera VIRCAM. One of the leading goals of the VVV survey is to contribute to the knowledge of the star cluster population of the Milky Way. To improve the census of the Galactic star clusters, we performed a systematic scan of the JHKs images of the Galactic plane section of the VVV survey. Our detection procedure is based on a combination of superficial density maps and visual inspection of promising features in the NIR images. The material examined are color-composite images corresponding to the DR1 of VVV. We report the discovery of 493 new star cluster candidates. The analysis of the spatial distribution show that the clusters are very concentrated in he Galactic plane, presenting some local maxima around the position of large star-forming complexes, such as G305, RCW 95, and RCW 106. The vast majority of the cluster candidates are quite compact and generally surrounded by bright and/or dark nebulosities. IRAS point sources are associated with 59% of the sample, while 88% are associated with MSX point sources. GLIMPSE 8 mum images of the cluster candidates show a variety of morphologies, with 292 clusters dominated by knotty sources, while 361 clusters show some kind of nebulosity. Spatial cross-correlation with young stellar objects, masers, and extended green-object catalogs suggest that a large sample of the new cluster candidates are extremely young. In particular, 104 star clusters associated to methanol masers are excellent candidates for ongoing massive star formation. Also, there is a special set of sixteen cluster candidates that present clear signspot of star-forming activity having associated simultaneosly dark nebulae, young stellar objects, EGOs, and masers.

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