arXiv subjects

Dante Minniti

Publications and source records attributed to Dante Minniti.

At least 163 records · Page 9Linked to original sources

arXiv2016

New variable stars discovered in the fields of three Galactic open clusters using the VVV Survey

This project is a massive near-infrared (NIR) search for variable stars in highly reddened and obscured open cluster (OC) fields projected on regions of the Galactic bulge and disk. The search is performed using photometric NIR data in the $J$-, $H$- and $K_s$- bands obtained from the Vista Variables in the Vía Láctea (VVV) Survey. We performed in each cluster field a variability search using Stetson's variability statistics to select the variable candidates. Later, those candidates were subjected to a frequency analysis using the Generalized Lomb-Scargle and the Phase Dispersion Minimization algorithms. The number of independent observations range between 63 and 73. The newly discovered variables in this study, 157 in total in three different known OCs, are classified based on their light curve shapes, periods, amplitudes and their location in the corresponding color-magnitude $(J-K_s,K_s)$ and color-color $(H-K_s,J-H)$ diagrams. We found 5 possible Cepheid stars which, based on the period-luminosity relation, are very likely type II Cepheids located behind the bulge. Among the newly discovered variables, there are eclipsing binaries, $δ$ Scuti, as well as background RR Lyrae stars. Using the new version of the Wilson & Devinney code as well as the "Physics Of Eclipsing Binaries" (PHOEBE) code, we analyzed some of the best eclipsing binaries we discovered. Our results show that these studied systems turn out to be ranging from detached to double-contact binaries, with low eccentricities and high inclinations of approximately $80^{\circ}$. Their surface temperatures range between $3500$K and $8000$K.

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arXiv2016

MagAO Imaging of Long-period Objects (MILO). I. A Benchmark M Dwarf Companion Exciting a Massive Planet around the Sun-like Star HD 7449

We present high-contrast Magellan adaptive optics (MagAO) images of HD 7449, a Sun-like star with one planet and a long-term radial velocity (RV) trend. We unambiguously detect the source of the long-term trend from 0.6-2.15 \microns ~at a separation of \about 0\fasec 54. We use the object's colors and spectral energy distribution to show that it is most likely an M4-M5 dwarf (mass \about 0.1-0.2 \msun) at the same distance as the primary and is therefore likely bound. We also present new RVs measured with the Magellan/MIKE and PFS spectrometers and compile these with archival data from CORALIE and HARPS. We use a new Markov chain Monte Carlo procedure to constrain both the mass ($> 0.17$ \msun ~at 99$\%$ confidence) and semimajor axis (\about 18 AU) of the M dwarf companion (HD 7449B). We also refine the parameters of the known massive planet (HD 7449Ab), finding that its minimum mass is $1.09^{+0.52}_{-0.19}$ \mj, its semimajor axis is $2.33^{+0.01}_{-0.02}$ AU, and its eccentricity is $0.8^{+0.08}_{-0.06}$. We use N-body simulations to constrain the eccentricity of HD 7449B to $\lesssim$ 0.5. The M dwarf may be inducing Kozai oscillations on the planet, explaining its high eccentricity. If this is the case and its orbit was initially circular, the mass of the planet would need to be $\lesssim$ 1.5 \mj. This demonstrates that strong constraints on known planets can be made using direct observations of otherwise undetectable long-period companions.

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arXiv2015

Interstellar Extinction Curve Variations Toward the Inner Milky Way: A Challenge to Observational Cosmology

