Early Stars Research Articles

A deep view of a fossil relic in the Galactic bulge: the Globular Cluster HP 1

HP$\,$1 is an $\alpha$-enhanced and moderately metal-poor bulge globular cluster with a blue horizontal branch. These combined characteristics make it a probable relic of the early star formation in the innermost Galactic regions. Here we present a detailed analysis of a deep near-infrared (NIR) photometry of HP$\,$1 obtained with the NIR GSAOI+GeMS camera at the Gemini-South telescope. $J$ and $K_{\rm S}$ images were collected with an exquisite spatial resolution (FWHM $\sim 0.1$ arcsec), reaching stars at two magnitudes below the MSTO. We combine our GSAOI data with archival F606W-filter $HST$ ACS/WFC images to compute relative proper motions and select bona fide cluster members. Results from statistical isochrone fits in the NIR and optical-NIR colour-magnitude diagrams indicate an age of $12.8^{+0.9}_{-0.8}$ Gyr, confirming that HP$\,$1 is one of the oldest clusters in the Milky Way. The same fits also provide apparent distance moduli in the $K_{\rm S}$ and $V$ filters in very good agreement with the ones from 11 RR Lyrae stars. By subtracting the extinction in each filter, we recover a heliocentric distance of $6.59^{+0.17}_{-0.15}$ kpc. Furthermore, we refine the orbit of HP$\,$1 using this accurate distance and update and accurate radial velocities (from high resolution spectroscopy) and absolute proper motions (from Gaia DR2), reaching mean perigalactic and apogalactic distances of $\sim$0.12 and $\sim$3 kpc respectively.

HD 2685 b: A hot Jupiter orbiting an early F-type star detected by TESS

HD 2685 b: A hot Jupiter orbiting an early F-type star detected by TESS

A Family Affair: Exploring Early Star Formation

A Family Affair: Exploring Early Star Formation

Validating the C i 5052.17 Å/Mg ii 4481 Å Equivalent Width Ratio as a Diagnostic for F-type Lambda Boo Stars

Abstract The Lambda Boo-type stars are chemically peculiar stars with deficiencies of iron-peak elements but near-solar C, N, O, and S abundances. Since the prototype Lambda Boötis was first reported as peculiar, this group has been expanded from a small group of early A-type stars to a larger group of late B to early F-type dwarfs. Although a detailed abundance analysis that supports the Lambda Boo-like abundance pattern is the definitive confirmation of this Lambda Boo characteristic, the rapid rotation of many Lambda Boo stars generally limits how much detail can be derived from an abundance analysis. Traditionally, Lambda Boo candidates have been classified by visually examining the difference between their spectra and spectra of standard stars. Therefore, some ambiguity remains especially for mild or borderline Lambda Boo stars. This is the third paper in a series that establishes a straightforward yet reliable way to identify Lambda Boo-type stars. In previous papers, we identified line equivalent width (EW) ratios in the ultraviolet and visible regions that can distinguish Lambda Boo stars from other metal-weak stars. In this paper, we apply the visible line EW ratio diagnostic to 25 Lambda Boo candidates and carry out a detailed abundance analysis of HD 81290, an F2 star with a C i/Mg ii EW ratio in the range expected for Lambda Boo-type stars. Our elemental abundance analysis results confirm HD 81290's Lambda Boo membership and demonstrate the utility of our EW ratio as a diagnostic for cooler F-type Lambda Boo stars.

