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R. A. Mendez

Publications and source records attributed to R. A. Mendez.

At least 19 recordsLinked to original sources
arXiv2024

Enabling discovery of solar system objects in large alert data streams

With the advent of large-scale astronomical surveys such as the Zwicky Transient Facility (ZTF), the number of alerts generated by transient, variable and moving astronomical objects is growing rapidly, reaching millions per night. Concerning solar system minor planets, their identification requires linking the alerts of many observations over a potentially large time, leading to a very large combinatorial number. This work aims to identify new candidates for solar system objects from massive alert data streams produced by large-scale surveys, such as the ZTF and the Vera C. Rubin Observatory's Legacy Survey of Space and Time. Our analysis used the Fink alert broker capabilities to reduce the 111,275,131 processed alerts from ZTF between November 2019 and December 2022 to only 389,530 new solar system alert candidates over the same period. We then implemented a linking algorithm, Fink-FAT, to create real-time trajectory candidates from alert data and extract orbital parameters. The analysis was validated on ZTF alert packets linked to confirmed solar system objects from the Minor Planet Center database. Finally, the results were confronted against follow-up observations. Between November 2019 and December 2022, Fink-FAT extracted 327 new orbits from solar system object candidates at the time of the observations, over which 65 were still unreported in the MPC database as of March 2023. After two late follow-up observation campaigns of six orbit candidates, four were associated with known solar system minor planets, and two remain unknown. Fink-FAT is deployed in the Fink broker and successfully analyzes in real time the alert data from the ZTF survey by regularly extracting new candidates for solar system objects. Our scalability tests also show that Fink-FAT can handle the even larger volume of alert data that the Rubin Observatory will send.

astro-ph.EP↗
arXiv2022

Characterization of exoplanetary atmospheres through a model-unbiased spectral survey methodology

Context. Collecting a large variety of exoplanetary atmosphere measurements is crucial to improve our understanding of exoplanets. In this context, it is likely that the field would benefit from broad species surveys, particularly using transit spectroscopy, which is the most successful technique of exoplanetary atmosphere characterization so far. Aims: Our goal is to develop a model-unbiased technique using transit spectroscopy to analyze every qualified atomic spectral line in exoplanetary transit data, and search for relative absorption, that is, a decrease in the flux of the line when the planet is transiting. Methods: We analyzed archive data from HDS at Subaru, HIRES at Keck, UVES at VLT, and HARPS at LaSilla to test our spectral survey methodology. It first filtered individual lines by relative noise levels. It also corrected for spectral offsets and telluric contamination. Our methodology performed an analysis along time and wavelength. The latter employed a bootstrap corroboration. Results: We highlight the possible detections of Mn I and V II in HD 209459b data taken by HDS at Subaru ($5.9σ$ at 5916.4 $\angstrom$, $5.1σ$ at 6021.8 $\angstrom$). The previous detection of Ca I in the same planet is classified as inconclusive by our algorithm, but we support the previous detection of Sc II ($3.5σ$ at 6604.6 $\angstrom$). We also highlight the possible detection of Ca I, Sc II, and Ti II in HD 189733 data taken by UVES at VLT ($4.4σ$ at $6572.8 \angstrom$, $6.8σ$ at $6604.6 \angstrom$, and $3.5σ$ at $5910.1 \angstrom$), in addition to the possible detection of Al I in WASP-74b data taken by UVES at VLT ($5.6 σ$ at $6696.0 \angstrom$).

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arXiv2019

On the origin of very massive stars around NGC 3603

The formation of the most massive stars in the Universe remains an unsolved problem. Are they able to form in relative isolation in a manner similar to the formation of solar-type stars, or do they necessarily require a clustered environment? In order to shed light on this important question, we study the origin of two very massive stars (VMS): the O2.5If*/WN6 star RFS7 ($\sim$100 $M_{\odot}$), and the O3.5If* star RFS8 ($\sim$70 $M_{\odot}$), found within $\approx$ 53 and 58 pc respectively from the Galactic massive young cluster NGC 3603, using Gaia data. RFS7 is found to exhibit motions resembling a runaway star from NGC 3603. This is now the most massive runaway star candidate known in the Milky Way. Although RFS8 also appears to move away from the cluster core, it has proper-motion values that appear inconsistent with being a runaway from NGC 3603 at the $3σ$ level (but with substantial uncertainties due to distance and age). Furthermore, no evidence for a bow-shock or a cluster was found surrounding RFS8 from available near-infrared photometry. In summary, whilst RFS7 is likely a runaway star from NGC 3603, making it the first VMS runaway in the Milky Way, RFS8 is an extremely young ($\sim$2 Myr) VMS, which might also be a runaway, but this would need to be established from future spectroscopic and astrometric observations, as well as precise distances. If RFS8 were still not meeting the criteria for being a runaway from NGC 3603 from such future data, this would have important ramifications for current theories of massive star formation, as well as the way the stellar initial mass function (IMF) is sampled.