We investigate interstellar extinction curve variations toward $\sim$4 deg$^{2}$ of the inner Milky Way in $VIJK_{s}$ photometry from the OGLE-III and $VVV$ surveys, with supporting evidence from diffuse interstellar bands and $F435W,F625W$ photometry. We obtain independent measurements toward $\sim$2,000 sightlines of $A_{I}$, $E(V-I)$, $E(I-J)$, and $E(J-K_{s})$, with median precision and accuracy of 2%. We find that the variations in the extinction ratios $A_{I}/E(V-I)$, $E(I-J)/E(V-I)$ and $E(J-K_{s})/E(V-I)$ are large (exceeding 20%), significant, and positively correlated, as expected. However, both the mean values and the trends in these extinction ratios are drastically shifted from the predictions of Cardelli and Fitzpatrick, regardless of how $R_{V}$ is varied. Furthermore, we demonstrate that variations in the shape of the extinction curve has at least two degrees of freedom, and not one (e.g. $R_{V}$), which we conform with a principal component analysis. We derive a median value of $ =13.44$, which is $\sim$60% higher than the "standard" value. We show that the Wesenheit magnitude $W_{I}=I-1.61(I-J)$ is relatively impervious to extinction curve variations. Given that these extinction curves are linchpins of observational cosmology, and that it is generally assumed that $R_{V}$ variations correctly capture variations in the extinction curve, we argue that systematic errors in the distance ladder from studies of type Ia supernovae and Cepheids may have been underestimated. Moreover, the reddening maps from the Planck experiment are shown to systematically overestimate dust extinction by $\sim$100%, and lack sensitivity to extinction curve variations.

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arXiv2015

The Araucaria Project: A study of the classical Cepheid in the eclipsing binary system OGLE LMC562.05.9009 in the Large Magellanic Cloud

We present a detailed study of the classical Cepheid in the double-lined, highly eccentric eclipsing binary system OGLE-LMC562.05.9009. The Cepheid is a fundamental mode pulsator with a period of 2.988 days. The orbital period of the system is 1550 days. Using spectroscopic data from three 4-8-m telescopes and photometry spanning 22 years, we were able to derive the dynamical masses and radii of both stars with exquisite accuracy. Both stars in the system are very similar in mass, radius and color, but the companion is a stable, non-pulsating star. The Cepheid is slightly more massive and bigger (M_1 = 3.70 +/- 0.03M_sun, R_1 = 28.6 +/- 0.2R_sun) than its companion (M_2 = 3.60 +/- 0.03M_sun, R_2 = 26.6 +/- 0.2R_sun). Within the observational uncertainties both stars have the same effective temperature of 6030 +/- 150K. Evolutionary tracks place both stars inside the classical Cepheid instability strip, but it is likely that future improved temperature estimates will move the stable giant companion just beyond the red edge of the instability strip. Within current observational and theoretical uncertainties, both stars fit on a 205 Myr isochrone arguing for their common age. From our model, we determine a value of the projection factor of p = 1.37 +/- 0.07 for the Cepheid in the OGLE-LMC562.05.9009 system. This is the second Cepheid for which we could measure its p-factor with high precision directly from the analysis of an eclipsing binary system, which represents an important contribution towards a better calibration of Baade-Wesselink methods of distance determination for Cepheids.

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arXiv2015

IAU Commission 37 "Star Clusters and Associations" Legacy report

It is widely accepted that stars do not form in isolation but result from the fragmentation of molecular clouds, which in turn leads to star cluster formation. Over time, clusters dissolve or are destroyed by interactions with molecular clouds or tidal stripping, and their members become part of the general field population. Star clusters are thus among the basic building blocks of galaxies. In turn, star cluster populations, from young associations and open clusters to old globulars, are powerful tracers of the formation, assembly, and evolutionary history of their parent galaxies. Although their importance had been recognised for decades, major progress in this area has only become possible in recent years, both for Galactic and extragalactic cluster populations. Star clusters are the observational foundation for stellar astrophysics and evolution, provide essential tracers of galactic structure, and are unique stellar dynamical environments. Star formation, stellar structure, stellar evolution, and stellar nucleosynthesis continue to benefit and improve tremendously from the study of these systems. Additionally, fundamental quantities such as the initial mass function can be successfully derived from modelling either the H-R diagrams or the integrated velocity structures of, respectively, resolved and unresolved clusters and cluster populations. Star cluster studies thus span the fields of Galactic and extragalactic astrophysics, while heavily affecting our detailed understanding of the process of star formation in dense environments.This report highlights science results of the last decade in the major fields covered by IAU Commission 37: Star clusters and associations.

