arXiv subjects

Eva Villaver

Publications and source records attributed to Eva Villaver.

At least 37 records · Page 2Linked to original sources

arXiv2021

Faint objects in motion: the new frontier of high precision astrometry

Sky survey telescopes and powerful targeted telescopes play complementary roles in astronomy. In order to investigate the nature and characteristics of the motions of very faint objects, a flexibly-pointed instrument capable of high astrometric accuracy is an ideal complement to current astrometric surveys and a unique tool for precision astrophysics. Such a space-based mission will push the frontier of precision astrometry from evidence of Earth-mass habitable worlds around the nearest stars, to distant Milky Way objects, and out to the Local Group of galaxies. As we enter the era of the James Webb Space Telescope and the new ground-based, adaptive-optics-enabled giant telescopes, by obtaining these high precision measurements on key objects that Gaia could not reach, a mission that focuses on high precision astrometry science can consolidate our theoretical understanding of the local Universe, enable extrapolation of physical processes to remote redshifts, and derive a much more consistent picture of cosmological evolution and the likely fate of our cosmos. Already several missions have been proposed to address the science case of faint objects in motion using high precision astrometry missions: NEAT proposed for the ESA M3 opportunity, micro-NEAT for the S1 opportunity, and Theia for the M4 and M5 opportunities. Additional new mission configurations adapted with technological innovations could be envisioned to pursue accurate measurements of these extremely small motions. The goal of this White Paper is to address the fundamental science questions that are at stake when we focus on the motions of faint sky objects and to briefly review instrumentation and mission profiles.

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arXiv2021

On the most luminous planetary nebulae of M31

To study the progenitors of PNe at the tip of the Planetary Nebula Luminosity Function (PNLF), we obtained the deepest existing spectra of a sample of PNe in the galaxy M31. Precise chemical abundances allow us to confront the theoretical yields for Asymptotic Giant Branch (AGB) stellar masses and metallicities expected at the bright end of the PNLF. Central star masses of the sampled PNe provide direct information on the controversial origin of the universal cutoff of the PNLF. Using the 10.4m Gran Telescopio Canarias (GTC) optical spectra of nine bright M31 PNe were obtained: four of them at the tip of the PNLF, and the other five some 0.5 magnitudes fainter. A control sample of 21 PNe with previous GTC spectra from literature is also included. Their physical properties and chemical abundances (He, N, O, Ar, Ne and S) are analysed. The central star masses are estimated with Cloudy modelling using the most recent evolutionary tracks. The studied PNe show a remarkable uniformity in all their nebular properties, the brightest PNe showing relatively large electron densities. Stellar characteristics also span a narrow range: ${ = 4300 \pm 310, = 122000 \pm 10600 K}$ for the central stars of the four brightest PNe, and ${ = 3300 \pm 370, < Teff> = 135000 \pm 26000 K}$ for those in the control set. This groups all the brightest PNe at the location of maximum temperature in the post-AGB tracks for stars with initial masses ${M = 1.5 M_{\odot}}$. These figures provide robust observational constraints about the stellar progenitors producing the PNLF cutoff in a star-forming galaxy such as M31, where a large range of initial masses is in principle available. Inconsistency is found, however, in the computed N/O abundance ratios of five nebulae, which are 1.5 to 3 times larger than predicted by the existing models for stars of these masses.

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arXiv2021

The search for gas in debris discs: ALMA detection of CO gas in HD 36546

Debris discs represent the last stages of planet formation and as such are expected to be depleted of primordial gas. Nonetheless, in the last few years the presence of cold gas has been reported in $\sim$ 20 debris discs from far-IR to (sub-)mm observations and hot gas has been observed in the optical spectra of debris discs for decades. While the origin of this gas is still uncertain, most evidences point towards a secondary origin, as a result of collisions and evaporation of small bodies in the disc. In this paper, we present ALMA observations aimed at the detection of CO gas in a sample of 8 debris discs with optical gas detections. We report the detection of CO ($^{12}$CO and $^{13}$CO) gas in HD 36546, the brightest and youngest disc in our sample, and provide upper limits to the presence of gas in the remaining seven discs.

