arXiv subjects

P. De Cat

Publications and source records attributed to P. De Cat.

At least 19 recordsLinked to original sources

arXiv2025

LAMOST medium-resolution observations of the Pleiades

In this work we present the results of our analysis of medium-resolution LAMOST spectra of late-type candidate members of the Pleiades. We have used the code ROTFIT to determine the atmospheric parameters (Teff, logg, and [Fe/H]), radial velocity (RV), and projected rotation velocity (vsini) for 1581 spectra of 283 stars. Moreover, for late-type stars (Teff<6500 K), we also calculated the H$\alpha$ and LiI-6708 net equivalent width by means of the subtraction of inactive photospheric templates. We have also used rotation periods available in the literature and we have determined them for 89 stars by analyzing TESS photometry. The RV distribution of the members peaks at 5.0 km/s with a dispersion of 1.4 km/s, while the average metallicity is [Fe/H]=-0.03$\pm$0.06, in line with previous determinations. Fitting empirical Lithium isochrones, we obtain a reliable age for the Pleiades of 118$\pm$6 Myr, in agreement with the recent literature. The activity indicators H$\alpha$ line flux (Fha) and luminosity ratio (R'ha) show the hottest stars to be the less active ones. The R'ha values display the typical activity-rotation trend with the Rossby number with a steep decay for $R_{\rm O}\geq$0.2 and a nearly flat (saturated) activity level for smaller values. However, we still see a slight dependence on $R_{\rm O}$ in the saturated regime which is well fitted by a power law with a slope of $-0.18\pm0.02$, in agreement with some previous work. For three sources we have found LAMOST spectra acquired during flares showing strong and broad H$\alpha$ profiles and the presence of the HeI-6678 emission line. We identify 39 possible SB1 and ten SB2 systems. We have also shown the potential of the LAMOST-MRS spectra, which allowed us to refine the orbital solution of some binary and to discover a new double-lined binary.

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arXiv2024

On the Existence of "Maia variables"

There are different classes of pulsating stars in the H-R diagram. While many of those classes are undisputed, some remain a mystery such as the objects historically called "Maia variables". Whereas the presence of such a class was suggested seven decades ago, no pulsational driving mechanism is known that could excite short-period oscillations in these late B to early A-type stars. Alternative hypotheses that would render the reports of variability of those stars erroneous have been proposed such as incorrect effective temperatures, binarity or rapid rotation, but no certain conclusions have been reached yet. Therefore the existence of these variables as a homogeneous class of pulsating star is still under discussion. Meanwhile, many new candidates of these variables have been claimed especially by using photometric observations of space telescopes. In this study, we examined 31 objects that are alleged members of this hypothetical group and carried out detailed spectroscopic and photometric analyses to test the proposed hypotheses for their cause of variability. The Teff, log g, v sin i, and chemical abundances of the targets were determined and the TESS photometric data were examined. As a result, we found that most of these targets are located inside the delta Scuti, beta Cep, or SPB star instability strips, a few show evidence for binarity and others for rapid rotation. We give arguments that none of the apparently rapid pulsations in our targets is caused by a star outside any known instability strip. By extrapolation, we argue that most stars proposed as pulsators outside well-established instability domains are misclassified. Hence there is no sufficient evidence justifying the existence of a class of pulsating stars formerly known as the "Maia variables".

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arXiv2023

TESS Cycle 2 observations of roAp stars with 2-min cadence data

We present the results of a systematic search of the Transiting Exoplanet Survey Satellite (TESS) 2-min cadence data for new rapidly oscillating Ap (roAp) stars observed during the Cycle 2 phase of its mission. We find seven new roAp stars previously unreported as such and present the analysis of a further 25 roAp stars that are already known. Three of the new stars show multiperiodic pulsations, while all new members are rotationally variable stars, leading to almost 70 per cent (22) of the roAp stars presented being $α^2$ CVn-type variable stars. We show that targeted observations of known chemically peculiar stars are likely to overlook many new roAp stars, and demonstrate that multi-epoch observations are necessary to see pulsational behaviour changes. We find a lack of roAp stars close to the blue edge of the theoretical roAp instability strip, and reaffirm that mode instability is observed more frequently with precise, space-based observations. In addition to the Cycle 2 observations, we analyse TESS data for all known roAp stars. This amounts to 18 further roAp stars observed by TESS. Finally, we list six known roAp stars that TESS is yet to observe. We deduce that the incidence of roAp stars amongst the Ap star population is just 5.5 per cent, raising fundamental questions about the conditions required to excite pulsations in Ap stars. This work, coupled with our previous work on roAp stars in Cycle 1 observations, presents the most comprehensive, homogeneous study of the roAp stars in the TESS nominal mission, with a collection of 112 confirmed roAp stars in total.