Planetary tidal interactions and the rotational evolution of low-mass stars

Context. The surface angular velocity evolution of low-mass stars is now globally understood and the main physical mechanisms involved in it are observationally quite constrained. However, while the general behaviour of these mechanisms is grasped, their theoretical description is still under ongoing work. This is the case, for instance, about the description of the physical process that extracts angular momentum from the radiative core, which could be described by several theoretical candidates. Additionally, recent observations showed anomalies in the rotation period distribution of open cluster, main sequence, early K-type stars that cannot be reproduced by current angular momentum evolution models. Aims. In this work, we study the parameter space of star-planet system’s configurations to investigate if including the tidal star-planet interaction in angular momentum evolution models could reproduce the anomalies of this rotation period distribution. Methods. To study this effect, we use a parametric angular momentum evolution model that allows for core-envelope decoupling and angular momentum extraction by magnetized stellar wind that we coupled to an orbital evolution code where we take into account the torque due to the tides raised on the star by the planet. We explore different stellar and planetary configurations (stellar mass from 0.5 to 1.0 M⊙ and planetary mass from 10 M⊕ to 13 Mjup) to study their effect on the planetary orbital and stellar rotational evolution. Results. The stellar angular momentum is the most impacted by the star-planet interaction when the planet is engulfed during the early main sequence phase. Thus, if a close-in Jupiter-mass planet is initially located at around 50% of the stellar corotation radius, a kink in the rotational period distribution opens around late and early K-type stars during the early main sequence phase. Conclusions. Tidal star-planet interactions can create a kink in the rotation period distribution of low-mass stars, which could possibly account for unexpected scatter seen in the rotational period distribution of young stellar clusters.

Suzaku and NuSTAR X-ray spectroscopy of γ Cassiopeiae and HD 110432

Abstract γ Cas and its dozen analogs comprise a small but distinct class of X-ray sources. They are early Be-type stars with an exceptionally hard thermal X-ray emission. The X-ray production mechanism has been under intense debate. Two competing ideas are (i) the magnetic activities in the Be star and its disk, and (ii) the mass accretion onto the unidentified white dwarf (WD). We adopt the latter as a working hypothesis and apply physical models developed to describe the X-ray spectra of classical WD binaries containing a late-type companion. Models of non-magnetic and magnetic accreting WDs were applied to γ Cas and its brightest analog HD 110432 using the Suzaku and NuSTAR data. The spectra were fitted by the two models, including the Fe fluorescence and the Compton reflection in a consistent geometry. The derived physical parameters are in a reasonable range in comparison to their classical WD binary counterparts. Additional pieces of evidence in the X-ray spectra—partial covering, Fe L lines, Fe i fluorescence—were not conclusive enough to classify these two sources into a subclass of accreting WD binaries. We discuss further observations, especially long-term temporal behaviors, which are important to elucidate the nature of these sources, if indeed they host accreting WDs.

Time–frequency-based non-harmonic analysis to reduce line-noise impact for LIGO observation system

Abstract Gravitational waves (GWs) are the subjects of a new type of space observation that is expected to reveal new information on the early universe, neutron stars, and black holes. To reduce the impact of the widely used notch filter on GW observations, we propose the use of non-harmonic analysis (NHA) for the high-accuracy visualization of GWs. A simulated signal is used to compare the results obtained using short-time Fourier transform, Hilbert spectrum analysis, Hilbert–Huang transformation, and NHA. We demonstrate that NHA exhibits superior performance. In addition, we verify the analytical precision of NHA using actual data measured by the Laser Interferometer Gravitational-Wave Observatory (LIGO). We found that the high time–frequency resolution of NHA and the small influence that the analysis window has on this technique facilitate effective quantitative analysis, even for LIGO signals containing mixing noise. In addition, we achieve GW visualization up to the noise limit without using a notch filter.

Significant uncertainties from calibrating overshooting with eclipsing binary systems

The precise measurement of the masses and radii of stars in eclipsing binary systems provides a window into uncertain processes in stellar evolution, especially mixing at convective boundaries. Recently, these data have been used to calibrate models of convective overshooting in the cores of main sequence stars. In this study we have used a small representative sample of eclipsing binary stars with 1.25 ≤ M/M⊙ < 4.2 to test how precisely this method can constrain the overshooting and whether the data support a universal stellar mass–overshooting relation. We do not recover the previously reported stellar mass dependence for the extent of overshooting and in each case we find there is a substantial amount of uncertainty, that is, the same binary pair can be matched by models with different amounts of overshooting. Models with a moderate overshooting parameter 0.013 ≤ fos ≤ 0.014 (using the scheme from Herwig et al. 1997, A&A, 324, L81) are consistent with all eight systems studied. Generally, a much larger range of f is suitable for individual systems. In the case of main sequence and early post-main sequence stars, large changes in the amount of overshooting have little effect on the radius and effective temperature, and therefore the method is of extremely limited utility.