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arXiv2014

Variable stars in the VVV globular clusters. I. 2MASS-GC02 and Terzan10

The VISTA Variables in the Via Lactea (VVV) ESO Public Survey is opening a new window to study the inner Galactic globular clusters using their variable stars. These globular clusters have been neglected in the past due to the difficulties caused by the presence of an elevated extinction and high field stellar densities in their lines of sight. However, the discovery and study of any present variables in these clusters, especially RRLyrae stars, can help to greatly improve the accuracy of their physical parameters. It can also help to shed some light on the interrogations brought by the intriguing Oosterhoff dichotomy in the Galactic globular cluster system. In a series of papers we plan to explore the variable stars in the globular clusters falling inside the field of the VVV survey. In this first paper we search and study the variables present in two highly-reddened, moderately metal-poor, faint, inner Galactic globular clusters: 2MASS-GC02 and Terzan10. We report the discovery of sizable populations of RR Lyrae stars in both globular clusters. We use near-infrared period-luminosity relations to determine the color excess of each RR Lyrae star, from which we obtain both accurate distances to the globular clusters and the ratios of the selective to total extinction in their directions. We find the extinction towards both clusters to be elevated, non-standard, and highly differential. We also find both clusters to be closer to the Galactic center than previously thought, with Terzan10 being on the far side of the Galactic bulge. Finally, we discuss their Oosterhoff properties, and conclude that both clusters stand out from the dichotomy followed by most Galactic globular clusters.

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arXiv2014

M dwarfs in the b201 tile of the VVV survey: Colour-based Selection, Spectral Types and Light Curves

The intrinsically faint M dwarfs are the most numerous stars in the Galaxy, have main-sequence lifetimes longer than the Hubble time, and host some of the most interesting planetary systems known to date. Their identification and classification throughout the Galaxy is crucial to unravel the processes involved in the formation of planets, stars and the Milky Way. The ESO Public Survey VVV is a deep near-IR survey mapping the Galactic bulge and southern plane. The VVV b201 tile, located in the border of the bulge, was specifically selected for the characterisation of M dwarfs. We used VISTA photometry to identify M dwarfs in the VVV b201 tile, to estimate their subtypes, and to search for transit-like light curves from the first 26 epochs of the survey. UKIDSS photometry from SDSS spectroscopically identified M dwarfs was used to calculate their expected colours in the $YJHK_s$ VISTA system. A colour-based spectral subtype calibration was computed. Possible giants were identified by a $(J-K_s, H_{J})$ reduced proper motion diagram. The light curves of 12.8<$K_s$<15.8 colour-selected M dwarfs were inspected for signals consistent with transiting objects. We identified 23,345 objects in VVV b201 with colours consistent with M dwarfs. We provided their spectral types and photometric distances, up to $\sim$ 300 pc for M9s and $\sim$ 1.2 kpc for M4s, from photometry. In the range 12<$K_s$<16, we identified 753 stars as possible giants out of 9,232 M dwarf candidates. While only the first 26 epochs of VVV were available, and 1 epoch was excluded, we were already able to identify transit-like signals in the light curves of 95 M dwarfs and of 12 possible giants. Thanks to its deeper photometry ($\sim$4 magnitudes deeper than 2MASS), the VVV survey will be a major contributor to the discovery and study of M dwarfs and possible companions towards the center of the Milky Way.

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arXiv2014

Confirmation of a cluster of galaxies hidden behind the Galactic bulge using the VVV Survey

Suzaku and Chandra X-ray observations detected a new cluster of galaxies, Suzaku J1759-3450, at a redshift z=0.13. It is located behind the Milky Way, and the high Galactic dust extinction renders it nearly invisible at optical wavelengths. We attempt here to confirm the galaxy cluster with near-infrared imaging observations, and to characterize its central member galaxies. Images from the VVV survey were used to detect candidate member galaxies of Suzaku J1759-3450 within the central region of the cluster, up to 350 kpc from the X-ray peak emission. Color-magnitude and color-color diagrams and morphology criteria allowed us to select the galaxies among the numerous foreground sources. Fifteen candidate cluster members were found very close to a modeled red-sequence at the redshift of the cluster. Five members are extremely bright, and one is possibly a cD galaxy. The asymmetry in the spatial distribution of the galaxies respect to the X-ray peak emission is an indicator of that this cluster is still suffering a virialization process. Our investigation of Suzaku J1759-3450 demonstrates the potential of the VVV Survey to study the hidden population of galaxies in the Zone of Avoidance.