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arXiv2023

MELCHIORS: The Mercator Library of High Resolution Stellar Spectroscopy

Over the past decades, libraries of stellar spectra have been used in a large variety of science cases, including as sources of reference spectra for a given object or a given spectral type. Despite the existence of large libraries and the increasing number of projects of large-scale spectral surveys, there is to date only one very high-resolution spectral library offering spectra from a few hundred objects from the southern hemisphere (UVES-POP) . We aim to extend the sample, offering a finer coverage of effective temperatures and surface gravity with a uniform collection of spectra obtained in the northern hemisphere. Between 2010 and 2020, we acquired several thousand echelle spectra of bright stars with the Mercator-HERMES spectrograph located in the Roque de Los Muchachos Observatory in La Palma, whose pipeline offers high-quality data reduction products. We have also developed methods to correct for the instrumental response in order to approach the true shape of the spectral continuum. Additionally, we have devised a normalisation process to provide a homogeneous normalisation of the full spectral range for most of the objects. We present a new spectral library consisting of 3256 spectra covering 2043 stars. It combines high signal-to-noise and high spectral resolution over the entire range of effective temperatures and luminosity classes. The spectra are presented in four versions: raw, corrected from the instrumental response, with and without correction from the atmospheric molecular absorption, and normalised (including the telluric correction).

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arXiv2023

Comprehensive spectroscopic and photometric study of pulsating eclipsing binary star AI Hya

The pulsating eclipsing binaries are remarkable systems that provide an opportunity to probe the stellar interior and to determine the fundamental stellar parameters precisely. Especially the detached eclipsing binary systems with (a) pulsating component(s) are significant objects to understand the nature of the oscillations since the binary effects in these systems are negligible. Recent studies based on space data have shown that the pulsation mechanisms of some oscillating stars are not completely understood. Hence, comprehensive studies of a number of pulsating stars within detached eclipsing binaries are important. In this study, we present a detailed analysis of the pulsating detached eclipsing binary system AI Hya which was studied by two independent groups with different methods. We carried out a spectroscopic survey to estimate the orbital parameters via radial velocity measurements and the atmospheric parameters of each binary component using the composite and/or disentangled spectra. We found that the more luminous component of the system is a massive, cool and chemically normal star while the hotter binary component is a slightly metal-rich object. The fundamental parameters of AI Hya were determined by the analysis of binary variations and subsequently used in the evolutionary modelling. Consequently, we obtained the age of the system as 850 $\pm$ 20 Myr and found that both binary components are situated in the Delta Scuti instability strip. The frequency analysis revealed pulsation frequencies between the 5.5 - 13.0 d$^{-1}$ and we tried to estimate which binary component is the pulsating one. However, it turned out that those frequencies could originate from both binary components.

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arXiv2022

Characterization of Kepler targets based on medium-resolution LAMOST spectra analyzed with ROTFIT

In this work we present the results of our analysis of 16,300 medium-resolution LAMOST spectra of late-type stars in the Kepler field with the aim of determining the stellar parameters, activity level, lithium atmospheric content, and binarity. We have used a version of the code ROTFIT specifically developed for these spectra. We provide a catalog with the atmospheric parameters (Teff, log(g), and [Fe/H]), radial velocity (RV), and projected rotation velocity (vsini). For cool stars (Teff < 6500 K), we also calculated the H-alpha and LiI-6708 equivalent width, which are important indicators of chromospheric activity and evolutionary stage, respectively. We have derived the RV and atmospheric parameters for 14,300 spectra of 7443 stars. Literature data were used for a quality control of the results. The Teff and log(g) values are in good agreement with the literature. The [Fe/H] values appear to be overestimated for metal-poor stars. We propose a relation to correct the [Fe/H] values derived with ROTFIT. We were able to identify double-lined binaries, stars with variable RVs, lithium-rich giants, and emission-line objects. Based on the H-alpha flux, we found 327 active stars. We detected the LiI-6708 line and measure its equivalent width for 1657 stars, both giants and stars on the main sequence. Regarding the latter, we performed a discrete age classification based on the atmospheric lithium abundance and the upper envelopes of a few open clusters. Among the giants, we found 195 Li-rich stars, 161 of which are reported here for the first time. No relationship is found between stellar rotation and lithium abundance, which allows us to rule out merger scenarios as the predominant explanation of the enrichment of Li in our sample. The fraction of Li-rich giants, about 4%, is higher than expected.

