arXiv subjects

M. Gomez

Publications and source records attributed to M. Gomez.

At least 19 recordsLinked to original sources

arXiv2025

Anomalous metallic phase and reduced critical current in superconducting nanowires due to inverse proximity effect

Superconductor-to-metal transitions (SMTs) are key probes of mesoscopic superconductivity, but their interpretation can be complicated by device geometry and measurement conditions. Here, we study epitaxial InAs-Al nanowires and show that metallic contacts induce an inverse proximity effect (IPE), creating weak spots in the superconductor that strongly suppress the critical current and give rise to an anomalous metallic phase. Using transport measurements supported by Usadel theory, we demonstrate that this phase originates from the contact-induced weakening of superconductivity together with Joule heating, rather than intrinsic material properties. Our findings reveal an overlooked observer effect in mesoscopic superconductors and provide essential guidance for interpreting SMTs and for designing devices based on these systems.

cond-mat.mes-hall↗

arXiv2025

Gaseous disks in white dwarfs: properties of the current sample

In recent years, the number of white dwarfs (WDs) showing infrared (IR) excesses, attributed to circumstellar dust disks, has grown significantly. Gaseous disks have also been detected in some WDs with dust. We obtained optical spectra with Gemini/GMOS for thirteen WDs exhibiting IR excesses to search for gaseous components. No Ca II emission was detected, suggesting the gas phase may be short-lived compared to the dusty stage. Combining our data with literature, we compiled the largest sample of WDs with and without gas disks. We find no significant differences in stellar properties between the two groups, although disks with gas may be, on average, brighter. The detection rate of Ca II emission supports the idea that the gas phase persists for only a fraction of the total disk lifetime. A more definitive assessment will likely require dedicated searches for both gas and dust across large WD samples.

astro-ph.SR↗

arXiv2024

Searching for globular clusters in the inner halo of the Circinus galaxy

In this study, we search for Globular Clusters (GCs) in the inner halo of the Circinus galaxy using a combination of observational data. Our dataset includes observations from the VISTA Variables in the V\'ia L\'actea Extended Survey (VVVX), optical data from Gaia Release 3 (DR3), and observations from the Dark Energy Camera (DECam). These multiple data sources provide a comprehensive basis for our analysis. Our search was concentrated within a 50 kpc radius from the centre, leading to the identification of 93 sources that met our established criteria. To ensure the reliability of our findings, we conducted multiple examinations for sample contamination. These examinations incorporated tests based on Gaia Astrometric Excess Noise (AEN), the Blue Photometer (BP)/Red Photometer (RP) Excess Factor (BRexcess), as well as comparisons with stellar population models. This analysis confidently classified 41 sources as genuine GCs, as they successfully passed both the 3$\sigma$ Gaia AEN and BRexcess tests. We used the ISHAPE program to determine the structural parameters (half-light radii) of the GC candidates, with a peak effective radius of 4$\pm$ 0.5 pc. The catalogue mainly consists of bright GCs. Relationships between colour, size, and distance were found in the GC candidates, alongside confirmation of bi-modality in colour distributions.

astro-ph.GA↗

arXiv2023

The globular cluster VVV CL002 falling down to the hazardous Galactic centre

Context. The Galactic centre is hazardous for stellar clusters because of the strong tidal force. Supposedly, many clusters were destroyed and contributed stars to the crowded stellar field of the bulge and the nuclear stellar cluster. However, it is hard to develop a realistic model to predict the long-term evolution of the complex inner Galaxy, and observing surviving clusters in the central region would provide crucial insights into destruction processes. Aims. Among hitherto-known Galactic globular clusters, VVV CL002 is the closest to the centre, 0.4 kpc, but has a very high transverse velocity, 400 km s$^{-1}$. The nature of this cluster and its impact on Galactic astronomy need to be addressed with spectroscopic follow-up. Methods. Here we report the first measurements of its radial velocity and chemical abundance based on near-infrared high-resolution spectroscopy. Results. We found that this cluster has a counterrotating orbit constrained within 1.0\,kpc of the centre, as close as 0.2 kpc at the perigalacticon, confirming that the cluster is not a passerby from the halo but a genuine survivor enduring the harsh conditions of the Galactic mill's tidal forces. In addition, its metallicity and $\alpha$ abundance ([$\alpha$/Fe] $\simeq +0.4$ and [Fe/H]$=-0.54$) are similar to some globular clusters in the bulge. Recent studies suggest that stars with such $\alpha$-enhanced stars were more common at 3 - 6 kpc from the centre around 10 Gyrs ago. Conclusions. We infer that VVV CL002 was formed outside but is currently falling down to the centre, exhibiting a real-time event that must have occurred to many clusters a long time ago.

