arXiv subjects

A. D. Romero

Publications and source records attributed to A. D. Romero.

At least 19 recordsLinked to original sources

arXiv2023

A 5.3-minute-period pulsing white dwarf in a binary detected from radio to X-rays

White dwarf stars are the most common stellar fossils. When in binaries, they make up the dominant form of compact object binary within the Galaxy and can offer insight into different aspects of binary formation and evolution. One of the most remarkable white dwarf binary systems identified to date is AR Scorpii (henceforth AR Sco). AR Sco is composed of an M-dwarf star and a rapidly-spinning white dwarf in a 3.56-hour orbit. It shows pulsed emission with a period of 1.97 minutes over a broad range of wavelengths, which led to it being known as a white dwarf pulsar. Both the pulse mechanism and the evolutionary origin of AR Sco provide challenges to theoretical models. Here we report the discovery of the first sibling of AR Sco, J191213.72-441045.1 (henceforth J1912-4410), which harbours a white dwarf in a 4.03-hour orbit with an M-dwarf and exhibits pulsed emission with a period of 5.30 minutes. This discovery establishes binary white dwarf pulsars as a class and provides support for proposed formation models for white dwarf pulsars.

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arXiv2023

Catalog of magnetic white dwarfs with hydrogen dominated atmospheres

White dwarfs are excellent research laboratories as they reach temperatures, pressures, and magnetic fields that are unattainable on Earth. To better understand how these three physical parameters interact with each other and with other stellar features, we determined the magnetic field strength for a total of 804 hydrogen-rich white dwarfs of which 287 are not in the literature. We fitted the spectra observed with the Sloan Digital Sky Survey using atmospheric models that consider the Zeeman effect due to the magnetic field at each point in the stellar disk. Comparing magnetic and non-magnetic WDs, the literature already shows that the magnetic ones have on average higher mass than the non-magnetic. In addition to that, magnetic fields are more common in cooler WDs than in hotter WDs. In consonance, we found that those with higher magnetic field strengths tend to have higher masses, and lower temperatures, for which models indicate the crystallization process has already started. This reinforces the hypothesis that the field is being generated and/or amplified in the cooling process of the white dwarf. Our sample constitutes the largest number of white dwarfs with determined magnetic fields to date.

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arXiv2022

The impact of the uncertainties in the 12C(α,γ)16O reaction rate on the evolution of low- to intermediate-mass stars

One of the largest uncertainties in stellar evolutionary computations is the accuracy of the considered reaction rates. The 12C(alpha,gamma)16O reaction is particularly important for the study of low- and intermediate-mass stars as it determines the final C/O ratio in the core which influences the white dwarf cooling evolution. Thus, there is a need for a study of how the computations of white dwarfs and their progenitors that are made to date may be affected by the uncertainties of the 12C(alpha,gamma)16O reaction rates. In this work we compute fully evolutionary sequences using the MESA code with initial masses in the range of 0.90 <= Mi/Msun <= 3.05. We consider different adopted reaction rates, obtained from the literature, as well as the extreme limits within their uncertainties. As expected, we find that previous to the core helium burning stage there are no changes to the evolution of the stars. However, the subsequent stages are all affected by the uncertainties of the considered reaction rate. In particular, we find differences to the convective core mass during the core helium burning stage which may affect pulsation properties of subdwarfs, the number of thermal pulses during the asymptotic giant branch and trends between final oxygen abundance in the core and the progenitor masses of the remnant white dwarfs.

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arXiv2022

Discovery of 74 new bright ZZ Ceti stars in the first three years of TESS

We report the discovery of 74 new pulsating DA white dwarf stars, or ZZ Cetis, from the data obtained by the Transiting Exoplanet Survey Satellite (TESS) mission, from Sectors 1 to 39, corresponding to the first 3 cycles. This includes objects from the Southern Hemisphere (Sectors 1-13 and 27-39) and the Northern Hemisphere (Sectors 14-26), observed with 120 s- and 20 s-cadence. Our sample likely includes 13 low-mass and one extremely low-mass white dwarf candidate, considering the mass determinations from fitting Gaia magnitudes and parallax. In addition, we present follow-up time series photometry from ground-based telescopes for 11 objects, which allowed us to detect a larger number of periods. For each object, we analysed the period spectra and performed an asteroseismological analysis, and we estimate the structure parameters of the sample, i.e., stellar mass, effective temperature and hydrogen envelope mass. We estimate a mean asteroseismological mass of _~ 0.635 +/-0.015 Msun, excluding the candidate low or extremely-low mass objects. This value is in agreement with the mean mass using estimates from Gaia data, which is ~ 0.631 +/- 0.040 Msun, and with the mean mass of previously known ZZ Cetis of = 0.644 +/-0.034 Msun. Our sample of 74 new bright ZZ~Cetis increases the number of known ZZ~Cetis by $\sim$20 per cent.

