Zero-age Main-sequence Stars Research Articles

Exploring massive star early evolution: the case of the Herschel 36 A triple system

Abstract Theoretical models show that some massive stars have not yet arrived at the zero-age main-sequence (ZAMS) at the end of the accretion phase. At that time, they have lost their thick envelopes and thus could be optically visible. Although some candidates to optically observable ZAMS stars have been reported, the evolutionary status of none of them has been confirmed yet. The O-type triple system Herschel 36 A (H36A) is one of these candidates. We present the quantitative spectral analysis of the individual stellar components of H36A and investigate the evolutionary status of the system by contrasting main sequence and pre-main sequence models. Overall, the derived parameters suggest that the components of H36A could be pre-main sequence stars going through the very last contraction to the ZAMS. However, the possibility of them already being on the main sequence is not yet ruled out. This study highlights the importance of considering multiple evolutionary models and shows that H36A represents a key object for understanding massive star formation and early evolution.

Chromospheric Mg I emission lines of pre-main-sequence stars

To reveal details of the internal structure, the relationship between chromospheric activity and the Rossby number has been extensively examined for main-sequence stars. For active pre-main-sequence (PMS) stars, it is suggested that the level of activity be assessed using optically thin emission lines, such as Mg I. We aim to detect Mg I chromospheric emission lines from PMS stars and to determine whether the chromosphere is activated by the dynamo process or by mass accretion from protoplanetary disks. We analyzed high-resolution optical spectra of $64\ $PMS stars obtained with the Very Large Telescope (VLT)/X-shooter and UVES and examined the infrared Ca II ($8542\ AA $) and Mg I ($8807\ AA $) emission lines. To detect the weak chromospheric emission lines, we determined the atmospheric parameters ($T_ eff $ and $ g$) and the degree of veiling of the PMS stars by comparing the observed spectra with photospheric model spectra. After subtracting the photospheric model spectrum from the PMS spectrum, we detected Ca II and Mg I as emission lines. The strengths of the Mg I emission lines in PMS stars with no veiling are comparable to those in zero-age main-sequence (ZAMS) stars if both types of stars have similar Rossby numbers. The Mg I emission lines in these PMS stars are thought to be formed by a dynamo process similar to that in ZAMS stars. In contrast, the Mg I emission lines in PMS stars with veiling are stronger than those in ZAMS stars. These objects are believed to have protoplanetary disks, where mass accretion generates shocks near the photosphere, heating the chromosphere. The chromosphere of PMS stars is activated not only by the dynamo process but also by mass accretion.

Characterizing the Rapid Hydrogen Disappearance in SN 2022crv: Evidence of a Continuum between Type Ib and IIb Supernova Properties

We present optical and near-infrared (NIR) observations of SN 2022crv, a stripped-envelope supernova in NGC 3054, discovered within 12 hr of explosion by the Distance Less Than 40 Mpc Survey. We suggest that SN 2022crv is a transitional object on the continuum between Type Ib supernovae (SNe Ib) and Type IIb supernovae (SNe IIb). A high-velocity hydrogen feature (∼ −20,000 to −16,000 km s−1) was conspicuous in SN 2022crv at early phases, and then quickly disappeared. We find that a hydrogen envelope of ∼10−3 M ⊙ can reproduce the observed behavior of the hydrogen feature. The lack of early envelope cooling emission implies that SN 2022crv had a compact progenitor with an extremely low amount of hydrogen. A nebular spectral analysis shows that SN 2022crv is consistent with the explosion of a He star with a final mass of ∼4.5–5.6 M ⊙ that evolved from a ∼16 to 22 M ⊙ zero-age main-sequence star in a binary system with ∼1.0–1.7 M ⊙ of oxygen finally synthesized in the core. In order to retain such a small amount of hydrogen, the initial orbital separation of the binary system is likely larger than ∼1000 R ⊙. The NIR spectra of SN 2022crv show a unique absorption feature on the blue side of the He i line at ∼1.005 μm. This is the first time such a feature has been observed in SNe Ib/IIb, and it could be due to Sr II. Further detailed modeling of SN 2022crv can shed light on the progenitor and the origin of the mysterious absorption feature in the NIR.