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arXiv2021

Tracking Advanced Planetary Systems (TAPAS) with HARPS-N VII. Elder suns with low-mass companions

We present the current status of and new results from our search for exoplanets in a sample of solar-mass, evolved stars observed with the HARPS-N and the 3.6-m Telescopio Nazionale Galileo (TNG), and the High Resolution Spectrograph (HRS) and the 9.2-m Hobby Eberly Telescope (HET). The aim of this project is to detect and characterise planetary-mass companions to solar-mass stars in a sample of 122 targets at various stages of evolution from the main sequence (MS) to the red giant branch (RGB), mostly sub-gaints and giants, selected from the Pennsylvania-Toruń Planet Search (PTPS) sample, and use this sample to study relations between stellar properties, such as metallicity, luminosity, and the planet occurrence rate. This work is based on precise radial velocity (RV) measurements. We have observed the program stars for up to 11 years with the HET/HRS and the TNG/HARPS-N. We present the analysis of RV measurements with the HET/HRS and the TNG/HARPS-N of four solar-mass stars, HD 4760, HD 96992 , BD+02 3313, and TYC 0434-04538-1. We found that: HD 4760 hosts a companion with a minimum mass of 13.9 MJ (a=1.14 au, e=0.23); HD 96992 is a host to a msin i=1.14 MJ companion on a a=1.24 au and e=0.41 orbit, and TYC 0434-04538-1 hosts an msin i=6.1MJ companion on a a=0.66 au and e=0.08$ orbit. In the case of BD+02 3313 we found a correlation between the measured RVs and one of the stellar activity indicators, suggesting that the observed RV variations may originate in either stellar activity or be caused by the presence of an unresolved companion. We also discuss the current status of the project and a statistical analysis of the RV variations in our sample of target stars.

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arXiv2020

White Paper on MAAT@GTC

MAAT is proposed as a visitor mirror-slicer optical system that will allow the OSIRIS spectrograph on the 10.4-m Gran telescopio CANARIAS (GTC) the capability to perform Integral Field Spectroscopy (IFS) over a seeing-limited FoV 14.20''x10'' with a slice width of 0.303''. MAAT@GTC will enhance the resolution power of OSIRIS by 1.6 times as compared to its 0.6'' wide long-slit. All the eleven OSIRIS grisms and volume-phase holographic gratings will be available to provide broad spectral coverage with moderate resolution (R=600 up to 4100) in the 3600 - 10000 Å wavelength range. MAAT unique observing capabilities will broaden its use to the needs of the GTC community to unveil the nature of most striking phenomena in the universe well beyond time-domain astronomy. The GTC equipped with OSIRIS+MAAT will also play a fundamental role in synergy with other facilities, some of them operating on the northern ORM at La Palma. This White Paper presents the different aspects of MAAT@GTC - including scientific and technical specifications, outstanding science cases, and an outline of the instrument concept.

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arXiv2019

NaCo polarimetric observations of Sz 91 transitional disk: a remarkable case of dust filtering

We present polarized light observations of the transitional disk around Sz 91 acquired with VLT/NaCo at $H$ (1.7$μ$m) and $K_s$ (2.2$μ$m) bands. We resolve the disk and detect polarized emission up to $\sim$0."5 ($\sim$80 au) along with a central cavity at both bands. We computed a radiative transfer model that accounts for the main characteristics of the polarized observations. We found that the emission is best explained by small, porous grains distributed in a disk with a $\sim$45 au cavity. Previous ALMA observations have revealed a large sub-mm cavity ($\sim$83 au) and extended gas emission from the innermost (<16 au) regions up to almost 400 au from the star. Dynamical clearing by multiple low-mass planets arises as the most probable mechanism for the origin of Sz 91's peculiar structure. Using new $L'$ band ADI observations we can rule out companions more massive than $M_p$ $\geq$ 8 $M_\mathrm{Jup}$ beyond 45 au assuming hot-start models. The disk is clearly asymmetric in polarized light along the minor axis, with the north side brighter than the south side. Differences in position angle between the disk observed at sub-mm wavelengths with ALMA and our NaCo observations were found. This suggests that the disk around Sz 91 could be highly structured. Higher signal-to-noise near-IR and sub-mm observations are needed to confirm the existence of such structures and to improve the current understanding in the origin of transitional disks.