astro-ph.GA↗

arXiv2021

FSR 1776: a new globular cluster in the Galactic bulge?

(ABRIDGED) Recent near-IR surveys have uncovered a plethora of new globular cluster (GC) candidates towards the Milky Way bulge. These new candidates need to be confirmed as real GCs and properly characterised. We investigate the physical nature of FSR 1776. This object was originally classified as an intermediate-age open cluster and has recently been re-discovered independently and classified as a GC candidate (Minni 23). Firstly, we aim at confirming its GC nature; secondly we determine its physical parameters. The confirmation of the cluster existence is checked using the radial velocity (RV) distribution of a MUSE data cube centred at FSR 1776. The cluster parameters are derived from isochrone fitting to the RV-cleaned colour-magnitude diagrams (CMDs) from visible and near-infrared photometry. The predicted RV distribution for the FSR 1776 coordinates, considering only contributions from the bulge and disc field stars, is not enough to explain the observed MUSE RV distribution. The extra population (12\% of the sample) is FSR 1776 with an average RV of $-103.7\pm 0.4~{\rm km}\,{\rm s}^{-1}$. The CMDs reveal that it is 10$\pm$1~Gyr old and metal-rich, with [Fe/H]$_{phot}\approx + 0.2\pm$0.2, [Fe/H]$_{spec}=~+0.02\pm0.01~(\sigma~=~0.14$~dex), located at the bulge distance of 7.24$\pm$0.5~kpc with A$_{\rm V}$ $\approx$ 1.1~mag. The mean cluster proper motions are ($\langle\mu_{\alpha}\rangle,\langle\mu_{\delta}\rangle$) $=$ ($-2.3\pm1.1,-2.6\pm0.8$) ${\rm mas\, yr^{-1}}$.} FSR 1776 is an old GC located in the Galactic bulge with a super-solar metallicity, among the highest for a Galactic GC. This is consistent with predictions for the age-metallicity relation of the bulge, being FSR 1776 the probable missing link between typical GCs and the metal-rich bulge field. High-resolution spectroscopy of a larger field of view and deeper CMDs are now required for a full characterisation.

astro-ph.GA↗

arXiv2008

A Catalog of Extended Green Objects (EGOs) in the GLIMPSE Survey: A new sample of massive young stellar object outflow candidates

Using images from the Spitzer GLIMPSE Legacy survey, we have identified more than 300 extended 4.5 micron sources (abbreviated EGO, Extended Green Object, for the common coding of the [4.5] band as green in 3-color composite IRAC images). We present a catalog of these EGOs, including integrated flux density measurements at 3.6, 4.5, 5.8, 8.0, and 24 microns from the GLIMPSE and MIPSGAL surveys. The average angular separation between a source in our sample and the nearest IRAS point source is >1 arcminute. The majority of EGOs are associated with infrared dark clouds (IRDCs), and where high-resolution 6.7 GHz methanol maser surveys overlap the GLIMPSE coverage, EGOs and 6.7 GHz methanol masers are strongly correlated. Extended 4.5 micron emission is thought to trace shocked molecular gas in protostellar outflows; the association of EGOs with IRDCs and 6.7 GHz methanol masers suggests that extended 4.5 micron emission may pinpoint outflows specifically from massive protostars. The mid-infrared colors of EGOs lie in regions of color-color space occupied by young protostars still embedded in infalling envelopes.