A star under multiple influences

Context.Close binaries with magnetically active components are astrophysical laboratories for studying the effects of binarity on activity. Of particular interest are binary and multiple star systems that contain a solar-type active component with an internal structure similar to the Sun, allowing us to study how the dynamo of a solar-type star would work under different conditions.Aims.We have conducted a comprehensive investigation of V815 Her using photometric and spectroscopic data to understand the origin of the activity and what influences it in the short and long term.Methods.Using space photometry we performed light curve modeling in order to derive astrophysical and orbital parameters for the eclipsing binary subsystem V815 Her B. Using archival photometric data covering a century we carried out a time frequency analysis. Spectral synthesis was applied to determine the basic astrophysical parameters of the rapidly rotating primary using high-resolution STELLA spectra recorded in 2018.Results.Photometric analysis of archived data revealed multiple cycles on timescales between ∼6.5 and ∼26 yr, some of which may be harmonic. From TESS photometry we obtained an orbital solution for the V815 Her B subsystem. By placing the primary component on the Hertzsprung–Russell-diagram, we could deduce an age of ≈30 Myr, in line with the high Li-6707 abundance. The STELLA spectra covering the 200 day-long observing season enabled us to create 19 time-series Doppler images, which revealed a constantly changing spotted surface on a timescale of a few weeks. From the consecutive image pairs we built up the average cross-correlation function map to measure the surface differential rotation of the spotted star, from which we derive a weak solar-type surface shear.Conclusions.We found evidence that the V815 Her B component previously apostrophized as a “third body” is actually an eclipsing close binary subsystem of two M dwarfs with a period of 0.5 d, that is, V815 Her is a 2+2 hierarchical quadruple system. The system is apparently young, only a few times ten million years old, consistent with the spotted primary V815 Her Aa being a zero-age main-sequence star. Spot activity on the primary was found to be vivid. Fast starspot decay suggests that convective-turbulent erosion plays a more significant role in such a rapidly rotating star. The weak surface shear of V815 Her Aa due to differential rotation is presumably confined by tidal forces of the close companion V815 Her Ab. The slowly increasing photometric cycle of about 6.5 yr on average is interpreted as a spot cycle of V815 Her Aa, which is probably modulated by the eccentric wide orbit.

Towards a self-consistent model of the convective core boundary in upper main sequence stars

There is strong observational evidence that the convective cores of intermediate-mass and massive main sequence stars are substantially larger than those predicted by standard stellar-evolution models. However, it is unclear what physical processes cause this phenomenon or how to predict the extent and stratification of stellar convective boundary layers. Convective penetration is a thermal-timescale process that is likely to be particularly relevant during the slow evolution on the main sequence. We use our low-Mach-number SEVEN-LEAGUE HYDRO code to study this process in 2.5D and 3D geometries. Starting with a chemically homogeneous model of a 15 M⊙ zero-age main sequence star, we construct a series of simulations with the luminosity increased and opacity decreased by the same factor, ranging from 103 to 106. After reaching thermal equilibrium, all of our models show a clear penetration layer; its thickness becomes statistically constant in time and it is shown to converge upon grid refinement. The penetration layer becomes nearly adiabatic with a steep transition to a radiative stratification in simulations at the lower end of our luminosity range. This structure corresponds to the adiabatic ‘step overshoot’ model often employed in stellar-evolution calculations. The simulations with the highest and lowest luminosity differ by less than a factor of two in the penetration distance. The high computational cost of 3D simulations makes our current 3D data set rather sparse. Depending on how we extrapolate the 3D data to the actual luminosity of the initial stellar model, we obtain penetration distances ranging from 0.09 to 0.44 pressure scale heights, which is broadly compatible with observations.

VY Scl-type cataclysmic variable SDSS J154453.60+255348.8: stellar and disc parameters

ABSTRACT We present a new study of the eclipsing cataclysmic variable SDSS J154453.60+255348.8 to determine the object’s nature and its system parameters together with the probe of the accretion flow structure in the system. Based on analyses of new simultaneous time-resolved photometric and spectroscopic observations of SDSS J154453.60+255348.8 and using our light-curve modelling techniques and the Doppler tomography method, we found that the system contains a white dwarf with the mass of MWD = 0.62(7) M⊙ and an evolved red dwarf as a secondary. The system inclination is close to 90° and the mass ratio is q = 0.49(2). The secondary has an effective temperature T2 = 3400(40) K and a radius about 1.35(15) times larger than a zero-age main-sequence star with similar mass. From observation of the system in high- and low-brightness states, we conclude that SDSS J154453.60+255348.8 is a long-orbital period VY Scl-type system. The accretion disc in the high state is about two times less than the truncation radius, and is completely missing during the low state of the system.