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arXiv2019

An apparently eccentric orbit of the exoplanet WASP-12 b as a radial velocity signature of planetary-induced tides in the host star

Massive exoplanets on extremely tight orbits, such as WASP-12 b, induce equilibrium tides in their host stars. Following the orbital motion of the planet, the tidal fluid flow in the star can be detected with the radial velocity method. Its signature manifests as the second harmonics of the orbital frequency that mimics a non-zero orbital eccentricity. Using the new radial velocity measurements acquired with the HARPS-N spectrograph at the Telescopio Nazionale Galileo and combining them with the literature data, we show that the apparent eccentricity of WASP-12 b's orbit is non-zero at a 5.8 sigma level, and the longitude of periastron of this apparently eccentric orbit is close to 270 degrees. This orbital configuration is compatible with a model composed of a circular orbit and a signature of tides raised in the host star. The radial velocity amplitude of those tides was found to be consistent with the equilibrium tide approximation. The tidal deformation is predicted to produce a flux modulation with an amplitude of 80 ppm which could be detected using space-born facilities.

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arXiv2019

A planetesimal orbiting within the debris disc around a white dwarf star

Many white dwarf stars show signs of having accreted smaller bodies, implying that they may host planetary systems. A small number of these systems contain gaseous debris discs, visible through emission lines. We report a stable 123.4min periodic variation in the strength and shape of the CaII emission line profiles originating from the debris disc around the white dwarf SDSSJ122859.93+104032.9. We interpret this short-period signal as the signature of a solid body held together by its internal strength.

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arXiv2019

Stellar Astrophysics and Exoplanet Science with the Maunakea Spectroscopic Explorer (MSE)

The Maunakea Spectroscopic Explorer (MSE) is a planned 11.25-m aperture facility with a 1.5 square degree field of view that will be fully dedicated to multi-object spectroscopy. A rebirth of the 3.6m Canada-France-Hawaii Telescope on Maunakea, MSE will use 4332 fibers operating at three different resolving powers (R ~ 2500, 6000, 40000) across a wavelength range of 0.36-1.8mum, with dynamical fiber positioning that allows fibers to match the exposure times of individual objects. MSE will enable spectroscopic surveys with unprecedented scale and sensitivity by collecting millions of spectra per year down to limiting magnitudes of g ~ 20-24 mag, with a nominal velocity precision of ~100 m/s in high-resolution mode. This white paper describes science cases for stellar astrophysics and exoplanet science using MSE, including the discovery and atmospheric characterization of exoplanets and substellar objects, stellar physics with star clusters, asteroseismology of solar-like oscillators and opacity-driven pulsators, studies of stellar rotation, activity, and multiplicity, as well as the chemical characterization of AGB and extremely metal-poor stars.