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arXiv2022

The near-core rotation of HD 112429: a gamma Doradus star with TESS photometry and legacy spectroscopy

The TESS space mission provides us with high-precision photometric observations of bright stars over more than 70% of the entire sky, allowing us to revisit and characterise well-known stars. We aim to conduct an asteroseismic analysis of the gamma Doradus star HD112429 using both the available ground-based spectroscopy and TESS photometry, and assess the conditions required to measure the near-core rotation rate and buoyancy travel time. We collect and reduce the available five sectors of short-cadence TESS photometry of this star, as well as 672 legacy observations from six medium- to high-resolution ground-based spectrographs. We determine the stellar pulsation frequencies from both data sets using iterative prewhitening, do asymptotic g mode modelling of the star and investigate the corresponding spectral line profile variations using the pixel-by-pixel method. We validate the pulsation frequencies from the TESS data up to $S/N \geq 5.6$, confirming recent reports in the literature that the classical criterion $S/N \geq 4$ does not suffice for space-based observations. We identify the pulsations as prograde dipole g modes and r-mode pulsations, and measure a near-core rotation rate of $1.536(3) d^{-1}$ and a buoyancy travel time $Π_0$ of 4190(50) s. These results are in agreement with the observed spectral line profile variations, which were qualitatively evaluated using a newly developed toy model. We establish a set of conditions that have to be fulfilled for an asymptotic asteroseismic analysis of g-mode pulsators. In the case of HD112429, two TESS sectors of space photometry suffice. Although a detailed asteroseismic modelling analysis is not viable for g-mode pulsators with only short or sparse light curves of space photometry, we find that it is possible to determine global asteroseismic quantities for a subset of these stars. (abbreviated.)

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arXiv2021

Mass transfer and tidally tilted pulsation in the Algol-type system TZ Dra

Oscillating eclipsing Algols (oEAs) are remarkable systems which allow us to determine accurate fundamental stellar parameters (mass, radius) and probe the stellar interiors through pulsations. TZ\,Dra is an oEA system containing a $δ$ Scuti component. To examine particular characteristics of such close systems including pulsations and mass transfer, we present a detailed photometric and spectroscopic study of TZ\,Dra. With the analysis of high-resolution spectra, the orbital parameters were determined by the radial velocity analysis and the atmospheric parameters were derived for the primary component. The binary modelling and the pulsational frequency analysis was carried out using the TESS data set. The H$α$ line profiles show the signature of mass transfer from the cool to the hot binary component. The conclusion of mass transfer/mass loss in the system was supported by the analysis of the orbital period changes. As a result, it was found that there is $3.52 \times 10^{-9}$ $M_\odot$/year mass loss from the system most probably through the hotspot and stellar winds. Additionally, most pulsation frequencies originating from the primary component were found to be spaced by harmonics of the orbital frequencies in particular, twelve doublets spaced by $2f_{\rm orb}$ were detected from which we infer that this star a tidally tilted pulsator. A mean p-mode frequency spacing of $\approx 7.2 $d$^{-1}$ was found as well.