Detection of magnetic fields in He-rich early B-type stars using HARPSpol

Aims. We focus on early-B type stars with helium overabundance, for which the presence of a magnetic field has not previously been reported. Methods. The measurements were carried out using high-spectral-resolution spectropolarimetric observations obtained with the High Accuracy Radial velocity Planet Searcher (HARPS) in polarimetric mode, installed at the ESO La Silla 3.6 m telescope. Results. For five He-rich stars, the longitudinal magnetic field was detected for the first time. For one target, HD 58260, the presence of a longitudinal magnetic field of the order of 1.8 kG has already been reported in the literature, but the magnetic field has remained constant over tens of years. Our measurement carried out using the polarimetric spectra obtained in 2015 March indicates a slight decrease of the longitudinal magnetic field strength compared to measurements reported in previous works. A search for periodic modulation in available photometric data allowed us to confidently establish a period of 2.64119 ± 0.00420 d in archival ASAS3 data for CPD–27°1791. No period could be determined for the other five stars. Conclusions. The obtained results support the scenario that all He-rich stars are detectably magnetic and form an extension of the Ap star phenomenon to higher temperatures.

The Properties of GRB 120923A at a Spectroscopic Redshift of z ≈ 7.8

Abstract Gamma-ray bursts (GRBs) are powerful probes of early stars and galaxies, during and potentially even before the era of reionization. Although the number of GRBs identified at z ≳ 6 remains small, they provide a unique window on typical star-forming galaxies at that time, and thus are complementary to deep field observations. We report the identification of the optical drop-out afterglow of Swift GRB 120923A in near-infrared Gemini-North imaging, and derive a redshift of from Very Large Telescope/X-shooter spectroscopy. At this redshift the peak 15–150 keV luminosity of the burst was 3.2 × 1052 erg s−1, and in this sense it was a rather typical long-duration GRB in terms of rest frame luminosity. This burst was close to the Swift/Burst Alert Telescope detection threshold, and the X-ray and near-infrared afterglow were also faint. We present ground- and space-based follow-up observations spanning from X-ray to radio, and find that a standard external shock model with a constant-density circumburst environment of density n ≈ 4 × 10−2 cm−3 gives a good fit to the data. The near-infrared light curve exhibits a sharp break at t ≈ 3.4 days in the observer frame which, if interpreted as being due to a jet, corresponds to an opening angle of . The beaming-corrected γ-ray energy is then erg, while the beaming-corrected kinetic energy is lower, erg, suggesting that GRB 120923A was a comparatively low kinetic energy event. We discuss the implications of this event for our understanding of the high-redshift population of GRBs and their identification.

Post-common envelope binary systems experiencing helium-shell driven stable mass transfer

We evolve stellar models to study the common envelope (CE) interaction of an early asymptotic giant branch star of initial mass $5\,\rm M_{\odot}$ with a companion star of mass ranging from $0.1$ to $2\,\rm M_{\odot}$. We model the CE as a fast stripping phase in which the primary experiences rapid mass loss and loses about 80 per cent of its mass. The post-CE remnant is then allowed to thermally readjust during a Roche-lobe overflow (RLOF) phase and the final binary system and its orbital period are investigated. We find that the post-CE RLOF phase is long enough to allow nuclear burning to proceed in the helium shell. By the end of this phase, the donor is stripped of both its hydrogen and helium and ends up as carbon-oxygen white dwarf of mass about $0.8\,\rm M_{\odot}$. We study the sensitivity of our results to initial conditions of different companion masses and orbital separations at which the stripping phase begins. We find that the companion mass affects the final binary separation and that helium-shell burning causes the star to refill its Roche lobe leading to post-CE RLOF. Our results show that double mass transfer in such a binary interaction is able to strip the helium and hydrogen layers from the donor star without the need for any special conditions or fine tuning of the binary parameters.

Dwarf galaxies at low and high redshift

AbstractThe overwhelming majority of galaxies in the Universe are dwarf galaxies. But although they are important components in understanding galaxy evolution, these systems are typically too faint to be observed at high redshifts. However, we are able to obtain an unobscured view of early star formation and chemical enrichment in these galaxies at low redshift and low-redshift analogs at high redshift. In this talk, I will review the mass-metallicity relation, the mass-star formation rate relation of galaxies, the classifications of dwarf galaxies, and the importance of dwarf galaxies for both astronomy and physics. Then I will introduce some work in our group on connections among between different types of dwarf galaxies,the mass-metallicity relations and the main sequence relations of dwarf galaxies, using the deep optical and near infrared images and spectra of large dwarf galaxy sample. At the end, I will talk about some projects of dwarf galaxies we are working on, including the spectroscopic survey for compact dwarf galaxies using the LAMOST.