Simulations of the Progenitors of Black Hole–Neutron Star Gravitational Wave Sources

Recent discoveries of gravitational wave (GW) events most likely originating from black hole (BH) + neutron star (NS) mergers reveal the existence of BH+NS binaries. The formation of BH+NS binaries and their merger rates through isolated binary evolution have been investigated extensively with population synthesis simulations. A detailed stellar evolution modeling of the formation of this population, however, is missing from the literature. In this work, we create the first complete 1D model of more than 30 BH+NS progenitor systems, which are calculated self-consistently until collapse of the iron core with infall velocity exceeding 1000 km s−1. Focusing on the progenitors of BH–NS GW sources, we apply the MESA code starting from a post-common-envelope binary with short orbital period (<1 day) consisting of a BH and a zero-age main-sequence helium star that experiences stable mass transfer. The (ultra)stripped supernova explosion is subsequently modeled using a semianalytic method to reveal final remnant masses and momentum kicks. Three example systems (A, B, and C) eventually evolve into BH+NS binaries with component masses of (M BH, M NS) = (8.80, 1.53), (8.92, 1.45), and (5.71, 1.34) M ⊙, respectively. These NS masses could be significantly larger depending on the exact mass cut during the supernova explosion. These BH+NS systems are likely to merge and produce GW events within a Hubble time. System C is a potential progenitor of a GW200115-like event, while Systems A and B are possible candidates for a GW200105-like event and may represent the final destiny of the X-ray binary SS 433.

A study of convective core overshooting as a function of stellar mass based on two-dimensional hydrodynamical simulations

ABSTRACT We perform two-dimensional (2D) numerical simulations of core convection for zero-age main-sequence stars covering a mass range from 3 to 20 M⊙. The simulations are performed with the fully compressible time-implicit code music. We study the efficiency of overshooting, which describes the ballistic process of convective flows crossing a convective boundary, as a function of stellar mass and luminosity. We also study the impact of artificially increasing the stellar luminosity for 3 M⊙ models. The simulations cover hundreds to thousands of convective turnover time-scales. Applying the framework of extreme plume events previously developed for convective envelopes, we derive overshooting lengths as a function of stellar masses. We find that the overshooting distance (dov) scales with the stellar luminosity (L) and the convective core radius (rconv). We derive a scaling law $d_{\rm ov} \propto L^{1/3} r_{\rm conv}^{1/2}$, which is implemented in a one-dimensional stellar evolution code and the resulting stellar models are compared to observations. The scaling predicts values for the overshooting distance that significantly increase with stellar mass, in qualitative agreement with observations. Quantitatively, however, the predicted values are underestimated for masses ≳10 M⊙. Our 2D simulations show the formation of a nearly adiabatic layer just above the Schwarzschild boundary of the convective core, as exhibited in recent three-dimensional simulations of convection. The most luminous models show a growth in size with time of the nearly adiabatic layer. This growth seems to slow down as the upper edge of the nearly adiabatic layer gets closer to the maximum overshooting length and as the simulation time exceeds the typical thermal diffusive time-scale in the overshooting layer.

Starspots, chromospheric emission lines, and flares of zero-age main-sequence stars