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arXiv2018

Parallaxes of southern extremely cool objects III: 118 L \& T dwarfs

We present new results from the Parallaxes of Southern Extremely Cool dwarfs program to measure parallaxes, proper motions and multi-epoch photometry of L and early T dwarfs. The observations were made on 108 nights over the course of 8 years using the Wide Field Imager on the ESO 2.2m telescope. We present 118 new parallaxes of L \& T dwarfs of which 52 have no published values and 24 of the 66 published values are preliminary estimates from this program. The parallax precision varies from 1.0 to 15.5 mas with a median of 3.8 mas. We find evidence for 2 objects with long term photometric variation and 24 new moving group candidates. We cross-match our sample to published photometric catalogues and find standard magnitudes in up to 16 pass-bands from which we build spectral energy distributions and H-R diagrams. This allows us to confirm the theoretically anticipated minimum in radius between stars and brown dwarfs across the hydrogen burning minimum mass. We find the minimum occurs between L2 and L6 and verify the predicted steep dependence of radius in the hydrogen burning regime and the gentle rise into the degenerate brown dwarf regime. We find a relatively young age of 2 Gyr from the kinematics of our sample.

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arXiv2017

Characterizing the astrometric precision limit for moving targets observed with digital-array detectors

Aims. We investigate the maximum astrometric precision that can be reached on moving targets observed with digital-sensor arrays, and provide an estimate for its ultimate lower limit based on the Cramér-Rao bound. Methods. We extend previous work on one-dimensional Gaussian point-spread functions (PSFs) focusing on moving objects and extending the scope to two-dimensional array detectors. In this study the PSF of a stationary point-source celestial body is replaced by its convolution with a linear motion, thus effectively modeling the spread function of a moving target. Results. The expressions of the Cramér-Rao lower bound deduced by this method allow us to study in great detail the limit of astrometric precision that can be reached for moving celestial objects, and to compute an optimal exposure time according to different observational parameters such as seeing, detector pixel size, decentering, and elongation of the source caused by its drift. Comparison to simulated and real data shows that the predictions of our simple model are consistent with observations.

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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.

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arXiv2016

Young stars in the periphery of the Large Magellanic Cloud

Despite their close proximity, the complex interplay between the two Magellanic Clouds, the Milky Way, and the resulting tidal features, is still poorly understood. Recent studies have shown that the Large Magellanic Cloud (LMC) has a very extended disk strikingly perturbed in its outskirts. We search for recent star formation in the far outskirts of the LMC, out to ~30 degrees from its center. We have collected intermediate-resolution spectra of thirty-one young star candidates in the periphery of the LMC and measured their radial velocity, stellar parameters, distance and age. Our measurements confirm membership to the LMC of six targets, for which the radial velocity and distance values match well those of the Cloud. These objects are all young (10-50 Myr), main-sequence stars projected between 7 and 13 degrees from the center of the parent galaxy. We compare the velocities of our stars with those of a disk model, and find that our stars have low to moderate velocity differences with the disk model predictions, indicating that they were formed in situ. Our study demonstrates that recent star formation occurred in the far periphery of the LMC, where thus far only old objects were known. The spatial configuration of these newly-formed stars appears ring-like with a radius of 12 kpc, and a displacement of 2.6 kpc from the LMC's center. This structure, if real, would be suggestive of a star-formation episode triggered by an off-center collision between the Small Magellanic Cloud and the LMC's disk.

astro-ph.GA↗
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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arXiv2014