The Belgian Repository of Fundamental Atomic Data and Stellar Spectra (BRASS) Identifying Fruitful Methods for Producing Atomic Data

The Belgian repository of fundamental atomic data and stellar spectra (BRASS) aims to provide the largest systematic and homogeneous quality assessment to-date of input atomic data required for stellar spectral synthesis. In addition to quality-assessed atomic data, BRASS shall also provide of a range of extremely high-quality benchmark stellar spectra spanning late B-type stars to early K-type stars. In this paper, we summarise the project’s progress and available results to-date. We provide a brief comparison between our results and the BRASS project’s compiled and cross-matched atomic literature, with the goal of providing useful feedback to the atomic community on which methods may produce more reliable and accurate atomic data. We hope that the examples presented here stimulate further investigation by the atomic physics community.

The Rotational Shear Layer inside the Early Red-giant Star KIC 4448777

Abstract We present the asteroseismic study of the early red-giant star KIC 4448777, complementing and integrating a previous work, aimed at characterizing the dynamics of its interior by analyzing the overall set of data collected by the Kepler satellite during the four years of its first nominal mission. We adopted the Bayesian inference code diamonds for the peak bagging analysis and asteroseismic splitting inversion methods to derive the internal rotational profile of the star. The detection of new splittings of mixed modes, which are more concentrated in the very inner part of the helium core, allowed us to reconstruct the angular velocity profile deeper into the interior of the star and to disentangle the details better than in Paper I: the helium core rotates almost rigidly about 6 times faster than the convective envelope, while part of the hydrogen shell seems to rotate at a constant velocity about 1.15 times lower than the He core. In particular, we studied the internal shear layer between the fast-rotating radiative interior and the slow convective zone and we found that it lies partially inside the hydrogen shell above r ≃ 0.05R and extends across the core–envelope boundary. Finally, we theoretically explored the possibility for the future capabilty to sound the convective envelope in the red-giant stars and we concluded that the inversion of a set of splittings with only low-harmonic degree l ≤ 3, even supposing a very large number of modes, will not allow us to resolve the rotational profile of this region in detail.

Implications of a prereionization 21-cm absorption signal for fuzzy dark matter

The EDGES experiment recently announced evidence for a broad absorption feature in the sky-averaged radio spectrum around 78 MHz, as may result from absorption in the 21 cm line by neutral hydrogen at z~15-20. If confirmed, one implication is that the spin temperature of the 21 cm line is coupled to the gas temperature by z=20. The known mechanism for accomplishing this is the Wouthuysen-Field effect, whereby Lyman-alpha photons scatter in the intergalactic medium (IGM) and impact the hyperfine level populations. This suggests that early star formation had already produced a copious Lyman-alpha background by z=20, and strongly constrains models in which the linear matter power spectrum is suppressed on small-scales, since halo and star formation are delayed in such scenarios. Here we consider the case that the dark matter consists of ultra-light axions with macroscopic de Broglie wavelengths (fuzzy dark matter, FDM). We assume that star formation tracks halo formation and adopt two simple models from the current literature for the halo mass function in FDM. We further suppose that the fraction of halo baryons which form stars is less than a conservative upper limit of $f_\star \leq 0.05$, and that ~10^4 Lyman-alpha to Lyman-limit photons are produced per stellar baryon. We find that the requirement that the 21 cm spin temperature is coupled to the gas temperature by $z=20$ places a lower-limit on the FDM particle mass of $m_a \geq 5 \times 10^{-21} {\rm eV}$. The constraint is insensitive to the precise minimum mass of halos where stars form. As the global 21 cm measurements are refined, the coupling redshift could change and we quantify how the FDM constraint would be modified. A rough translation of the FDM mass bound to a thermal relic warm dark matter (WDM) mass bound is also provided.