astro-ph↗

arXiv2008

Spectroscopic metallicities of Vega-like stars

Aims: To determine the metallicities of 113 Southern Hemisphere Vega-like candidate stars in relation to the Exoplanet host group and field stars. Methods: We applied two spectroscopic methods of abundance determinations: equivalent width measurements together with the ATLAS9 (Kurucz 1993) model atmospheres and the WIDTH9 program, and a comparison of observed spectra with the grid of synthetic spectra of Munari et al. (2005). Results: For the Vega-like group, the metallicities are indistinguishable from those of field stars not known to be associated with planets or disks. This result is quite different from the metallicities of Exoplanet host stars which are metal-rich in comparison to field stars.

astro-ph↗

arXiv2007

Science with the new generation high energy gamma- ray experiments

This Conference is the fifth of a series of Workshops on High Energy Gamma- ray Experiments, following the Conferences held in Perugia 2003, Bari 2004, Cividale del Friuli 2005, Elba Island 2006. This year the focus was on the use of gamma-ray to study the Dark Matter component of the Universe, the origin and propagation of Cosmic Rays, Extra Large Spatial Dimensions and Tests of Lorentz Invariance.

astro-ph↗

arXiv2007

Sizes of Confirmed Globular Clusters in NGC 5128: A Wide-Field High-Resolution Study

Using Magellan/IMACS images covering a 1.2 x 1.2 sq. degree FOV with seeing of 0.4"-0.6", we have applied convolution techniques to analyse the light distribution of 364 confirmed globular cluster in the field of NGC 5128 and to obtain their structural parameters. Combining these parameters with existing Washington photometry from Harris et al. (2004), we are able to examine the size difference between metal-poor (blue) and metal-rich (red) globular clusters. For the first time, this can be addressed on a sample of confirmed clusters that extends to galactocentric distances about 8 times the effective radius, R$_{eff}$, of the galaxy. Within 1 R$_{eff}$, red clusters are about 30% smaller on average than blue clusters, in agreement with the vast majority of extragalactic globular cluster systems studied. As the galactocentric distance increases, however, this difference becomes negligible. Thus, our results indicate that the difference in the clusters' effective radii, r$_e$, could be explained purely by projection effects, with red clusters being more centrally concentrated than blue ones and an intrinsic r$_e$--R$_{gc}$ dependence, like the one observed for the Galaxy.

astro-ph↗

arXiv2007

GAW - An Imaging Atmospheric Cherenkov Telescope with Large Field of View

GAW, acronym for Gamma Air Watch, is a Research and Development experiment in the TeV range, whose main goal is to explore the feasibility of large field of view Imaging Atmospheric Cherenkov Telescopes. GAW is an array of three relatively small telescopes (2.13 m diameter) which differs from the existing and presently planned projects in two main features: the adoption of a refractive optics system as light collector and the use of single photoelectron counting as detector working mode. The optics system allows to achieve a large field of view (24x24 squared degrees) suitable for surveys of large sky regions. The single photoelectron counting mode in comparison with the charge integration mode improves the sensitivity by permitting also the reconstruction of events with a small number of collected Cherenkov photons. GAW, which is a collaboration effort of Research Institutes in Italy, Portugal and Spain, will be erected in the Calar Alto Observatory (Sierra de Los Filabres - Andalucia, Spain), at 2150 m a.s.l.). The first telescope will be settled within Autumn 2007. This paper shows the main characteristics of the experiment and its expected performance.

astro-ph↗

arXiv2005

Structural Parameters from Ground-based Observations of Newly Discovered Globular Clusters in NGC 5128