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arXiv2014

VISTA Variables in the Vía Láctea (VVV): Halfway Status and Results

The VISTA Variables in the Vía Láctea (VVV) survey is one of six public ESO surveys, and is now in its 4th year of observing. Although far from being complete, the VVV survey has already delivered many results, some directly connected to the intended science goals (detection of variables stars, microlensing events, new star clusters), others concerning more exotic objects, e.g. novae. Now, at the end of the fourth observing period, and comprising roughly 50% of the proposed observations, the actual status of the survey, as well some of the results based on the VVV data, are presented.

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arXiv2014

Ground-based transit observations of the super-Earth GJ 1214b

GJ 1214b is one of the few known transiting super-Earth-sized exoplanets with a measured mass and radius. It orbits an M-dwarf, only 14.55 pc away, making it a favorable candidate for follow-up studies. However, the composition of GJ 1214b's mysterious atmosphere has yet to be fully unveiled. Our goal is to distinguish between the various proposed atmospheric models to explain the properties of GJ 1214b: hydrogen-rich or hydrogen-He mix, or a heavy molecular weight atmosphere with reflecting high clouds, as latest studies have suggested. Wavelength-dependent planetary radii measurements from the transit depths in the optical/NIR are the best tool to investigate the atmosphere of GJ 1214b. We present here (i) photometric transit observations with a narrow-band filter centered on 2.14 microns and a broad-band I-Bessel filter centered on 0.8665 microns, and (ii) transmission spectroscopy in the H and K atmospheric windows that cover three transits. The obtained photometric and spectrophotometric time series were analyzed with MCMC simulations to measure the planetary radii as a function of wavelength. We determined radii ratios of 0.1173 for I-Bessel and 0.11735 at 2.14 microns. Our measurements indicate a flat transmission spectrum, in agreement with last atmospheric models that favor featureless spectra with clouds and high molecular weight compositions.

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arXiv2014

VISTA variables in the Sagittarius dwarf spheroidal galaxy: pulsation- vs. dust-driven winds on the giant branches

Variability is examined in over 2.6 million stars covering 11 square degrees of the core of the Sagittarius dwarf spheroidal galaxy (Sgr dSph) from VISTA Z-band observations. Generally, pulsation on the Sgr dSph giant branches appears to be excited by the internal kappa mechanism. Pulsation amplitudes appear identical between red and asymptotic (RGB/AGB) giant stars, and between unreddened carbon and oxygen-rich stars at the same luminosity. The lack of correlation between infrared excess and variability among oxygen-rich stars indicates pulsations do not contribute significantly to wind driving in oxygen-rich stars in the Sgr dSph, though the low amplitudes of these stars mean this may not apply elsewhere. The dust-enshrouded carbon stars have the highest amplitudes of the stars we observe. Only in these stars does an external kappa-mechanism-driven pulsation seem likely, caused by variations in their more-opaque carbon-rich molecules or dust. This may allow pulsation driving of winds to be effective in carbon stars. Variability can be simplified to a power law (amplitude proportional to L/T^2), as in other systems. In total, we identify 3026 variable stars (with r.m.s. variability of delta Z >~ 0.015 magnitudes), of which 176 are long-period variables associable with the upper giant branches of the Sgr dSph. We also identify 324 candidate RR Lyrae variables in the the Sgr dSph and 340 in the outer Galactic Bulge.

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arXiv2013

Investigating potential planetary nebula/cluster pairs

Fundamental parameters characterizing the end-state of intermediate-mass stars may be constrained by discovering planetary nebulae (PNe) in open clusters (OCs). Cluster membership may be exploited to establish the distance, luminosity, age, and physical size for PNe, and the intrinsic luminosity and mass of its central star. Four potential PN-OC associations were investigated, to assess the cluster membership for the PNe. Radial velocities were measured from intermediate-resolution optical spectra, complemented with previous estimates in the literature. When the radial velocity study supported the PN/OC association, we analyzed if other parameters (e.g., age, distance, reddening, central star brightness) were consistent with this conclusion. Our measurements imply that the PNe VBe3 and HeFa1 are not members of the OCs NGC5999 and NGC6067, respectively, and likely belong to the background bulge population. Conversely, consistent radial velocities indicate that NGC2452/NGC2453 could be associated, but our results are not conclusive and additional observations are warranted. Finally, we demonstrate that all the available information point to He2-86 being a young, highly internally obscured PN member of NGC4463. New near-infrared photometry acquired via the Vista Variables in the Via Lactea ESO public survey was used in tandem with existing UBV photometry to measure the distance, reddening, and age of NGC4463, finding d=1.55+-0.10 kpc, E(B-V)=0.41+-0.02, and tau=65+-10 Myr, respectively. The same values should be adopted for the PN if the proposed cluster membership will be confirmed.