Constraints on early star formation from the 21-cm global signal

Abstract The tentative detection by the EDGES experiment of a global 21-cm absorption trough centred at redshift 17 opens up the opportunity to study the birth of the first luminous sources, the intensity of radiation backgrounds at cosmic dawn, the thermal and ionization history of the young intergalactic medium. Here, we focus on the astrophysical implications of the Ly α photon field needed to couple the spin temperature to the kinetic temperature of the gas at these early epochs. Under the basic assumption that the 21-cm signal is activated by extremely metal-poor stellar systems, we show that the EDGES results are consistent with an extrapolation of the declining galaxy UV luminosity density measured at $4\lesssim z\lesssim 9$ by deep Hubble Space Telescope observations. A substantially enhanced star formation rate density or new exotic sources of UV photons are not required at the redshifts of the EDGES signal. The amount of ionizing radiation produced by the same stellar systems that induce Ly α coupling is significant, of order 0.5 LyC photons per H-atom per 100 Myr. To keep hydrogen largely neutral and delay the reionization process consistently with recent Planck cosmic microwave background results, mean escape fractions of $f_{\rm esc}\lesssim 20$ per cent are required at $z$ > 15.

An extended hydrogen envelope of the extremely hot giant exoplanet KELT-9b

Giant exoplanets orbiting close to their host stars have high temperatures because of the immense stellar irradiation which they receive. The extreme energy input leads to the expansion of the atmosphere and the escape of neutral hydrogen. A particularly intriguing case among the hot giant planets is KELT-9b -- an exoplanet orbiting very close to an early A-type star with the highest temperature (~4600 K at day-side) among all the exoplanets known so far. The atmospheric composition and dynamic of such a unique planet have been unknown. Here we report the first detection of an extended hot hydrogen atmosphere around KELT-9b. The detection was achieved by measuring the atomic hydrogen absorption during transit with the Balmer H{\alpha} line, which is unusually strong mainly due to the high level of extreme-ultraviolet radiation from the star. We detected a significant wavelength shift of the H{\alpha} absorption which is mostly attributed to the planetary orbital motion. The obtained transmission spectrum has a significant line contrast (1.15% extra absorption at the H{\alpha} line centre). The observation implies that the effective radius at the H{\alpha} line centre is ~1.64 times the size of the planetary radius, indicating the planet has a largely extended hydrogen envelope close to the size of the Roche lobe (1.91 Rplanet) and is probably undergoing dramatic atmosphere escape.

A Refined Sample of Lyman Excess H ii Regions

A large number (67) of the compact/ultra-compact H II regions identified in the Coordinated Radio and Infrared Survey for High-Mass Star Formation catalogue were determined to be powered by a Lyman continuum flux in excess of what was expected given their corresponding luminosity. In this study we attempt to reasonably explain the Lyman excess phenomenon in as many of the 67 H II regions as possible through a variety of observational and astrophysical means including new luminosity estimates, new Herschel photometry, new distance determinations, the use of different models for dust and ionized gas covering factors, and the use of different stellar calibrations. This phenomenon has been observed before; however, the objects shown to exhibit this behaviour in the literature have decidedly different physical properties than the regions in our sample, and thus the origin of the excess is not the same. We find that the excess can be reproduced using OB stellar atmosphere models that have been slightly modified in the extreme ultraviolet. Though the exact mechanism producing the excess is still uncertain, we do find that a scaled up magnetospheric accretion model, often used to explain similar emission from T Tauri stars, is unable to match our observations. Our results suggest that the Lyman excess may be associated with younger H II regions, and that it is more commonly found in early B-type stars. Our refined sample of 24 Lyman excess H II regions provides an ideal sample for comparative studies with regular H II regions, and can act as the basis for the further detailed study of individual regions.