Abstract Zero-age main-sequence (ZAMS) stars are considered to have enormous starspots and show strong chromospheric emission lines because of their strong surface magnetic field. We discuss the dynamo activities of ZAMS stars with respect to their periodic light variation caused by a starspot and with respect to the strength of the chromospheric emission lines. The light curves of 33 ZAMS stars in IC 2391 and IC 2602 were obtained from TESS (Transiting Exoplanet Survey Satellite) photometric data. The light curves can be grouped into the following four categories: single frequency, possible shape changer, beater, and complex variability. The amplitudes of the light curves are 0.001–0.145 mag, similar to those of ZAMS stars in Pleiades. The starspot coverages are $0.1\%$–$21\%$. We found that the light variations and Ca ii emission line strength of ZAMS stars in IC 2391, IC 2602, and the Pleiades cluster are as large as those of the most active superflare stars and two orders larger than those of the Sun, and are located on the extensions of the superflare stars. These results suggest that superflare stars link the properties of the Sun to those of the ZAMS stars of ages between 30 and 120 Myr. ZAMS stars with a single frequency or possible shape change in the light curve tend to have both large light variation, indicating large spot coverage, and saturated Ca ii emission line strength. ZAMS stars with beat or complex variability have small spot coverage and a faint Ca ii emission line. We also detected 21 flares in the TESS light curves of 12 ZAMS stars in IC 2391 and IC 2602, where most of these stars have saturated chromospheric Ca ii emission lines. The energies of the flares are estimated to be ∼1033–1035 erg, which is comparable with the energy of a superflare.

SN 2016iyc: a Type IIb supernova arising from a low-mass progenitor

ABSTRACT In this work, photometric and spectroscopic analyses of a very low-luminosity Type IIb supernova (SN) 2016iyc have been performed. SN 2016iyc lies near the faint end among the distribution of similar supernovae (SNe). Given lower ejecta mass (Mej) and low nickel mass (MNi) from the literature, combined with SN 2016iyc lying near the faint end, one-dimensional stellar evolution models of 9–14 M⊙ zero-age main-sequence (ZAMS) stars as the possible progenitors of SN 2016iyc have been performed using the publicly available code mesa. Moreover, synthetic explosions of the progenitor models have been simulated, using the hydrodynamic evolution codes stella and snec. The bolometric luminosity light curve and photospheric velocities produced through synthetic explosions of ZAMS stars of mass in the range of 12–13 M⊙ having a pre-supernova radius R0 = (204–300) R⊙, with Mej = (1.89–1.93) M⊙, explosion energy Eexp = (0.28–0.35) × 1051 erg, and MNi &amp;lt; 0.09 M⊙, are in good agreement with observations; thus, SN 2016iyc probably exploded from a progenitor near the lower mass limits for SNe IIb. Finally, hydrodynamic simulations of the explosions of SN 2016gkg and SN 2011fu have also been performed to compare intermediate- and high-luminosity examples among well-studied SNe IIb. The results of progenitor modelling and synthetic explosions for SN 2016iyc, SN 2016gkg, and SN 2011fu exhibit a diverse range of mass for the possible progenitors of SNe IIb.

Measurements of chromospheric Mg i emission lines of zero-age main-sequence stars

Abstract The chromosphere is the active atmosphere in which energetic eruption events, such as flares, occur. Chromospheric activity is driven by the magnetic field generated by stellar rotation and convection. The relationship between chromospheric activity and the Rossby number, the ratio of the rotational period to the convective turnover time, has been extensively examined for many types of stars, by using narrow chromospheric emission lines, such as the Ca ii lines and the Mg ii H and K lines. However, the stars with small Rossby numbers, i.e., stars with rapid rotations and/or long convective turnover times, show constant strengths of such lines against the Rossby number. In this study, we investigate the infrared Mg i emission lines at 8807 Å of 47 zero-age main-sequence (ZAMS) stars in IC 2391 and IC 2602 using the archive data of the Anglo-Australian Telescope at the University College London Echelle Spectrograph. After subtracting the photospheric absorption component, the Mg i line is detected as an emission line for 45 ZAMS stars, the equivalent widths of which are between 0.02 Å and 0.52 Å. A total of 42 ZAMS stars show the narrower Mg i emission lines instead of the Ca ii infrared triplet emission lines, suggesting that they are formed at different depths. The ZAMS stars with smaller Rossby numbers show stronger Mg i emission lines. The Mg i emission line is not saturated even in the saturated regime of the Ca ii emission lines, i.e., Rossby number &amp;lt;10−1.1. The Mg i emission line is considered to be a good indicator of chromospheric activity, particularly for active objects.