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arXiv2013

Discovery of New Companions to High Proper Motion Stars from the VVV Survey

[abridged] The severe crowding in the direction of the inner Milky Way suggests that the census of stars within a few tens of parsecs in that direction may not be complete. We search for new nearby objects companions of known high proper motion (HPM) stars located towards the densest regions of the Southern Milky Way where the background contamination presented a major problem to previous works. The common proper motion (PM) method was used--we inspected the area around 167 known HPM (>=200 mas/yr) stars: 67 in the disk and 100 in the bulge. Multi-epoch images were provided by 2MASS and the VISTA Variables in Via Lactea (VVV). The VVV is a new on-going ZYJHKs plus multi-epoch Ks survey of ~562 deg^2 of Milky Way's bulge and inner Southern disk. Seven new co-moving companions were discovered around known HPM stars; six known co-moving pairs were recovered; a pair of stars that was thought to be co-moving was found to have different proper motions; published HPMs of eight stars were not confirmed; last but not least, spectral types ranging from G8V to M5V were derived from new infrared spectroscopy for seventeen stars, members of the co-moving pairs. The seven newly discovered stars constitute ~4% of the nearby HPM star list but this is not a firm limit on the HPM star incompleteness because our starting point--the HPM list assembled from the literature--is incomplete itself, missing many nearby HPM M and L type objects, and it is contaminated with non-HPM stars. We have demonstrated, that the superior sub-arcsec spatial resolution, with respect to previous surveys, allows the VVV to examine further the binary nature nature of known HPM stars. The >=5 yr span of VVV will provide sufficient baseline for finding new HPM stars from VVV data alone.

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arXiv2013

The long bar as seen by the VVV Survey: II. Star counts

Context: There is still some debate about the presence and the morphological properties of the long bar in the inner Galaxy. Aims: We investigate the morphological properties of the long Galactic bar using the VVV survey extending star counts at least 3 mag deeper than 2MASS. Our study covers the relatively unexplored negative longitudes of the Galactic bar. We obtain a detailed description of the spatial distribution of star counts towards the long Galactic bar as well as to measure its parameters. Methods: We performed star counts towards -20<l<0 deg., |b|< 2 deg. using VVV, 2MASS, and GLIMPSE data. We applied an average interstellar extinction correction. We also adjusted latitudinal profiles to obtain the centroid variation and bar thickness. Results: We probe the structure of long Galactic bar, as well as its far edge at l=-14 deg. The differences between counts with and without extinction correction allow us to produce a crude extinction map showing regions with high extinction, mainly beyond the end of long Galactic bar. The latitudinal profiles show evidence of the centroid vertical variation with Galactic longitude reaching a minimum at l=-13.8 deg. The bar has an inclination angle 43+/-5 deg with respect to the line Sun-Galactic center. In addition, we have determined the bar parameters, such as thickness, length, and stellar distribution.

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arXiv2013

VISTA's view of the Sagittarius dwarf spheroidal galaxy and southern Galactic Bulge