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arXiv2021

Uncovering the chemical structure of the pulsating low-mass white dwarf SDSS J115219.99+024814.4

Pulsating low-mass white dwarf stars are white dwarfs with stellar masses between 0.30~M$_{\odot}$ and 0.45~M$_{\odot}$ that show photometric variability due to gravity-mode pulsations. Within this mass range, they can harbour both a helium- and hybrid-core, depending if the progenitor experienced helium-core burning during the pre-white dwarf evolution. SDSS J115219.99$+$024814.4 is an eclipsing binary system where both components are low-mass white dwarfs, with stellar masses of 0.362$\pm$0.014~M$_{\odot}$ and 0.325$\pm$0.013~M$_{\odot}$. In particular, the less massive component is a pulsating star, showing at least three pulsation periods of $\sim$1314 s, $\sim$1069 s and $\sim$582.9 s. This opens the way to use asteroseismology as a tool to uncover its inner chemical structure, in combination with the information obtained using the light-curve modelling of the eclipses. To this end, using binary evolutionary models leading to helium- and hybrid-core white dwarfs, we compute adiabatic pulsations for $\ell=1$ and $\ell=2$ gravity modes with \texttt{Gyre}. We found that the pulsating component of the SDSS J115219.99$+$024814.4 system must have a hydrogen envelope thinner that the value obtained from binary evolution computations, independently of the inner composition. Finally, from our asteroseismological study, we find a best fit model characterised by T$_{\rm e ff}=10\, 917$ K, M=0.338~M$_{\odot}$, M$_{\rm H}=10^{-6}$~M$_{\odot}$ with the inner composition of a hybrid WD.

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arXiv2020

Evidence of Spectral Evolution on the white dwarf sample from the Gaia Mission

Since the Gaia data release 2, several works were published describing a bifurcation in the observed white dwarf colour$-$magnitude diagram for $\mbox{$G_{\mathrm{BP}}$}-\mbox{$G_{\mathrm{RP}}$} > 0$. Some possible explanations in the literature include the existence of a double population with different initial mass function or two distinct populations, one formed by hydrogen$-$ and one formed by helium$-$envelope white dwarfs. We propose instead spectral evolution to explain the bifurcation. From a population synthesis approach, we find that the spectral evolution occurs for effective temperature below ${\simeq}11\,000\,\mathrm{K}$ and masses mainly between $0.64\,\mathrm{M}_\odot$ and $0.74\,\mathrm{M}_\odot$, which correspond to around $16$ per cent of all DA white dwarfs. We also find the Gaia white dwarf colour-magnitude diagram indicates a star formation history that decreases abruptly for objects younger than $1.4\,\mathrm{Gyr}$ and a top-heavy initial mass function for the white dwarf progenitors.

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arXiv2018

A Study of Cool White Dwarfs in the Sloan Digital Sky Survey Data Release 12

In this work we study white dwarfs where $30\,000\,\text{K} {>} \mathrm{T}_{\rm{eff}} {>} 5\,000\,\text{K}$ to compare the differences in the cooling of DAs and non-DAs and their formation channels. Our final sample is composed by nearly $13\,000$ DAs and more than $3\,000$ non-DAs that are simultaneously in the SDSS DR12 spectroscopic database and in the \textit{Gaia} survey DR2. We present the mass distribution for DAs, DBs and DCs, where it is found that the DCs are ${\sim}0.15\,\mathrm{M}_\odot$ more massive than DAs and DBs on average. Also we present the photometric effective temperature distribution for each spectral type and the distance distribution for DAs and non-DAs. In addition, we study the ratio of non-DAs to DAs as a function of effective temperature. We find that this ratio is around ${\sim}0.075$ for effective temperature above ${\sim}22\,000\,\text{K}$ and increases by a factor of five for effective temperature cooler than $15\,000\,\text{K}$. If we assume that the increase of non-DA stars between ${\sim}22\,000\,\text{K}$ to ${\sim}15\,000\,\text{K}$ is due to convective dilution, $14{\pm}3$ per cent of the DAs should turn into non-DAs to explain the observed ratio. Our determination of the mass distribution of DCs also agrees with the theory that convective dilution and mixing are more likely to occur in massive white dwarfs, which supports evolutionary models and observations suggesting that higher mass white dwarfs have thinner hydrogen layers.