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arXiv2021

Study of Chemically Peculiar Stars-I : High-resolution Spectroscopy and K2 Photometry of Am Stars in the Region of M44

We present a study based on the high-resolution spectroscopy and K2 space photometry of five chemically peculiar stars in the region of the open cluster M44. The analysis of the high-precision photometric K2 data reveals that the light variations in HD 73045 and HD 76310 are rotational in nature and caused by spots or cloud-like co-rotating structures, which are non-stationary and short-lived. The time-resolved radial velocity measurements, in combination with the K2 photometry, confirm that HD 73045 does not show any periodic variability on timescales shorter than 1.3 d, contrary to previous reports in the literature. In addition to these new rotational variables, we discovered a new heartbeat system, HD 73619, where no pulsational signatures are seen. The spectroscopic and spectropolarimetric analyses indicate that HD 73619 belongs to the peculiar Am class, with either a weak or no magnetic field considering the 200 G detection limit of our study. The Least-Squares Deconvolution (LSD) profiles for HD 76310 indicate a complex structure in its spectra suggesting that this star is either part of a binary system or surrounded by a cloud shell. When placed in the Hertzsprung-Russell diagram, all studied stars are evolved from main-sequence and situated in the $δ$ Scuti instability strip. The present work is relevant for further detailed studies of CP stars, such as inhomogeneities (including spots) in the absence of magnetic fields and the origin of the pulsational variability in heartbeat systems.

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arXiv2021

Orbital solutions derived from radial velocities and time delays for four {\it Kepler} systems with A/F-type (candidate) hybrid pulsators

The presence of A/F-type {\it Kepler} hybrid stars extending across the entire $δ$ Sct-$γ$ Dor instability strips and beyond remains largely unexplained. In order to better understand these particular stars, we performed a multi-epoch spectroscopic study of 49 candidate A/F-type hybrid stars and one cool(er) hybrid object detected by the {\it Kepler} mission. We determined a lower limit of 27 % for the multiplicity fraction. For six spectroscopic systems, we also reported long-term variations of the time delays. For four systems, the time delay variations are fully coherent with those of the radial velocities and can be attributed to orbital motion. We aim to improve the orbital solutions for those systems with long orbital periods (order of 4-6 years) among the {\it Kepler} hybrid stars. The orbits are computed based on a simultaneous modelling of the RVs obtained with high-resolution spectrographs and the photometric time delays derived from time-dependent frequency analyses of the {\it Kepler} light curves. We refined the orbital solutions of four spectroscopic systems with A/F-type {\it Kepler} hybrid component stars: KIC 4480321, 5219533, 8975515 and KIC 9775454. Simultaneous modelling of both data types analysed together enabled us to improve the orbital solutions, obtain more robust and accurate information on the mass ratio, and identify the component with the short-period $δ$ Sct-type pulsations. In several cases, we were also able to derive new constraints for the minimum component masses. From a search for regular frequency patterns in the high-frequency regime of the Fourier transforms of each system, we found no evidence of tidal splitting among the triple systems with close (inner) companions. However, some systems exhibit frequency spacings which can be explained by the mechanism of rotational splitting.

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arXiv2020

Overview of the LAMOST-$Kepler$ project

The NASA $Kepler$ mission obtained long-term high-quality photometric observations for a large number of stars in its original field of view from 2009 to 2013. In order to provide reliable stellar parameters in a homogeneous way, the LAMOST telescope began to carry out low-resolution spectroscopic observations for as many stars as possible in the $Kepler$ field in 2012. By September 2018, 238,386 low-resolution spectra with SNR$_g \geq 6$ had been collected for 155,623 stars in the $Kepler$ field, enabling the determination of atmospheric parameters and radial velocities, as well as spectral classification of the target stars. This information has been used by astronomers to carry out research in various fields, including stellar pulsations and asteroseismology, exoplanets, stellar magnetic activity and flares, peculiar stars and the Milky Way, binary stars, etc. We summarize the research progress in these fields where the usage of data from the LAMOST-$Kepler$ (LK) project has played a role. In addition, time-domain medium-resolution spectroscopic observations have been carried out for about 12,000 stars in four central plates of the $Kepler$ field since 2018. The currently available results show that the LAMOST-$Kepler$ medium resolution (LK-MRS) observations provide qualified data suitable for research in additional science projects including binaries, high-amplitude pulsating stars, etc. As LAMOST is continuing to collect both low- and medium-resolution spectra of stars in the $Kepler$ field, we expect more data to be released continuously and new scientific results to appear based on the LK project data.