On local dark matter density

In 2012, we applied a three-dimensional formulation to kinematic measurements of the Galactic thick disk and derived a surprisingly low dark matter density at the solar position. This result was challenged by Bovy & Tremaine (2012, ApJ, 756, 89), who claimed that the observational data are consistent with the expected dark matter density if a one-dimensional approach is adopted. We analyze the assumption at the bases of their formulation and their claim that this returns a lower limit for the local dark matter density, which is accurate within 20%. We find that the validity of their formulation depends on the underlying mass distribution. We therefore analyze the predictions that their hypothesis casts on the radial gradient of the azimuthal velocity dV/dR and compare it with observational data as a testbed for the validity of their formulation. We find that their hypothesis requires too steep a profile of dV(Z)/dR, which is inconsistent with the observational data both in the Milky Way and in external galaxies. As a consequence, their results are biased and largely overestimate the mass density. Dynamical simulations also show that, contrary to their claims, low values of dV/dR are compatible with a Milky Way like potential with radially constant circular velocity. We nevertheless confirm that, according to their criticism, our assumption dV/dR=0 is only an approximation. If this hypothesis is released, and the available information about dV/dR in the thick disk is used, the resulting local dark matter density increases by a tiny amount, from 0+-1 to 2+-3 mM_sun pc^(-3), with an upper limit of ~3.5 mM_sun pc^(-3). Hence, this approximation has negligible influence on our results. Our analysis shows that their criticism is not a viable explanation for the inferred lack of dark matter at the solar position detected by us. More studies are required to understand these unexpected results.

astro-ph.GA↗
arXiv2014

Temperature constraints on the coldest brown dwarf known WISE 0855-0714

Context. Nearby isolated planetary mass objects are beginning to be discovered, but their individual properties are poorly constrained because their low surface temperatures and strong molecular self-absorption make them extremely faint. Aims. We aimed to detect the near infrared emission of the coldest brown dwarf (BD) found so far, WISE0855$-$0714, located $\sim$2.2 pc away, and to improve its temperature estimate (T$_{\rm eff}$= 225-260 K) from a comparison with state-of-the-art models of BD atmospheres. Methods. We observed the field containing WISE0855-0714 with HAWK-I at the VLT in the $Y$ band. For BDs with T$_{\rm eff}<$500\,K theoretical models predict strong signal (or rather less molecular absorption) in this band. Results. WISE0855-0714 was not detected in our Y-band images, thus placing an upper limit on its brightness to Y>24.4 mag at 3-$σ$ level, leading to Y-[4.5]>10.5. Combining this limit with previous detections and upper limits at other wavelengths, WISE0855$-$0714 is confirmed as the reddest BD detected, further supporting its status as the coldest known brown dwarf. We applied spectral energy distribution fitting with collections of models from two independent groups for extremely cool BD atmospheres leading to an effective temperature of T$_{\rm eff}<$250\,K,.

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arXiv2013

OB stars in the Leading Arm of the Magellanic Stream

We present our spectroscopic program aimed to study some new interesting features recently discovered in the Magellanic Cloud System. These were revealed by the spatial distribution of OB-type candidate stars, selected based on UV, optical and IR photometry and proper motions from existing large-area catalogs. As a pilot study of our project, we are studying OB-star candidates in the Leading Arm (LA) of the Magellanic Stream, a gaseous tidal structure with no stellar counterpart known so far. Our targets group in three clumps near regions of high HI density in the LA. If confirmed, these young stars would evidence recent star formation in the LA, and they would help better understand and constrain the formation of the LA and its interactions with the Milky Way.

astro-ph.GA↗
arXiv2012

No evidence of dark matter in the solar neighborhood

We measured the surface mass density of the Galactic disk at the solar position, up to 4 kpc from the plane,by means of the kinematics of ~400 thick disk stars. The results match the expectations for the visible mass only, and no dark matter is detected in the volume under analysis. The current models of dark matter halo are excluded with a significance higher than 5sigma, unless a highly prolate halo is assumed, very atypical in cold dark matter simulations. The resulting lack of dark matter at the solar position challenges the current models.

astro-ph.GA↗
arXiv2012

Kinematical and chemical vertical structure of the Galactic thick disk II. A lack of dark matter in the solar neighborhood

We estimated the dynamical surface mass density Sigma at the solar position between Z=1.5 and 4 kpc from the Galactic plane, as inferred from the kinematics of thick disk stars. The formulation is exact within the limit of validity of a few basic assumptions. The resulting trend of Sigma(Z) matches the expectations of visible mass alone, and no dark component is required to account for the observations. We extrapolate a dark matter (DM) density in the solar neighborhood of 0+-1 mM_sun pc^-3, and all the current models of a spherical DM halo are excluded at a confidence level higher than 4sigma. A detailed analysis reveals that a small amount of DM is allowed in the volume under study by the change of some input parameter or hypothesis, but not enough to match the expectations of the models, except under an exotic combination of non-standard assumptions. Identical results are obtained when repeating the calculation with kinematical measurements available in the literature. We demonstrate that a DM halo would be detected by our method, and therefore the results have no straightforward interpretation. Only the presence of a highly prolate (flattening q>2) DM halo can be reconciled with the observations, but this is highly unlikely in LambdaCDM models. The results challenge the current understanding of the spatial distribution and nature of the Galactic DM. In particular, our results may indicate that any direct DM detection experiment is doomed to fail, if the local density of the target particles is negligible.

