Stellar Luminosity Research Articles

Transition disc nature of post-AGB binary systems confirmed by mid-infrared interferometry

Context. Many properties of circumbinary discs around evolved post-asymptotic giant branch (post-AGB) binary systems are similar to those of protoplanetary discs around young stars. The deficits of near-infrared (near-IR) flux in the spectral energy distributions (SEDs) of these systems hints towards large dust-free cavities that are reminiscent of transition discs as are commonly observed around young stars. Aims. We aim to assess the size of the inner rim of six post-AGB binary systems with lack in the near-IR like this. We used resolved mid-infrared (mid-IR) high-angular resolution observations of VLTI/MATISSE and VLTI/MIDI. The inner rim of only one such system was previously resolved. We compared these inner rim sizes to five systems with available MATISSE data that were identified to host a disc starting at the dust sublimation radius. Methods. We used geometric ring models to estimate the inner rim sizes, the relative flux contributions of the star, the ring, and an over-resolved emission, the orientation of the ring, and the spectral dependences of the components. Results. We find that the inner dust rims of the targets with a lack of near-IR excess in their SEDs are ∼2.5 to 7.5 times larger than the theoretical dust sublimation radii, and inner rim sizes of the systems that do not show this deficit are similar to those of their theoretical dust sublimation radii. The physical radii of the inner rims of these transition discs around post-AGB binaries are 3–25 au, which are larger than the disc sizes inferred for transition discs around young stars with VLTI/MIDI. This is due to the higher stellar luminosities of post-AGB systems compared to young stars, implying larger dust sublimation radii and thus larger physical inner radii of the transition disc. Conclusions. With mid-IR interferometric data, we directly confirm the transition disc nature of six circumbinary discs around post-AGB binary systems. Future observational and modelling efforts are needed to progress in our understanding of the structure, origin, and evolution of these transition discs.

The Way to Detect Exoplanetary Systems and the Limitations of the Doppler Method

According to current astronomical developments, Doppler and lensing methods are the two most important methods used by astronomers to detect extrasolar planetary systems. Unlike the lensing method, which requires the use of high-precision astronomical telescopes to measure stellar luminosity variations, the Doppler method can rely on simple stellar redshift phenomena and calculations to accurately predict the existence of extraterrestrial planets, their masses, orbital radii, and other basic characteristics. Therefore, Doppler's method will be the most widely used method to measure planetary systems. Based on the derivation of Kepler's third law, the formulas for calculating the masses and orbital distances of planets are discussed and a python program for easy calculation is presented. Also, the limitations of Doppler's method and possible calculation errors are collected, organized, and explained.

Exploring the impact of IMF and binary parameter stochasticity with a binary population synthesis code

ABSTRACT Low-mass star formation regions are unlikely to fully populate their initial mass functions (IMFs), leading to a deficit of massive stars. In binary stellar populations, the full range of binary separations and mass ratios will also be underpopulated. To explore the effects of stochastic sampling in the integrated light of stellar clusters, we calculate models at a broad range of cluster masses, from 102 to 107 M⊙, using a binary stellar population synthesis code. For clusters with stellar masses less than 105 M⊙, observable quantities show substantial scatter and their mean properties reflect the expected deficit of massive stars. In common with previous work, we find that purely stochastic sampling of the IMF appears to underestimate the mass of the most massive star in known clusters. However, even with this constraint, the majority of clusters likely inject sufficient kinetic energy to clear their birth clusters of gas. For quantities that directly measure the impact of the most massive stars, such as Nion, ξion, and βUV, uncertainties due to stochastic sampling dominate over those from the IMF shape or distribution of binary parameters, while stochastic sampling has a negligible effect on the stellar continuum luminosity density.

Planetary Parameters, XUV Environments, and Mass-loss Rates for Nearby Gaseous Planets with X-Ray-detected Host Stars

We leverage Gaia DR2 parallactic distances to deliver new or revised estimates of planetary parameters and X-ray irradiation for a distance-limited (≲100 pc) sample of 27 gaseous planets (from super-Earths to hot Jupiters) with publicly available Chandra and/or XMM observations, for which we carry out a homogeneous data reduction. For 20 planets with X-ray-detected host stars we make use of the photoionization hydrodynamics code ATES to derive updated atmospheric mass outflow rates. The newly derived masses/radii are not consistent with the exoplanet.eu values for five systems: HD 149026b and WASP-38, for mass, and Au Mic b, HAT-P-20, and HAT-P-2 for radii. Notably, the lower mass implies a (Saturn-like) density of 0.86 ± 0.09 g cm−3 for HD 149026b. This independent estimate is consistent with the lowest values reported in the literature. Separately, we report on the X-ray detection of GJ 9827, HD 219134, and LHS 1140 for the first time. The inferred stellar X-ray luminosity of LHS 1140 ( erg s−1) implies that LHS 1140 b is the least irradiated transiting super-Earth known to orbit within the habitable zone of a nearby M dwarf.