We present the deepest near-infrared (ZJKs) photometry yet obtained of the Sagittarius dwarf spheroidal (Sgr dSph), using VISTA to survey 11 square degrees centred on its core. We list locations and ZJKs-band magnitudes for over 2.9 million sources in the field. We discuss the isolation of the Sgr dSph from the foreground and Galactic Bulge populations, identify the Sgr dSph's horizontal branch in the near-infrared for the first time, and map the density of the galaxy's stars. We present isochrones for the Sgr dSph and Bulge populations. These are consistent with the previously-reported properties of the Sgr dSph core: namely that it is dominated by a population between [Fe/H] ~ -1 dex and solar, with a significant [alpha/Fe] versus [Fe/H] gradient. While strong contamination from the Galactic Bulge prevents accurate measurement of the (Galactic) north side of the Sgr dSph, the dwarf galaxy can be well-approximated by a roughly ovaloid projection of characteristic size 4 x 2 degrees, beyond which the projected stellar density is less than half that of the region surrounding the core. The galaxy's major axis is perpendicular to the Galactic Plane, as in previous studies. We find slight evidence to confirm a metallicity gradient in the Sgr dSph and use isochrones to fit a distance of 24.3 +/- 2.3 kpc. We were unable to fully constrain the metallicity distribution of the Sgr dSph due to the Bulge contamination and strong correlation of [alpha/Fe] with metallicity, however we find that metal-poor stars ([Fe/H] <~ --1) make up <~29 per cent of the Sgr dSph's upper-RGB population. The Bulge population is best fit by a younger population with [Fe/H] ~ 0 and [alpha/Fe] ~ 0 or slightly higher. We find no evidence for a split, peanut- or X-shaped Bulge population in this line of sight (l = 5.6 +/- ~1 deg, b = -14.1 +/- ~3 deg).

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arXiv2013

Searching for variable stars in the VVV globular clusters

The VVV is currently surveying the central regions of the Milky Way at near-infrared wavelengths, including 36 known Galactic globular clusters. We already have data in several filters to generate their color-magnitude diagrams. We also have enough epochs to begin producing the light-curves of the cluster stars and look for any possible member variable stars. We are especially interested in RR Lyrae stars, since they are abundant in Galactic globular clusters, and the period-luminosity relation they show at near-infrared wavelengths, can help enormously in providing accurate distances and reddenings for the most extincted and poorly studied inner Galactic globular clusters. We center our attention here in the preliminary analysis of three of these clusters: NGC 6441, Terzan 10 and 2MASS-GC02.

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arXiv2013

Two planetary companions around the K7 dwarf GJ 221 : a hot super-Earth and a candidate in the sub-Saturn desert range

We report two low mass companions orbiting the nearby K7 dwarf GJ 221 that have emerged from re-analyzing 4.4 years of publicly available HARPS spectra complemented with 2 years of high precision Doppler measurements with Magellan/PFS. The HARPS measurements alone contain the clear signal of a low mass companion with a period of 125 days and a minimum mass of 53.2 \mearth (GJ 221b), falling in a mass range where very few planet candidates have been found (sub-Saturn desert). The addition of 17 PFS observations allow the confident detection of a second low mass companion (6.5 \mearth) in a hot orbit (3.87 days period, GJ 221c). Spectrocopic and photometric calibrations suggest that GJ 221 is slightly depleted ([Fe/H]$\sim$ -0.1) compared to the Sun so the presence of two low mass companions in the system confirms the trend that slightly reduced stellar metallicity does not prevent the formation of planets in the super-Earth to sub-Saturn mass regime.

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arXiv2012

Double Horizontal Branches in NGC 6440 and NGC 6569 unveiled by the VVV Survey

We report the discovery of a peculiar horizontal branch (HB) in NGC 6440 and NGC 6569, two massive and metal-rich Galactic globular clusters (GGCs) located in the Galactic bulge, within 4 kpc from the Galactic Center. In both clusters, two distinct clumps are detected at the level of the cluster HB, separated by only ~ 0.1 magnitudes in the Ks band. They were detected with IR photometric data collected with the "VISTA Variables in the Via Lactea" (VVV) Survey, and confirmed in independent IR catalogs available in the literature, and HST optical photometry. Our analysis demonstrates that these clumps are real cluster features, not a product of field contamination or interstellar reddening. The observed split HBs could be a signature of two stellar sub-populations with different chemical composition and/or age, as recently found in Terzan 5, but it cannot be excluded that they are caused by evolutionary effects, in particular for NGC 6440. This interpretation, however, requires an anomalously high helium content (Y > 0.30). Our discovery suggests that such a peculiar HB morphology could be a common feature of massive, metal-rich bulge GGCs.

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