Novel Model of an Ultra-stripped Supernova Progenitor of a Double Neutron Star

Abstract Recent discoveries of double neutron star (DNS) mergers and ultra-stripped supernovae (SNe) raise the questions of their origin and connection. We present the first 1D model of a DNS progenitor system that is calculated self-consistently until an ultra-stripped iron core collapse. We apply the MESA code starting from a post-common-envelope binary consisting of a 1.35 M ⊙ NS and a 3.20 M ⊙ zero-age main-sequence helium star and continue the modeling via Case BB Roche-lobe overflow until the infall velocity of the collapsing iron core exceeds 1000 km s−1. The exploding star has a total mass of ∼1.90 M ⊙, consisting of a ∼0.29 M ⊙ He-rich envelope embedding a CO core of ∼1.61 M ⊙ and an iron-rich core of ∼1.50 M ⊙. The resulting second-born NS has an estimated mass of ∼1.44 M ⊙, and we discuss the fate of the post-SN system, as well as the mild recycling of the first-born NS. Depending on the initial conditions, this family of systems is anticipated to reproduce the DNS mergers detected by the LIGO network.

Lithium abundance in a sample of active stars: High-resolution spectrograph observation with the 1.8 m telescope

Context. Observations of young, low-mass, main-sequence, and zero-age main-sequence stars show evidence of a correlation between lithium abundance and chromospheric activity, albeit with a very large scatter. Fast rotation stars (including T Tauri stars, RS CVn, and BY Dra systems) show the Li I doublet at 6707.8 Å in their spectra. The lithium depletion is probably related to the rotation (turbulent diffusion induced by rotation). Because the flare activity of stars increases with decreasing rotation period, a correlation can be expected between lithium abundance and chromospheric activity for active stars. Aims. The aim of this paper is to investigate the relation between lithium abundance and the Ca II H and K emission index (R′HK = LHK/Lbol) for a sample of active stars. Methods. Based on the high-resolution spectroscopic observations, we calculated lithium abundance for 14 chromospherically active late-type stars using the comparison of the measured Li I λ670.8 nm equivalent width with curve of growth calculations in non-local-thermodynamic-equilibrium conditions. We also searched the correlation between lithium abundance and the Ca II H &amp; K emission index (log R′HK) for the 14 chromospherically active late-type stars. Results. The study of the relationship between lithium abundance and the Ca II H &amp; K emission index (log R′HK) found that the activity of sample stars increases with increasing lithium abundance. Next, the lithium abundance analogs progressively decrease as the rotation periods increase (rotation becomes slow) and the large values of the log R′HK along with small values of Rossby numbers for the sample of chromospherically active stars. Conclusions. The lithium abundance (log N(Li)) versus the chromospheric activity and log N(Li) against the rotation period both indicate that the lithium abundance analogs progressively increase as the chromospheric activity index increases and/or the rotation velocity increase (rotation period becomes small) for our sample of active stars. On the other hand, the log R′HK against the Rossby number RO shows that there is a clear trend of increasing activity with increasing rotation velocity for these active stars. Considering that the lithium abundance decreases with increasing stellar age in late-type stars, we can deduce that the chromospheric activity and the rotation velocity both decrease with the increase of stellar age for our sample active stars.

Evaluating modern system dynamics software for use in astrophysical simulations

System dynamics (SD) software provides a diagrammatic way of modelling integral and differential relations in a system. In the current literature, SD software is typically applied to problems in the social sciences. However, the methods utilised in SD are not constrained to these fields, warranting novel applications in the physical sciences. Previous literature has applied SD software to time-dependent systems, but this method of modelling problems in physics has not proved popular in research, possibly due to limitations within the software available. The primary contribution of this paper is to demonstrate the application of current SD software to astrophysical phenomena in which the independent variable is not time, namely in modelling the internal structure of zero-age main-sequence stars. This demonstrates an innovative model application with the possibility for future adaptations, and extension to other astrophysical phenomena. The ease of the modelling process and validity of results for such a system is demonstrated, however a number of complications associated with current SD software were encountered. Suggestions are made as to how these may be overcome in future software iterations to make SD software more accessible to researchers in astrophysics.

The stellar system HIP 101227: is it a binary, a triple or a quadruple system?