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arXiv2018

Herschel Planetary Nebula Survey (HerPlaNS): Hydrogen Recombination Laser Lines in Mz 3

The bipolar nebula Menzel 3 (Mz 3) was observed as part of the \textit{Herschel} Planetary Nebula Survey (\textit{HerPlaNS}), which used the PACS and SPIRE instruments aboard the \textit{Herschel Space Observatory} to study a sample of planetary nebulae (PNe). In this paper, one of the series describing \textit{HerPlaNS} results, we report the detection of H I recombination lines (HRLs) in the spectrum of Mz 3. Inspection of the spectrum reveals the presence of 12 HRLs in the 55 to 680 $μ$m range covered by the PACS and SPIRE instruments (H11$α$ to H21$α$ and H14$β$). The presence of HRLs in this range is unusual for PNe and has not been reported in Mz 3 before. Our analysis indicates that the HRLs we observed are enhanced by laser effect occurring in the core of Mz 3. Our arguments for this are: (i) the available Mz 3 optical to submillimetre HRL $α$ line intensity ratios are not well reproduced by the spontaneous emission of optically thin ionized gas, as would be typical for nebular gas in PNe; (ii) the compact core of Mz 3 is responsible for a large fraction of the Herschel HRLs emission; (iii) the line intensity ratios for Mz 3 are very similar to those in the core emission of the well known star MWC 349A, where laser effect is responsible for the enhancement of HRLs in the Herschel wavelength range; (iv) the physical characteristics relevant to cause laser effect in the core of MWC 349A are very similar to those in the core of Mz 3.

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arXiv2018

Unstable low-mass planetary systems as drivers of white dwarf pollution

At least 25% of white dwarfs show atmospheric pollution by metals, sometimes accompanied by detectable circumstellar dust/gas discs or (in the case of WD 1145+017) transiting disintegrating asteroids. Delivery of planetesimals to the white dwarf by orbiting planets is a leading candidate to explain these phenomena. Here, we study systems of planets and planetesimals undergoing planet-planet scattering triggered by the star's post-main sequence mass loss, and test whether this can maintain high rates of delivery over the several Gyr that they are observed. We find that low-mass planets (Earth to Neptune mass) are efficient deliverers of material and can maintain the delivery for Gyr. Unstable low-mass planetary systems reproduce the observed delayed onset of significant accretion, as well as the slow decay in accretion rates at late times. Higher-mass planets are less efficient, and the delivery only lasts a relatively brief time before the planetesimal populations are cleared. The orbital inclinations of bodies as they cross the white dwarf's Roche limit are roughly isotropic, implying that significant collisional interactions of asteroids, debris streams and discs can be expected. If planet-planet scattering is indeed responsible for the pollution of white dwarfs, many such objects, and their main-sequence progenitors, can be expected to host (currently undetectable) super-Earth planets on orbits of several au and beyond.

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arXiv2017

The Pan-Pacific Planet Search VII: The most eccentric planet orbiting a giant star

Radial velocity observations from three instruments reveal the presence of a 4 M_jup planet candidate orbiting the K giant HD 76920. HD 76920b has an orbital eccentricity of 0.856$\pm$0.009, making it the most eccentric planet known to orbit an evolved star. There is no indication that HD 76920 has an unseen binary companion, suggesting a scattering event rather than Kozai oscillations as a probable culprit for the observed eccentricity. The candidate planet currently approaches to about four stellar radii from its host star, and is predicted to be engulfed on a $\sim$100 Myr timescale due to the combined effects of stellar evolution and tidal interactions.

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arXiv2017

The Herschel Planetary Nebula Survey (HerPlaNS) - a comprehensive dusty photoionization model of NGC6781

We perform a comprehensive analysis of the planetary nebula (PN) NGC6781 to investigate the physical conditions of each of its ionized, atomic, and molecular gas and dust components and the object's evolution, based on panchromatic observational data ranging from UV to radio. Empirical nebular elemental abundances, compared with theoretical predictions via nucleosynthesis models of asymptotic giant branch (AGB) stars, indicate that the progenitor is a solar-metallicity, 2.25-3.0 Msun initial-mass star. We derive the best-fit distance of 0.46 kpc by fitting the stellar luminosity (as a function of the distance and effective temperature of the central star) with the adopted post-AGB evolutionary tracks. Our excitation energy diagram analysis indicate high excitation temperatures in the photodissociation region (PDR) beyond the ionized part of the nebula, suggesting extra heating by shock interactions between the slow AGB wind and the fast PN wind. Through iterative fitting using the Cloudy code with empirically-derived constraints, we find the best-fit dusty photoionization model of the object that would inclusively reproduce all of the adopted panchromatic observational data. The estimated total gas mass (0.41 Msun) corresponds to the mass ejected during the last AGB thermal pulse event predicted for a 2.5 Msun initial-mass star. A significant fraction of the total mass (about 70 percent) is found to exist in the PDR, demonstrating the critical importance of the PDR in PNe that are generally recognized as the hallmark of ionized/H+ regions.