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arXiv2012

Kinematical and chemical vertical structure of the Galactic thick disk I. Thick disk kinematics

The variation of the kinematical properties of the Galactic thick disk with Galactic height Z are studied by means of 412 red giants observed in the direction of the south Galactic pole up to 4.5 kpc from the plane. We confirm the non-null mean radial motion toward the Galactic anticenter found by other authors, but we find that it changes sign at |Z|=3 kpc, and the proposed inward motion of the LSR alone cannot explain these observations. The rotational velocity decreases with |Z| by -30 km/s/kpc, but the data are better represented by a power-law with index 1.25, similar to that proposed from the analysis of SDSS data. All the velocity dispersions increase with |Z|, but the vertical gradients are small. The dispersions grow proportionally, with no significant variation of the anisotropy. The ratio sigma_U/sigma_W=2 suggests that the thick disk could have formed from a low-latitude merging event. The vertex deviation increases with Galactic height, reaching ~20 degrees at |Z|=3.5 kpc. The tilt angle also increases, and the orientation of the ellipsoid in the radial-vertical plane is constantly intermediate between the alignment with the cylindrical and the spherical coordinate systems. The tilt angle at |Z|=2 kpc coincides with the expectations of MOND, but an extension of the calculations to higher |Z| is required to perform a conclusive test. Finally, between 2.5 and 3.5 kpc we detect deviations from the linear trend of many kinematical quantities, suggesting that some kinematical substructure could be present.

astro-ph.GA↗
arXiv2010

No evidence for a dark matter disk within 4 kpc from the Galactic plane

We estimated the dynamical surface mass density (Sigma) at the solar Galactocentric distance between 2 and 4 kpc from the Galactic plane, as inferred from the observed kinematics of the thick disk. We find Sigma(z=2 kpc)=57.6+-5.8 Mo pc^-2, and it shows only a tiny increase in the z-range considered by our investigation. We compared our results with the expectations for the visible mass, adopting the most recent estimates in the literature for contributions of the Galactic stellar disk and interstellar medium, and proposed models of the dark matter distribution. Our results match the expectation for the visible mass alone, never differing from it by more than 0.8 $Mo pc^-2 at any z, and thus we find little evidence for any dark component. We assume that the dark halo could be undetectable with our method, but the dark disk, recently proposed as a natural expectation of the LambdaCDM models, should be detected. Given the good agreement with the visible mass alone, models including a dark disk are less likely, but within errors its existence cannot be excluded. In any case, these results put constraints on its properties: thinner models (scale height lower than 4 kpc) reconcile better with our results and, for any scale height, the lower-density models are preferred. We believe that successfully predicting the stellar thick disk properties and a dark disk in agreement with our observations could be a challenging theoretical task.

astro-ph.GA↗
arXiv2008

Trumpler 20 - an old and rich open cluster

We show that the open cluster Trumpler 20, contrary to the earlier findings, is actually an old Galactic open cluster. New CCD photometry and high-resolution spectroscopy are used to derive the main parameters of this cluster. At [Fe/H]=-0.11 for a single red giant star, the metallicity is slightly subsolar. The best fit to the color-magnitude diagrams is achieved using a 1.3 Gyr isochrone with convective overshoot. The cluster appears to have a significant reddening at E(B-V)=0.46 (for B0 spectral type), although for red giants this high reddening yields the color temperature exceeding the spectroscopic T_eff by about 200 K. Trumpler 20 is a very rich open cluster, containing at least 700 members brighter than M_V=+4. It may extend over the field-of-view available in our study at 20'x20'.

astro-ph↗
arXiv2007

The vertical velocity dispersion profile of the Galactic thick disk

We present the results of radial velocity measurements of 770 thick disk red giants toward the South Galactic Pole, vertically distributed from 0.5 kpc to 5 kpc with respect to the Galactic plane. We find a small gradient in the vertical velocity dispersion (sigma_W) of 3.8+/-0.8 km/s kpc. Even more noteworthy, our values of $σ_W$ are small compared to literature values: in the middle of the vertical height range we find sigma_W(z=2kpc)=30 km/s. We found no possible explanation for this small value of sigma_W in terms of sample contamination by thin disk stars, nor by wrong assumptions regarding the metallicity distribution and the derived distances.

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