In this paper, we present the analysis of the stellar system HIP 101227 to determine the actual number of components in the system, and their properties. We use dynamical modeling and complex spectrophotometric (involving atmospheric modeling) techniques with recent data, to determine the physical properties and orbital solution for the system, respectively, with better accuracy than past studies. Based on our analysis, we found that the system is more consistent with being a quadruple rather than a binary or a triple system as suggested by previous studies. The total mass of the system determined from our SED analysis is 3.42 ± 0.20 M ⊙, which is distributed almost equally between the four stars. The stars are found to be zero-age main sequence stars; i.e., at the last stage of pre-main sequence, with age less than 200 Myr and spectral types K0. All four stars have very similar physical characteristics, suggesting that the fragmentation process is the most likely theory for the formation and evolution of the system.

Progenitor mass constraints for the type Ib intermediate-luminosity SN 2015ap and the highly extinguished SN 2016bau

ABSTRACT Photometric and spectroscopic analyses of the intermediate-luminosity Type Ib supernova (SN) 2015ap and of the heavily reddened Type Ib SN 2016bau are discussed. Photometric properties of the two SNe, such as colour evolution, bolometric luminosity, photospheric radius, temperature, and velocity evolution, are also constrained. The ejecta mass, synthesized nickel mass, and kinetic energy of the ejecta are calculated from their light-curve analysis. We also model and compare the spectra of SN 2015ap and SN 2016bau at various stages of their evolution. The P Cygni profiles of various lines present in the spectra are used to determine the velocity evolution of the ejecta. To account for the observed photometric and spectroscopic properties of the two SNe, we have computed 12 M⊙ zero-age main-sequence (ZAMS) star models and evolved them until the onset of core-collapse using the publicly available stellar-evolution codeMESA. Synthetic explosions were produced using the public version of STELLA and another publicly available code, SNEC, utilizing the MESA models. SNEC and stella provide various observable properties such as the bolometric luminosity and velocity evolution. The parameters produced by SNEC/STELLA and our observations show close agreement with each other, thus supporting a 12 M⊙ ZAMS star as the possible progenitor for SN 2015ap, while the progenitor of SN 2016bau is slightly less massive, being close to the boundary between SN and non-SN as the final product.

Rotation-Activity Relation of Ca II and Mg I Infrared Emission Lines of Young Stars

To reveal detail of internal structure, relationship between chromospheric activity and Rossby number, N_R (= rotational period P / convective turnover time tau_c), has been extensively examined for main-sequence stars. The goal of our work is to apply same methods to pre-main-sequence (PMS) stars and identify appropriate model of tau_c for them. Yamashita et al. (2020) investigated relationship between N_R and strengths of Ca II infrared triplet (IRT; lambda 8498, 8542, 8662 A) emission lines of 60 PMS stars. Their equivalent widths are converted into emission line to stellar bolometric luminosity ratio (R'). The 54 PMS stars have N_R < 10^{-1.0} and show R' \sim 10^{-4.2} as large as maximum R' of zero-age main-sequence (ZAMS) stars. However, because all R' was saturated against N_R, it was not possible to estimate appropriate tau_c model for PMS stars. We noticed that Mg I emission lines at 8808 A is an optically thin chromospheric line, appropriate for determination of adequate tau_c for PMS stars. Using archive data of Anglo-Australian Telescope (AAT)/the University College London Echelle Spectrograph (UCLES), we investigated Mg I line of 52 ZAMS stars. After subtracting photospheric absorption component, Mg I line is detected as an emission line in 45 ZAMS stars, whose R' is between 10^{-5.9} and 10^{-4.1}. The Mg I line is not saturated yet in the saturated regime for Ca II emission lines, i.e. 10^{-1.6} < N_R < 10^{-0.8}. Therefore, adequate tau_c for PMS stars can be determined by measuring of their R' values.

Photometric analysis of three totally eclipsing W UMa stars with increasing periods: TYC 3700-1384-1, V1511 Her, and V1179 Her