We have investigated a number of globular cluster candidates from a recent wide-field study by Harris et al. (2004a) of the giant elliptical galaxy NGC 5128. We used the Magellan I telescope + MagIC camera under excellent seeing (0.3"-0.6") and obtained very high resolution images for a sample of 44 candidates. Our images allow us to study the light profiles of the likely clusters. This is the first ground-based study of structural parameters for globular clusters outside the Local Group. We compare the psf-deconvolved profiles with King models and derive structural parameters, ellipticities and surface brightnesses. In general, our clusters are similar in size, ellipticity, core radius and central surface brightness to their counterparts in other galaxies, in particular those in NGC 5128 observed with HST by Harris et al. (2002). However, they extend to higher ellipticities and larger half-light radii than their Galactic counterparts. Combining our results with those of Harris et al. fills in the gaps previously existing in $r_h - M_V$ parameter space and indicates that any substantial difference between presumed distinct cluster types in this diagram, including for example the Faint Fuzzies of Larsen & Brodie (2000) and the `extended, luminous' M31 clusters of Huxor et al. (2005) is now removed and that clusters form a continuum in this diagram. Indeed, this continuum now extends to the realm of the Ultra Compact Dwarfs. We have carried out additional analysis to quantify the contamination by background galaxies. This shows that, although galaxies cannot be easily told apart from clusters in some of the structural diagrams, the combination of excellent image quality and Washington photometry should limit the contamination to roughly 10% of the population of cluster candidates.

astro-ph↗

arXiv2005

Distances to six Cepheids in the LMC cluster NGC1866 from the near-IR surface-brightness method

We derive individual distances to six Cepheids in the young populous star cluster NGC1866 in the Large Magellanic Cloud employing the near-IR surface brightness technique. With six stars available at the exact same distance we can directly measure the intrinsic uncertainty of the method. We find a standard deviation of 0.11 mag, two to three times larger than the error estimates and more in line with the estimates from Bayesian statistical analysis by Barnes et al. (2005). Using all six distance estimates we determine an unweighted mean cluster distance of 18.30+-0.05. The observations indicate that NGC1866 is close to be at the same distance as the main body of the LMC. If we use the stronger dependence of the p-factor on the period as suggested by Gieren et al. (2005) we find a distance of 18.50+-0.05 (internal error) and the PL relations for Galactic and MC Cepheids are in very good agreement.

astro-ph↗

arXiv2005

On the Ages of Exoplanet Host Stars

We obtained spectra, covering the CaII H and K region, for 49 exoplanet host (EH) stars, observable from the southern hemisphere. We measured the chromospheric activity index, Rhk. We compiled previously published values of this index for the observed objects as well as the remaining EH stars in an effort to better smooth temporal variations and derive a more representative value of the average chromospheric activity for each object. We used the average index to obtain ages for the group of EH stars. In addition we applied other methods, such as: Isochrone, lithium abundance, metallicity and transverse velocity dispersions, to compare with the chromospheric results. The kinematic method is a less reliable age estimator because EH stars lie red-ward of Parenago's discontinuity in the transverse velocity dispersion vs dereddened B-V diagram. The chromospheric and isochrone techniques give median ages of 5.2 and 7.4 Gyr, respectively, with a dispersion of 4 Gyr. The median age of F and G EH stars derived by the isochrone technique is 1--2 Gyr older than that of identical spectral type nearby stars not known to be associated with planets. However, the dispersion in both cases is large, about 2--4 Gyr. We searched for correlations between the chromospheric and isochrone ages and Lir/L* (the excess over the stellar luminosity) and the metallicity of the EH stars. No clear tendency is found in the first case, whereas the metallicy dispersion seems to slightly increase with age.

astro-ph↗

arXiv2005

A tiny Galactic dust cloud projected onto NGC 3269?