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arXiv2018

New full evolutionary sequences of H and He atmosphere massive white dwarf stars using MESA

We explore the evolution of hydrogen-rich and hydrogen-deficient white dwarf stars with masses between 1.012 and 1.307 Msun, and initial metallicity of Z=0.02. These sequences are the result of main sequence stars with masses between 8.8 and 11.8 Msun. The simulations were performed with MESA, starting at the zero-age main sequence, through thermally pulsing and mass-loss phases, ending at the white dwarfs cooling sequence. We present reliable chemical profiles for the whole mass range considered, covering the different expected central compositions, i.e. C/O, O/Ne and Ne/O/Mg, and its dependence with the stellar mass. In addition, we present detailed chemical profiles of hybrid C/O-O/Ne core white dwarfs, found in the mass range between 1.024 and 1.15 Msun. We present the initial-to-final mass relation, mass-radius relation, and cooling times considering the effects of atmosphere and core composition.

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arXiv2018

The impact of pre-white dwarf evolution on the pulsational properties and asteroseismological inferences of ZZ Ceti stars

ZZ Ceti stars are pulsating white dwarfs with a carbon-oxygen core (or possibly ONe for the most massive stars) build up during the core helium burning (CHeB) and thermally pulsing Asymptotic Giant Branch (TP-AGB) phases. Through the interpretation of their pulsation periods by means of asteroseismology, details about their origin and evolution can be inferred. The whole pulsation spectrum exhibited by ZZ Ceti stars strongly depend on the inner chemical structure. At present, there are several processes affecting the chemical profiles that are still not accurately determined. We present a study of the impact of current uncertainties in the evolution of white dwarf progenitor on the expected pulsation properties and on the stellar parameters inferred from asteroseismological fits of ZZ Ceti stars. Our analysis is based on a set of carbon-oxygen core white dwarf models that are derived from full evolutionary computations from the ZAMS to the ZZ Ceti domain. We considered models in which we varied the number of thermal pulses, the amount of overshooting, and the carbon-alpha reaction rate within their uncertainties. We explore the impact of these major uncertainties in prior evolution on the chemical structure and expected pulsation spectrum. We find that these uncertainties yield significant changes in the g-mode pulsation periods being those found during the TP-AGB phase the most relevant for the pulsational properties and the asteroseismological derived stellar parameters of ZZ Ceti stars. We conclude that the uncertainties in the white dwarf progenitor evolution should be taken into account in detailed asteroseismological analyses of these pulsating stars.

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arXiv2018

Evolution and Asteroseismology of Pulsating Low-Mass White Dwarfs

Many low-mass white dwarfs are being discovered in the field of our galaxy and some of them exhibit $g$-mode pulsations, comprising the extremely low-mass variable (ELMV) stars class. Despite it is generally believed that these stars are characterized by thick H envelopes, from stellar evolution considerations, the existence of low-mass WDs with thin H envelopes is also possible. We have performed detailed asteroseismological fits to all the known ELMVs to search for a representative model by employing a set of fully evolutionary models that are representative of low-mass He-core white dwarf stars with a range of stellar masses $[0.1554-0.4352]\ M_{\odot}$, effective temperatures $[6000-10000]\ $K, and also with a range of H envelope thicknesses $-5.8 \lesssim \log(M_{\rm H}/M_{\star}) \lesssim -1.7$, hence expanding the space of parameters. We found that some of the stars under analysis are characterized by thick H envelopes, but others are better represented by models with thin H envelope.

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arXiv2018

The sdA problem - II. Photometric and Spectroscopic Follow-up

Subdwarf A star (sdA) is a spectral classification given to objects showing H-rich spectra and sub-main sequence surface gravities, but effective temperature lower than the zero-age horizontal branch. Their evolutionary origin is an enigma. In this work, we discuss the results of follow-up observations of selected sdAs. We obtained time resolved spectroscopy for 24 objects, and time-series photometry for another 19 objects. For two targets, we report both spectroscopy and photometry observations. We confirm seven objects to be new extremely-low mass white dwarfs (ELMs), one of which is a known eclipsing star. We also find the eighth member of the pulsating ELM class.