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arXiv2017

Theia: Faint objects in motion or the new astrometry frontier

In the context of the ESA M5 (medium mission) call we proposed a new satellite mission, Theia, based on relative astrometry and extreme precision to study the motion of very faint objects in the Universe. Theia is primarily designed to study the local dark matter properties, the existence of Earth-like exoplanets in our nearest star systems and the physics of compact objects. Furthermore, about 15 $\%$ of the mission time was dedicated to an open observatory for the wider community to propose complementary science cases. With its unique metrology system and "point and stare" strategy, Theia's precision would have reached the sub micro-arcsecond level. This is about 1000 times better than ESA/Gaia's accuracy for the brightest objects and represents a factor 10-30 improvement for the faintest stars (depending on the exact observational program). In the version submitted to ESA, we proposed an optical (350-1000nm) on-axis TMA telescope. Due to ESA Technology readiness level, the camera's focal plane would have been made of CCD detectors but we anticipated an upgrade with CMOS detectors. Photometric measurements would have been performed during slew time and stabilisation phases needed for reaching the required astrometric precision.

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arXiv2016

High surface magnetic field in red giants as a new signature of planet engulfment?

Context. Red-giant stars may engulf planets. This may increase the rotation rate of their convective envelope, which could lead to strong dynamo-triggered magnetic fields. Aims. We explore the possibility of generating magnetic fields in red giants that have gone through the process of a planet engulfment. We compare them with similar models that evolve without any planets. We discuss the impact of stellar wind magnetic braking on the evolution of the surface velocity of the parent star. Methods. With rotating stellar models with and without planets and an empirical relation between the Rossby number and the surface magnetic field, we deduce the evolution of the surface magnetic field along the red-giant branch. The effects of wind magnetic braking is explored using a relation deduced from MHD simulations. Results. The stellar evolution model of a 1.7 M$_\odot$ without planet engulfment and that has a time-averaged rotation velocity during the Main-Sequence equal to 100 km s$^{-1}$, shows a surface magnetic field triggered by convection larger than 10 G only at the base of the red giant branch, that means for gravities log $g > 3$. When a planet engulfment occurs, such magnetic field can also appear at much lower gravities, i.e. at much higher luminosities along the red giant branch. Typically the engulfment of a 15 M$_J$ planet produces a dynamo triggered magnetic field larger than 10 G for gravities between 2.5 and 1.9. We show that for reasonable wind magnetic braking laws, the high surface velocity reached after a planet engulfment may be maintained sufficiently long for being observable. Conclusions. High surface magnetic fields for red giants in the upper part of the red giant branch is a strong indication of a planet engulfment or of an interaction with a companion. Our theory can be tested by observing fast rotating red giants and check whether they show magnetic fields.