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arXiv2019

The first view of $δ$ Scuti and $γ$ Doradus stars with the TESS mission

We present the first asteroseismic results for $δ$ Scuti and $γ$ Doradus stars observed in Sectors 1 and 2 of the TESS mission. We utilise the 2-min cadence TESS data for a sample of 117 stars to classify their behaviour regarding variability and place them in the Hertzsprung-Russell diagram using Gaia DR2 data. Included within our sample are the eponymous members of two pulsator classes, $γ$ Doradus and SX Phoenicis. Our sample of pulsating intermediate-mass stars observed by TESS also allows us to confront theoretical models of pulsation driving in the classical instability strip for the first time and show that mixing processes in the outer envelope play an important role. We derive an empirical estimate of 74% for the relative amplitude suppression factor as a result of the redder TESS passband compared to the Kepler mission using a pulsating eclipsing binary system. Furthermore, our sample contains many high-frequency pulsators, allowing us to probe the frequency variability of hot young $δ$ Scuti stars, which were lacking in the Kepler mission data set, and identify promising targets for future asteroseismic modelling. The TESS data also allow us to refine the stellar parameters of SX Phoenicis, which is believed to be a blue straggler.

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arXiv2019

Rotation and pulsation in Ap stars: first light results from TESS sectors 1 and 2

We present the first results from the Transiting Exoplanet Survey Satellite (TESS) on the rotational and pulsational variability of magnetic chemically peculiar A-type stars. We analyse TESS 2-min cadence data from sectors 1 and 2 on a sample of 83 stars. Five new rapidly oscillating Ap (roAp) stars are announced. One of these pulsates with periods around 4.7 min, making it the shortest period roAp star known to date. Four out of the five new roAp stars are multiperiodic. Three of these, and the singly-periodic one show the presence of rotational mode splitting. Individual frequencies are provided in all cases. In addition, seven previously known roAp stars are analysed. Additional modes of oscillation are found in some stars, while in others we are able to distinguish the true pulsations from possible aliases present in the ground-based data. We find that the pulsation amplitude in the TESS filter is typically a factor 6 smaller than that in the $B$ filter which is usually used for ground-based observations. For four roAp stars we set constraints on the inclination angle and magnetic obliquity, through the application of the oblique pulsator model. We also confirm the absence of roAp-type pulsations down to amplitude limits of 6 and 13 micromag, respectively, in two of the best characterised non-oscillating Ap (noAp) stars. We announce 27 new rotational variables along with their rotation periods, and provide different rotation periods for seven other stars. Finally, we discuss how these results challenge state-of-the-art pulsation models for roAp stars.

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arXiv2018

Frequency and mode identification of γ Doradus from photometric and spectroscopic observations

The prototype star for the γ Doradus class of pulsating variables was studied em- ploying photometric and spectroscopic observations to determine the frequencies and modes of pulsation. The four frequencies found were self-consistent between the obser- vation types and almost identical to those found in previous studies (1.3641 d-1 ,1.8783 d-1 , 1.4742 d-1 and 1.3209 d-1). Three of the frequencies are classified as l, m = (1, 1) pulsations and the other is ambiguous between l = 2 modes. Two frequencies are shown to be stable over twenty years since their first identification. The agreement in ground-based work makes this star an excellent calibrator for the upcoming TESS observations and a standard for continued asteroseismic modelling.

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arXiv2018

K2 photometry and HERMES spectroscopy of the blue supergiant rho Leo: rotational wind modulation and low-frequency waves

We present an 80-d long uninterrupted high-cadence K2 light curve of the B1Iab supergiant rho Leo (HD 91316), deduced with the method of halo photometry. This light curve reveals a dominant frequency of $f_{\rmrot}=0.0373$d$^{-1}$ and its harmonics. This dominant frequency corresponds with a rotation period of 26.8d and is subject to amplitude and phase modulation. The K2 photometry additionally reveals multiperiodic low-frequency variability ($<1.5 $d$^{-1}$) and is in full agreement with low-cadence high-resolution spectroscopy assembled during 1800 days. The spectroscopy reveals rotational modulation by a dynamic aspherical wind with an amplitude of about 20km s$^{-1}$ in the H$α$ line, as well as photospheric velocity variations of a few km s$^{-1}$ at frequencies in the range 0.2 to 0.6 d$^{-1}$ in the SiIII 4567Å line. Given the large macroturbulence needed to explain the spectral line broadening of the star, we interpret the detected photospheric velocity as due to travelling super-inertial low-degree large-scale gravity waves with dominant tangential amplitudes and discuss why $ρ$~Leo is an excellent target to study how the observed photospheric variability propagates into the wind.