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arXiv2017

Evidence from K2 for rapid rotation in the descendant of an intermediate-mass star

Using patterns in the oscillation frequencies of a white dwarf observed by K2, we have measured the fastest rotation rate, 1.13(02) hr, of any isolated pulsating white dwarf known to date. Balmer-line fits to follow-up spectroscopy from the SOAR telescope show that the star (SDSSJ0837+1856, EPIC 211914185) is a 13,590(340) K, 0.87(03) solar-mass white dwarf. This is the highest mass measured for any pulsating white dwarf with known rotation, suggesting a possible link between high mass and fast rotation. If it is the product of single-star evolution, its progenitor was a roughly 4.0 solar-mass main-sequence B star; we know very little about the angular momentum evolution of such intermediate-mass stars. We explore the possibility that this rapidly rotating white dwarf is the byproduct of a binary merger, which we conclude is unlikely given the pulsation periods observed.

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arXiv2017

Four New Massive Pulsating White Dwarfs Including an Ultramassive DAV

We report the discovery of four massive ($M > 0.8\,M_\odot$) ZZ Ceti white dwarfs, including an ultramassive $1.16\,M_\odot$ star. We obtained ground based, time-series photometry for thirteen white dwarfs from the Sloan Digital Sky Survey Data Release 7 and Data Release 10 whose atmospheric parameters place them within the ZZ Ceti instability strip. We detect mono-periodic pulsations in three of our targets (J1053, J1554, and J2038) and identify three periods of pulsation in J0840 (173, 327, and 797 s). Fourier analysis of the remaining nine objects do not indicate variability above the $4\langle{A}\rangle$ detection threshold. Our preliminary asteroseismic analysis of J0840 yields a stellar mass $M=1.14\pm 0.01\,M_{\odot}$, hydrogen and helium envelope masses of $M_H = 5.8 \times 10^{-7}\,M_{\odot}$ and $M_{He}=4.5 \times 10^{-4}\,M_{\odot}$, and an expected core crystallized mass ratio of 50-70\%. J1053, J1554, and J2038 have masses in the range $0.84-0.91 M_\odot$ and are expected to have a CO core; however, the core of J0840 could consist of highly crystallized CO or ONeMg given its high mass. These newly discovered massive pulsators represent a significant increase in the number of known ZZ Ceti white dwarfs with mass $M > 0.85\,M_\odot$, and detailed asteroseismic modeling of J0840 will allow for significant tests of crystallization theory in CO and ONeMg core white dwarfs.

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arXiv2016

What's the nature of sdA stars?

White dwarfs with log g lower than 7.0 are called Extremely Low Mass white dwarfs (ELMs). They were first found as companions to pulsars, then to other white dwarfs and main sequence stars (The ELM Survey: 2010 to 2016), and can only be formed in interacting binaries in the age of the Universe. In our SDSS DR12 white dwarf catalog (Kepler et al. 2016), we found a few thousand stars in the effective temperature and surface gravity ranges attributed to ELMs. We have called these objects sdAs, alluding to their narrow hydrogen line spectra showing sub-main sequence log g. One possible explanation for the sdAs is that they are ELMs. Increasing the ELMs sample would help constrain the number of close binaries in the Galaxy. Interestingly, if they turn out to be A stars with an overestimated log g, the distance modulus would put these young stars in the Galaxy's halo.

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arXiv2015

A comparative analysis of the observed white dwarf cooling sequence from globular clusters

We report our study of features at the observed red end of the white dwarf cooling sequences for three Galactic globular clusters: NGC\,6397, 47\,Tucanae and M\,4. We use deep colour-magnitude diagrams constructed from archival Hubble Space Telescope (ACS) to systematically investigate the blue turn at faint magnitudes and the age determinations for each cluster. We find that the age difference between NGC\,6397 and 47\,Tuc is 1.98$^{+0.44}_{-0.26}$\,Gyr, consistent with the picture that metal-rich halo clusters were formed later than metal-poor halo clusters. We self-consistently include the effect of metallicity on the progenitor age and the initial-to-final mass relation. In contrast with previous investigations that invoked a single white dwarf mass for each cluster, the data shows a spread of white dwarf masses that better reproduce the shape and location of the blue turn. This effect alone, however, does not completely reproduce the observational data - the blue turn retains some mystery. In this context, we discuss several other potential problems in the models. These include possible partial mixing of H and He in the atmosphere of white dwarf stars, the lack of a good physical description of the collision-induced absorption process and uncertainties in the opacities at low temperatures. The latter are already known to be significant in the description of the cool main sequence. Additionally, we find that the present day local mass function of NGC\,6397 is consistent with a top-heavy type, while 47\,Tuc presents a bottom-heavy profile.

