arXiv subjects

Ana Escorza

Publications and source records attributed to Ana Escorza.

25 records · Page 2Linked to original sources

arXiv2019

The Detailed Science Case for the Maunakea Spectroscopic Explorer, 2019 edition

(Abridged) The Maunakea Spectroscopic Explorer (MSE) is an end-to-end science platform for the design, execution and scientific exploitation of spectroscopic surveys. It will unveil the composition and dynamics of the faint Universe and impact nearly every field of astrophysics across all spatial scales, from individual stars to the largest scale structures in the Universe. Major pillars in the science program for MSE include (i) the ultimate Gaia follow-up facility for understanding the chemistry and dynamics of the distant Milky Way, including the outer disk and faint stellar halo at high spectral resolution (ii) galaxy formation and evolution at cosmic noon, via the type of revolutionary surveys that have occurred in the nearby Universe, but now conducted at the peak of the star formation history of the Universe (iii) derivation of the mass of the neutrino and insights into inflationary physics through a cosmological redshift survey that probes a large volume of the Universe with a high galaxy density. MSE is positioned to become a critical hub in the emerging international network of front-line astronomical facilities, with scientific capabilities that naturally complement and extend the scientific power of Gaia, the Large Synoptic Survey Telescope, the Square Kilometer Array, Euclid, WFIRST, the 30m telescopes and many more.

astro-ph.IM↗

arXiv2019

The post-common-envelope X-ray binary nucleus of the planetary nebula NGC 2392

The Chandra X-ray Observatory has detected relatively hard X-ray emission from the central stars of several planetary nebulae (PNe). A subset have no known late-type companions, making it very difficult to isolate which of several competing mechanisms may be producing the X-ray emission. The central star of NGC 2392 is one of the most vexing members, with substantial indirect evidence for a hot white dwarf (WD) companion. Here we report on the results of a radial velocity (RV) monitoring campaign of its central star with the HERMES echelle spectrograph of the Flemish 1.2 m Mercator telescope. We discover a single-lined spectroscopic binary with an orbital period of $1.902208\pm0.000013$ d and a RV semi-amplitude of $9.96\pm0.13$ km/s. The high degree of nebula ionisation requires a WD companion ($M\gtrsim0.6 M_\odot$), which the mass-function supports at orbital inclinations $\lesssim$7 deg, in agreement with the nebula orientation of 9 deg. The hard component of the X-ray spectrum may be explained by the companion accreting mass from the wind of the Roche lobe filling primary, while the softer component may be due to colliding winds. A companion with a stronger wind than the primary could produce the latter and would be consistent with models of the observed diffuse X-ray emission detected in the nebula. The diffuse X-rays may also be powered by the jets of up to 180 km/s and active accretion would imply that they could be the first active jets of a post-common-envelope PN, potentially making NGC 2392 an invaluable laboratory to study jet formation physics. The 1.9 d orbital period rules out a double-degenerate merger leading to a Type Ia supernova and the weak wind of the primary likely also precludes a single-degenerate scenario. We suggest that a hard X-ray spectrum, in the absence of a late-type companion, could be a powerful tool to identify accreting WD companions.

astro-ph.SR↗

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.

astro-ph.SR↗

arXiv2018

S stars and s-process in the Gaia era I. Stellar parameters and chemical abundances in a sub-sample of S stars with new MARCS model atmospheres

S stars are transition objects between M-type giants and carbon stars on the asymptotic giant branch (AGB). They are characterized by overabundances of s-process elements. Roughly half of them are enhanced in technetium (Tc), an s-process element with no stable isotope, while the other half lack technetium. This dichotomy arises from the fact that Tc-rich S stars are intrinsically producing s-process elements and have undergone third dredge-up (TDU) events, while Tc-poor S stars owe their s-process overabundances to a past pollution by a former AGB companion. Our aim is to analyse the abundances of S stars and gain insights into their evolutionary status and on the nucleosynthesis of heavy s-process elements taking place in their interior. In particular, the location of extrinsic and intrinsic S stars in the HR diagram will be compared with the theoretical onset of the TDU on the thermally-pulsing AGB. A sample of 19 S-type stars was analysed by combining HERMES high-resolution spectra, accurate Gaia Data Release 2 (GDR2) parallaxes, stellar-evolution models, and newly-designed MARCS model atmospheres for S-type stars. Combining the derived parameters with GDR2 parallaxes allows a joint analysis of the location of the stars in the Hertzsprung-Russell diagram and of their surface abundances. For all 19 stars, Zr and Nb abundances are derived, complemented by abundances of other s-process elements for the three Tc-rich S stars. These abundances agree within the uncertainties with nucleosynthesis predictions for stars of corresponding mass, metallicity and evolutionary stage. Most extrinsic S stars lie close to the tip of the red giant branch (RGB), and a few are located along the early AGB. The location of intrinsic S stars in the HR diagram is compatible with them being thermally-pulsing AGB stars.