We report on new observations obtained with the Magellan Clay-telescope of a tiny dust patch in the Sa-galaxy NGC 3269 in the Antlia galaxy cluster. It has already been suspected to be a projected Galactic foreground cloud. In this case a size of about 4 arcsec and a maximum absorption of ~1 mag in the B-band would make it unique so far. We present further evidence for its Galactic nature from imaging under excellent seeing conditions (0.3-0.6 arcsec). This dust cloud could be the first optically identified counterpart of a new type of molecular cloud recently discovered by Heithausen (2004).

astro-ph↗

arXiv2003

The Globular Cluster System of NGC4374

We study the globular cluster system (GCS) of the giant elliptical NGC4374 (M84) in the Virgo cluster using B and R photometry. The colour distribution is bimodal with peaks at B-R=1.11 and B-R=1.36, fitting well to those found in other early-type galaxies. The radial profile of the cluster number density is flatter than the galaxy light. Using the luminosity function we derive a distance modulus of $μ=31.61\pm0.2$, which within the uncertainty agrees with the distance from surface brightness fluctuations. Blue and red clusters show similar radial concentrations and azimuthal distributions. The total number of clusters is $N=1775\pm150$, which together with our distance modulus leads to a specific frequency of $S_{N}=1.6\pm0.3$. This value is surprisingly low for a giant elliptical, but resembles the case of merger remnants like NGC1316, where the low specific frequency is probably caused by the luminosity contribution of an intermediate-age population. A further common property is the high rate of type Ia supernovae which also may indicate the existence of a younger population. However, unlike in the case of NGC1316, one cannot find any further evidence that NGC4374 indeed hosts younger populations. The low specific frequency would also fit to a S0 galaxy seen face-on.

astro-ph↗

arXiv2003

Optical spectra of selected Chamaeleon I young stellar objects

We present optical spectra of eight candidate brown dwarfs and a previously known T Tauri star (Sz 33) of the Chamaeleon I dark cloud. We derived spectral types based on the strength of the TiO or VO absorption bands present in the spectra of these objects as well as on the PC3 index of Martin et al. (1999). Photometric data from the literature are used to estimate the bolometric luminosities for these sources. We apply D'Antona & Mazzitelli (1997) pre-main sequence evolutionary tracks and isochrones to derive masses and ages. Based on the presence of Halpha in emission, we confirm that most of the candidates are young objects. Our sample however includes two sources for which we can only provide upper limits for the emission in Halpha; whereas these two objects are most likely foreground/background stars, higher resolution spectra are required to confirm their true nature. Among the likely cloud members, we detect one new sub-stellar object and three transition stellar/sub-stellar sources.

astro-ph↗

arXiv2003

Near-infrared Spectra of Chamaeleon I Stars

We present low resolution (R = 500) near-infrared spectra of 46 candidate young stellar objects in the Chamaeleon I star-forming region recently detected in several deep photometric surveys of the cloud. Most of these stars have K < 12. In addition, we present spectra of 63 previously known southern hemisphere young stars mainly belonging to the Chamaeleon I and Lupus dark clouds. We describe near-infrared spectroscopic characteristics of these stars and use the water vapor indexes to derive spectral types for the new objects. Photometric data from the literature are used to estimate the bolometric luminosities of all sources. We apply D'Antona & Mazzitelli (1998) pre-main sequence evolutionary tracks and isocrones to derive masses and ages. We detect two objects with mass below the H burning limit among the 46 new candidates. One of this object (PMK99 IR Cha INa1) is the likely driving source of a bipolar outflow in the northern region of the cloud.

astro-ph↗

arXiv2000

A Near-Infrared Imaging Survey of the Chamaeleon I Dark Cloud

We describe a near-infrared imaging survey covering approximately 1 square deg of the Chamaeleon I dark cloud. The survey is complete for K < 15.0, H < 16.0, and J < 16.5, roughly two magnitudes more sensitive than previous large scale surveys. We use the large number of background stars detected to derive an accurate near-infrared extinction law for the cloud and select new candidate members with near-infrared color excesses. We list about 100 candidates of the cloud with K > 12.0, based on their positions in the J-H, H-K color-color diagram. These new stars have low luminosities (K > 12 -- 16, H-K > 0.5 -- 1.5) and may have masses close to or even below the hydrogen burning limit.

astro-ph↗