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arXiv2015

The age-metallicity dependence for white dwarfs

We present a theoretical study on the metallicity dependence of the initial$-$to$-$final mass relation and its influence on white dwarf age determinations. We compute a grid of evolutionary sequences from the main sequence to $\sim 3\, 000$ K on the white dwarf cooling curve, passing through all intermediate stages. During the thermally-pulsing asymptotic giant branch no third dredge-up episodes are considered and thus the photospheric C/O ratio is below unity for sequences with metallicities larger than $Z=0.0001$. We consider initial metallicities from $Z=0.0001$ to $Z=0.04$, accounting for stellar populations in the galactic disk and halo, with initial masses below $\sim 3M_{\odot}$. We found a clear dependence of the shape of the initial$-$to$-$final mass relation with the progenitor metallicity, where metal rich progenitors result in less massive white dwarf remnants, due to an enhancement of the mass loss rates associated to high metallicity values. By comparing our theoretical computations with semi empirical data from globular and old open clusters, we found that the observed intrinsic mass spread can be accounted for by a set of initial$-$to$-$final mass relations characterized by different metallicity values. Also, we confirm that the lifetime spent before the white dwarf stage increases with metallicity. Finally, we estimate the mean mass at the top of the white dwarf cooling curve for three globular clusters NGC 6397, M4 and 47 Tuc, around $0.53 M_{\odot}$, characteristic of old stellar populations. However, we found different values for the progenitor mass, lower for the metal poor cluster, NGC 6397, and larger for the younger and metal rich cluster 47 Tuc, as expected from the metallicity dependence of the initial$-$to$-$final mass relation.

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arXiv2013

Asteroseismological study of massive ZZ Ceti stars with fully evolutionary models

We present the first asteroseismological study for 42 massive ZZ Ceti stars based on a large set of fully evolutionary carbon$-$oxygen core DA white dwarf models characterized by a detailed and consistent chemical inner profile for the core and the envelope. Our sample comprise all the ZZ Ceti stars with spectroscopic stellar masses between 0.72 and $1.05M_{\odot}$ known to date. The asteroseismological analysis of a set of 42 stars gives the possibility to study the ensemble properties of the massive pulsating white dwarf stars with carbon$-$oxygen cores, in particular the thickness of the hydrogen envelope and the stellar mass. A significant fraction of stars in our sample have stellar mass high enough as to crystallize at the effective temperatures of the ZZ Ceti instability strip, which enables us to study the effects of crystallization on the pulsation properties of these stars. Our results show that the phase diagram presented in Horowitz et al. (2010) seems to be a good representation of the crystallization process inside white dwarf stars, in agreement with the results from white dwarf luminosity function in globular clusters.

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arXiv2012

The seismic properties of low-mass He-core white dwarf stars

We present here a detailed pulsational study applied to low-mass He-core white dwarfs, based on full evolutionary models representative of these objects. The background stellar models on which our pulsational analysis was carried out were derived by taking into account the complete evolutionary history of the progenitor stars, with special emphasis on the diffusion processes acting during the white dwarf cooling phase. We computed nonradial $g$-modes to assess the dependence of the pulsational properties of these objects with stellar parameters such as the stellar mass and the effective temperature, and also with element diffusion processes. We also performed a g- and p-mode pulsational stability analysis on our models and found well-defined blue edges of the instability domain, where these stars should start to exhibit pulsations. We found substantial differences in the seismic properties of white dwarfs with $M_* \gtrsim 0.20 M_{\odot}$ and the extremely low-mass (ELM) white dwarfs ($M_* \lesssim 0.20 M_{\odot}$). Specifically, $g$-mode pulsation modes in ELM white dwarfs mainly probe the core regions and are not dramatically affected by mode-trapping effects by the He/H interface, whereas the opposite is true for more massive He-core white dwarfs. We found that element diffusion processes substantially affects the shape of the He/H chemical transition region, leading to non-negligible changes in the period spectrum of low-mass white dwarfs. Our stability analysis successfully predicts the pulsations of the only known variable low-mass white dwarf (SDSS J184037.78+642312.3), and also predicts both $g$- and $p$-mode pulsational instabilities in a significant number of known low-mass and ELM white dwarfs.

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