ABSTRACT The first multicolour light-curve models and period studies for the totally eclipsing W UMa stars TYC 3700-1384-1, V1511 Her, and V1179 Her are presented. All three stars are A-subtype W UMa stars of spectral type F. The light-curve solutions show that TYC 3700-1384-1 has a moderately low mass ratio of q = 0.182 ± 0.001 and a degree of overcontact of $f = 49 {{\ \rm per\ cent}}$. For V1179 Her a mass ratio q = 0.153 ± 0.001 and a degree of overcontact of $f = 48 {{\ \rm per\ cent}}$ is derived. The solution for V1511 Her is inconclusive, however the mass ratio is expected to be between 0.13 &amp;lt; q &amp;lt; 0.15. The evolutionary status is compared with zero-age main sequence stars taking into account energy transfer from the primary to the secondary component. The primary component of TYC 3700-1384-1 fits well in the main-sequence, while V1179 Her is more evolved. The period study reveals for all three stars a continuously increasing period at a rate of ${{\mathrm{d}}P/{\mathrm{d}}t} = 6.1\times 10^{-7}\, \mathrm{d\, yr}^{-1}$, ${{\mathrm{d}}P/{\mathrm{d}}t} = 5.0\times 10^{-7}\, \mathrm{d\, yr}^{-1}$, and ${{\mathrm{d}}P/{\mathrm{d}}t} = 9.6\times 10^{-7}\, \mathrm{d\, yr}^{-1}$ for TYC 3700-1384-1, V1511 Her, and V1179 Her, respectively. The estimated mass transfer rates derived from these period changes are $\dot{M}=1.6 \times 10^{-7} {\, {\rm M}_\odot } \, \mathrm{yr}^{-1}$ for TYC 3700-1384-1 and $\dot{M}= 1.9 \times 10^{-7} {\, {\rm M}_\odot } \, \mathrm{yr}^{-1}$ for V1179 Her.

Measurements of the Ca ii infrared triplet emission lines of pre-main-sequence stars

Abstract We investigated the chromospheric activity of 60 pre-main-sequence (PMS) stars in four molecular clouds and five moving groups. It is considered that strong chromospheric activity is driven by the dynamo processes generated by stellar rotation. In contrast, several researchers have pointed out that the chromospheres of PMS stars are activated by mass accretion from their protoplanetary disks. In this study, the Ca ii infrared triplet (IRT) emission lines were investigated utilizing medium- and high-resolution spectroscopy. The observations were conducted with Nayuta/MALLS and Subaru/HDS. Additionally, archive data obtained by Keck/HIRES, VLT/UVES, and VLT/X-Shooter were used. The small ratios of the equivalent widths indicate that Ca ii IRT emission lines arise primarily in dense chromospheric regions. Seven PMS stars show broad emission lines. Among them, four PMS stars have more than one order of magnitude brighter emission line fluxes compared to the low-mass stars in young open clusters. The four PMS stars have a high mass accretion rate, which indicates that the broad and strong emission results from a large mass accretion. However, most PMS stars exhibit narrow emission lines. No significant correlation was found between the accretion rate and flux of the emission line. The ratios of the surface flux of the Ca ii IRT lines to the stellar bolometric luminosity, $R^{\prime }_{\rm IRT}$, of the PMS stars with narrow emission lines are as large as the largest $R^{\prime }_{\rm IRT}$ of the low-mass stars in the young open clusters. This result indicates that most PMS stars, even in the classical T Tauri star stage, have chromospheric activity similar to zero-age main-sequence stars.

The Bimodal Distribution in Exoplanet Radii: Considering Varying Core Compositions and H2 Envelope’s Sizes

Abstract Several models have been introduced in order to explain the radius distribution in exoplanet radii observed by Fulton et al. with one peak at , the other at , and the minimum at . In this paper we focus on the hypothesis that the exoplanet size distribution is caused by stellar X-ray and ultraviolet (XUV)-induced atmospheric loss. We evolve 106 synthetic exoplanets by exposing them to XUV irradiation from synthetic zero-age main-sequence stars. For each planet we set a different interior composition, which ranged from 100 wt% Fe (very dense), through to 100 wt% MgSiO3 (average density), and to 100 wt% ice (low density), with varying hydrogen envelope sizes that varied from 0 wt% (a negligible envelope) to 100 wt% (a negligible core). Our simulations were able to replicate the bimodal distribution in exoplanet radii. We argue that in order to reproduce the distribution by Fulton et al. it is mandatory for there to be a paucity of exoplanets with masses above . Furthermore, our best-fit result predicts an initial flat distribution in exoplanet occurrence for with a strong deficiency for planets with . Our results are consistent with the radius peak mostly encompassing denuded exoplanets, while the radius peak is mainly comprised of exoplanets with large hydrogen envelopes.