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arXiv2017

Multi-technique investigation of the binary fraction among A-F type candidate hybrid variable stars discovered by Kepler

Hundreds of candidate hybrid pulsators of intermediate type A-F were revealed by the recent space missions. Hybrid pulsators allow to study the full stellar interiors, where p- and g-modes are simultaneously excited. The true hybrid stars must be identified since other processes, due to stellar multiplicity or rotation, might explain the presence of (some) low frequencies observed in their periodograms. We measured the radial velocities of 50 candidate Delta Sct - Gamma Dor hybrid stars from the Kepler mission with the Hermes/Ace spectrographs over a span of months to years. We aim to derive the fraction of binary and multiple systems and to provide an independent and homogeneous determination of the atmospheric properties and vsini for all targets. The objective is to identify the physical cause of the low frequencies. We computed 1-D cross-correlation functions (CCFs) in order to find the best parameters in terms of the number of components, spectral type and vsini for each target. Radial velocities were measured from spectrum synthesis and by using a 2-D cross-correlation technique in the case of double- and triple-lined systems. Fundamental parameters were determined by fitting (composite) synthetic spectra to the normalised median spectra corrected for the appropriate Doppler shifts. We report on the analysis of 478 high-resolution Hermes and 41 Ace spectra of A/F-type candidate hybrid pulsators from the Kepler field. We determined their radial velocities, projected rotational velocities, atmospheric properties and classified our targets based on the shape of the CCFs and the temporal behaviour of the radial velocities. We derived orbital solutions for seven new systems. Three long-period preliminary orbital solutions are confirmed by a photometric time-delay analysis. Finally, we determined a global multiplicity fraction of 27% in our sample of candidate hybrid stars.

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arXiv2016

Pulsation versus metallicism in Am stars as revealed by LAMOST and WASP

We present the results of a study of a large sample of A and Am stars with spectral types from LAMOST and light curves from WASP. We find that, unlike normal A stars, $δ$ Sct pulsations in Am stars are mostly confined to the effective temperature range 6900 $<$ $T_{\rm eff}$ $<$ 7600 K. We find evidence that the incidence of pulsations in Am stars decreases with increasing metallicism (degree of chemical peculiarity). The maximum amplitude of the pulsations in Am stars does not appear to vary significantly with metallicism. The amplitude distributions of the principal pulsation frequencies for both A and Am stars appear very similar and agree with results obtained from Kepler photometry. We present evidence that suggests turbulent pressure is the main driving mechanism in pulsating Am stars, rather than the $κ$-mechanism, which is expected to be suppressed by gravitational settling in these stars.

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arXiv2016

Exoplanet Orbital Eccentricities Derived From LAMOST-Kepler Analysis

The nearly circular (mean eccentricity ~0.06) and coplanar (mean mutual inclination ~3 deg) orbits of the Solar System planets motivated Kant and Laplace to put forth the hypothesis that planets are formed in disks, which has developed into the widely accepted theory of planet formation. Surprisingly, the first several hundred extrasolar planets (mostly Jovian) discovered using the Radial Velocity (RV) technique are commonly on eccentric orbits ( ~ 0.3). This raises a fundamental question: Are the Solar System and its formation special? The Kepler mission has found thousands of transiting planets dominated by sub-Neptunes, but most of their orbital eccentricities remain unknown. By using the precise spectroscopic host star parameters from the LAMOST observations, we measure the eccentricity distributions for a large (698) and homogeneous Kepler planet sample with transit duration statistics. Nearly half of the planets are in systems with single transiting planets (singles), while the other half are multiple-transiting planets (multiples). We find an eccentricity dichotomy: on average, Kepler singles are on eccentric orbits with ~0.3, while the multiples are on nearly circular ( = 0.04^{+0.03}_{-0.04}) and coplanar ( = 1.4^{+0.8}_{-1.1} deg) orbits similar to the Solar System planets. Our results are consistent with previous studies of smaller samples and individual systems. We also show that Kepler multiples and solar system objects follow a common relation ~(1-2)x between mean eccentricities and mutual inclinations. The prevalence of circular orbits and the common relation may imply that the solar system is not so atypical in the galaxy after all.

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