astro-ph.SR↗

arXiv2018

Orbital properties of binary post-AGB stars

Binary post-asymptotic giant branch (post-AGB) stars are thought to be the products of a strong but poorly-understood interaction during the AGB phase. The aim of this contribution is to update the orbital elements of a sample of galactic post-AGB binaries observed in a long-term radial-velocity monitoring campaign. Radial velocities are computed from high signal-to-noise spectra by use of a cross-correlation method. The radial-velocity curves are fitted by using both a least-squares algorithm and a Nelder-Mead simplex algorithm. We use a Monte Carlo method to compute uncertainties on the orbital elements. The resulting mass functions are used to derive a companion mass distribution by optimising the predicted to the observed cumulative mass-function distributions, after correcting for observational bias. As a result, we derive and update orbital elements for 33 galactic post-AGB binaries, among which 3 are new orbits. The orbital periods of the systems range from 100 to about 3000 days. Over 70 percent (23 out of 33) of our binaries have significant non-zero eccentricities ranging over all periods. Their orbits are non-circular despite the fact that the Roche-lobe radii are smaller than the maximum size of a typical AGB star and tidal circularisation should have been strong when the objects were on the AGB. We derive a distribution of companion masses that is peaked around 1.09 $M_\odot$ with a standard deviation of 0.62 $M_\odot$. The large spread in companion masses highlights the diversity of post-AGB binary systems. Furthermore, we find that only post-AGB stars with high effective temperatures (> 5500 K) in wide orbits are depleted in refractory elements, suggesting that re-accretion of material from a circumbinary disc is an ongoing process. It appears, however, that chemical depletion is inefficient for the closest orbits irrespective of the actual surface temperature.

astro-ph.SR↗

arXiv2018

Binary interaction along the Red Giant Branch: The Barium Star perspective

Barium (Ba) stars form via mass-transfer in binary systems, and can subsequently interact with their white dwarf companion in a second stage of binary interaction. We used observations of main-sequence Ba systems as input for our evolutionary models, and try to reproduce the orbits of the Ba giants. We show that to explain short and sometimes eccentric orbits, additional interaction mechanisms are needed along the RGB.

astro-ph.SR↗

arXiv2016

Establishing binarity amongst Galactic RV Tauri stars with a disc

This study is a contribution in comprehending the role of binarity upon late stages of stellar evolution. We determine the binary status of six Galactic RV Tauri stars, namely DY Ori, EP Lyr, HP Lyr, IRAS 17038-4815, IRAS 09144-4933 and TW Cam, which are surrounded by a dusty disc. We also place them on the HR diagram, thereby establishing their evolutionary nature. All the six Galactic RV Tauri stars included in this study are binaries with orbital periods ranging between $\sim$ 650 and 1700 days and with eccentricities between 0.2 and 0.6. The mass functions range between 0.08 to 0.55 M$_\odot$ which points to an unevolved low mass companion. In the photometric time series we detect a long-term variation on the time-scale of the orbital period for IRAS 17038-4815, IRAS 09144-4933 and TW Cam. Our derived stellar luminosities obtained from a calibrated PLC relation indicates that all except DY Ori and EP Lyr, are post-AGB stars. DY Ori and EP Lyr are likely examples of the recently discovered dusty post-RGB stars. The orbital parameters strongly suggest that the evolution of these stars was interrupted by a strong phase of binary interaction during or even prior to the AGB. The observed eccentricities and long orbital periods among these stars provides a challenge to the standard theory of binary evolution.

astro-ph.SR↗