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arXiv2016

Compact Planetary Nebulae in the Galactic Disk: Analysis of the Central Stars

We have obtained multi-wavelength observations of compact Galactic planetary nebulae (PNe) to probe post-Asymptotic Giant Branch (AGB) evolution from the onset of nebular ejection. We analyze new observations from HST to derive the masses and evolutionary status of their central stars (CSs) in order to better understand the relationship between the CS properties and those of the surrounding nebulae. We also compare this sample with others we obtained using the same technique in different metallicity environments: the Large and Small Magellanic Clouds. We work with HST/WFC3 images of 51 targets obtained in a snapshot survey (GO-11657). The high spatial resolution of HST allows us to resolve these compact PNe and distinguish the CS emission from that of their surrounding PNe. The targets were imaged through the filters F200LP, F350LP, and F814W from which we derive Johnson V and I magnitudes. We derive CS bolometric luminosities and effective temperatures using the Zanstra technique, from a combination of HST photometry and ground-based spectroscopic data. We present new unique photometric measurements of 50 CSs, and derived effective temperatures and luminosities for most of them. Central star masses for 23 targets were derived by placing the stars on a temperature-luminosity diagram and compare their location with the best available single star post-AGB evolutionary tracks, the remaining masses were indeterminate most likely because of underestimates of the stellar temperature, or because of substantial errors in the adopted statistical distances to these objects. The distribution of CS masses in the sample of compact PNe is different than sample in the LMC and SMC, but with a median mass of 0.59 solar masses it is similar to other Galactic samples. We conclude that the compact nature of many of the PNe is a result of their large distance, rather than their physical dimension.

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arXiv2016

The Effect of Tides on the Population of PN from Interacting Binaries

We have used the tidal equations of Zahn to determine the maximum orbital distance at which companions are brought into Roche lobe contact with their giant primary, when the primary expands during the giant phases. This is a key step when determining the rates of interaction between giants and their companions. Our stellar structure calculations are presented as maximum radii reached during the red and asymptotic giant branch (RGB and AGB, respectively) stages of evolution for masses between 0.8 and 4.0 Mo (Z=0.001 - 0.04) and compared with other models to gauge the uncertainty on radii deriving from details of these calculations. We find overall tidal capture distances that are typically 1-4 times the maximum radial extent of the giant star, where companions are in the mass range from 1 Jupiter mass to a mass slightly smaller than the mass of the primary. We find that only companions at initial orbital separations between ~320 and ~630 Ro will be typically captured into a Roche lobe-filling interaction or a common envelope on the AGB. Comparing these limits with the period distribution for binaries that will make PN, we deduce that in the standard scenario where all ~1-8 Mo stars make a PN, at most 2.5 per cent of all PN should have a post-common envelope central star binary, at odds with the observational lower limit of 15-20 per cent. The observed over-abundance of post-interaction central stars of PN cannot be easily explained considering the uncertainties. We examine a range of explanations for this discrepancy.

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arXiv2016

Star-planet interactions: II. Is planet engulfment the origin of fast rotating red giants?

Context. Fast rotating red giants in the upper part of the red giant branch have surface velocities that cannot be explained by single star evolution. Aims. We check whether tides between a star and a planet followed by planet engulfment can indeed accelerate the surface rotation of red giants for a sufficient long time in order to produce these fast rotating red giants. Methods. Using rotating stellar models, accounting for the redistribution of the angular momentum inside the star by different transport mechanisms, for the exchanges of angular momentum between the planet orbit and the star before the engulfment and for the deposition of angular momentum inside the star at the engulfment, we study how the surface rotation velocity at the stellar surface evolves. Results. We show that the surface velocities reached at the end of the orbital decay due to tidal forces and planet engulfment can be similar to values observed for fast rotating red giants. This surface velocity then decreases when the star evolves along the red giant branch but at a sufficiently slow pace for allowing stars to be detected with such a high velocity. More quantitatively, star-planet interaction can produce a rapid acceleration of the surface of the star, above values equal to 8 km s$^{-1}$, for periods lasting up to more than 30% the red giant branch phase. The changes of the surface carbon isotopic ratios produced by the dilution of the planetary material into the convective envelope is quite modest. Much more important might be the increase of the lithium abundance due to this effect. However lithium may be affected by many different, still uncertain, processes. Conclusions. The acceleration of the stellar surface to rotation velocities above limits that depend on the surface gravity does appear at the moment as the clearest signature of a star-planet interaction.[abridged]

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