The Late-type Eclipsing Binaries in the Large Magellanic Cloud: Catalog of Fundamental Physical Parameters

Abstract We present a determination of the precise fundamental physical parameters of 20 detached, double-lined, eclipsing binary stars in the Large Magellanic Cloud (LMC) containing G- or early K-type giant stars. Eleven are new systems; the remaining nine are systems already analyzed by our team for which we present updated parameters. The catalog results from our long-term survey of eclipsing binaries in the Magellanic Clouds suitable for high-precision determination of distances (the Araucaria Project). The V-band brightnesses of the systems range from 15.4 to 17.7 mag, and their orbital periods range from 49 to 773 days. Six systems have favorable geometry showing total eclipses. The absolute dimensions of all eclipsing binary components are calculated with a precision of better than 3%, and all systems are suitable for a precise distance determination. The measured stellar masses are in the range 1.4 to 4.6 M ⊙, and comparison with the MESA isochrones gives ages between 0.1 and 2.1 Gyr. The systems show an age–metallicity relation with no evolution of metallicity for systems older than 0.6 Gyr, followed by a rise to a metallicity maximum at age 0.5 Gyr and then a slow metallicity decrease until 0.1 Gyr. Two systems have components with very different masses: OGLE LMC-ECL-05430 and OGLE LMC-ECL-18365. Neither system can be fitted by a single stellar evolution isochrone, explained by a past mass transfer scenario in the case of ECL-18365 and a gravitational capture or hierarchical binary merger scenario in the case of ECL-05430. The longest-period system, OGLE LMC SC9_230659, shows a surprising apsidal motion that shifts the apparent position of the eclipses. This is a clear sign of a physical companion to the system; however, neither investigation of the spectra nor light-curve analysis indicates a third-light contribution larger than 2%–3%. In one spectrum of OGLE LMC-ECL-12669, we noted a peculiar dimming of one of the components by 65% well outside of the eclipses. We interpret this observation as arising from an extremely rare occultation event, as a foreground Galactic object covers only one component of an extragalactic eclipsing binary.

Light Chemical Elements in Stars: Mysteries and Unsolved Problems

The first eight elements of the periodic table are discussed: H, He, Li, Be, B, C, N, and O. They are referred to as key elements, given their important role in stellar evolution. It is noteworthy that all of them were initially synthesized in the Big Bang. The primordial abundances of these elements calculated using the Standard Model of the Big Bang (SMBB) are presented in this review. The good agreement between the SMBB and observations of the primordial abundances of the isotopes of hydrogen and helium, D, 3He, and 4He, is noted, but there is a difference of ~0.5 dex for lithium (the isotope 7Li) between the SMBB and observations of old stars in the galactic halo that has not yet been explained. The abundances of light elements in stellar atmospheres depends on the initial rotation velocity, so the typical rotation velocities of young Main Sequence (MS) stars are examined. Since the data on the abundances of light elements in stars are very extensive, the main emphasis in this review is on several unsolved problems. The helium abundance He/H in early B-type of the MS stars shows an increment with age; in particular, for the most massive B stars with masses M = 12−19M⦿, He/H increases by more than a factor of two toward the end of the MS. Theoretical models of stars with rotation cannot explain such a large increase in He/H. For early B- and late O-type MS stars that are components of close binary systems, He/H undergoes a sharp jump in the middle of the MS stage that is a mystery for the theory. The anomalous abundance of helium (and lithium) in the atmospheres of chemically peculiar stars (types He-s, He-w, HgMn, Ap, and Am) is explained in terms of the diffusion of atoms in surface layers of the stars, but this hypothesis cannot yet explain all the features of the chemical composition of these stars. The abundances of lithium, beryllium, and boron in FGK-dwarfs manifest a trend with decreasing effective temperature T eff as well as a dip at T eff ~ 6600 K in the Hyades and other old clusters. The two effects are among the unsolved problems. In the case of lithium, there is special interest in FGK-giants and supergiants that are rich in lithium (they have logε(Li)≥ 2). Most of them cannot be explained in terms of the standard theory of stellar evolution, so nonstandard hypotheses are invoked: the recent synthesis of lithium in a star and the engulfment by a star of a giant planet with mass equal to that of Jupiter or greater. An analysis of the abundances of carbon, nitrogen, and oxygen in early B- and late O-stars of the MS indicates that the C II, N II, and O II ions are overionized in their atmospheres. For early B-type MS stars, good agreement is found between observations of the N/O ratio and model calculations for rotating stars. A quantitative explanation of the well-known “nitrogen-oxygen” anticorrelation in FGK-giants and supergiants is found. It reflects the dependence of the anomalies in N and C on the initial rotation velocity V0. The stellar rotation models which yield successful explanations for C, N. and O cannot, however, explain the observed helium enrichment in early B-type MS stars.