Debris disk color with the Hubble Space Telescope

Context. Multiwavelength scattered light imaging of debris disks may inform dust properties including typical size and mineral composition. Existing studies have investigated a small set of individual systems across a variety of imaging instruments and filters, calling for uniform comparison studies to systematically investigate dust properties. Aims. We obtain the surface brightness of dust particles in debris disks by post-processing coronagraphic imaging observations, and compare the multiwavelength reflectance of dust. For a sample of resolved debris disks, we perform a systematic analysis on the reflectance properties of their birth rings. Methods. We reduced the visible and near-infrared images of 23 debris disk systems hosted by A through M stars using two coron-agraphs on board the Hubble Space Telescope: the STIS instrument observations centered at 0.58 µm, and the NICMOS instrument at 1.12 µm or 1.60 µm. For proper recovery of debris disks, we used classical reference differential imaging for STIS, and adopted non-negative matrix factorization with forward modeling for NICMOS. By dividing disk signals by stellar signals to take into account intrinsic stellar color effects, we systematically obtained and compared the reflectance of debris birth rings at ≈90º scattering angle. Results. Debris birth rings typically exhibit a blue color at ≈90º scattering angle. As the stellar luminosity increases, the color tends to be more neutral. A likely L-shaped color–albedo distribution indicates a clustering of scatterer properties. Conclusions. The observed color trend correlates with the expected blow-out size of dust particles. The color-albedo clustering likely suggests different populations of dust in these systems. More detailed radiative transfer models with realistic dust morphology will contribute to explaining the observed color and color–albedo distribution of debris systems.

The energetic particle environment of a GJ 436 b-like planet

ABSTRACTA key first step to constrain the impact of energetic particles in exoplanet atmospheres is to detect the chemical signature of ionization due to stellar energetic particles and Galactic cosmic rays. We focus on GJ 436, a well-studied M dwarf with a warm Neptune-like exoplanet. We demonstrate how the maximum stellar energetic particle momentum can be estimated from the stellar X-ray luminosity. We model energetic particle transport through the atmosphere of a hypothetical exoplanet at orbital distances between $a=0.01\text{ and }0.2\,$au from GJ 436, including GJ 436 b’s orbital distance (0.028 au). For these distances, we find that, at the top of atmosphere, stellar energetic particles ionize molecular hydrogen at a rate of $\zeta _{\rm StEP,H_2} \sim 4\times 10^{-10}\text{ to }2\times 10^{-13}\, \mathrm{s^{-1}}$. In comparison, Galactic cosmic rays alone lead to $\zeta _{\rm GCR, H_2}\sim 2\times 10^{-20}\!-\!10^{-18} \, \mathrm{s^{-1}}$. At 10 au, we find that ionization due to Galactic cosmic rays equals that of stellar energetic particles: $\zeta _{\rm GCR,H_2} = \zeta _{\rm StEP,H_2} \sim 7\times 10^{-18}\, \rm {s^{-1}}$ for the top-of-atmosphere ionization rate. At GJ 436 b’s orbital distance, the maximum ion-pair production rate due to stellar energetic particles occurs at pressure $P\sim 10^{-3}\,$bar, while Galactic cosmic rays dominate for $P\gt 10^2\,$bar. These high pressures are similar to what is expected for a post-impact early Earth atmosphere. The results presented here will be used to quantify the chemical signatures of energetic particles in warm Neptune-like atmospheres.

Planetary geodynamics and age constraints on circumstellar habitable zones around main sequence stars

AbstractPlanetary geodynamics may have an important influence over planetary habitability and the boundaries of the circumstellar habitable zone (CHZ) in space and time. To investigate this we use a minimal parameterized model of the co-evolution of the geosphere and atmosphere of Earth-like planets around F, G, K and M main sequence stars. We found the CHZ for the present Solar System located between 0.92 and 1.09 au for a 1.0 M$_{\oplus }$Earth-like planet, extendible to 1.36 au for a 4.0 M$_{\oplus }$planet. In the literature, the CHZ varies considerably in width and border location, but the outer edges tend to be more spread out than the inner edges, showing a higher difficulty in determining the outer edge. Planetary mass has a considerable effect on planetary geodynamics, with low-mass planets cooling down faster and being less capable of maintaining a rich carbon dioxide atmosphere for several billions of years. Age plays a particularly important role in the width of the CHZ as the CHZ contracts in both directions: from the inner edge (as stellar luminosity increases with time), and from the outer edge (as planetary heat flux and seafloor spreading rate decrease with time). This strongly affects long-lived habitability as the 5 Gyr continuous CHZ may be very narrow or even non-existent for low-mass planets (<0.5 M$_{\oplus }$) and fast-evolving high-mass stars (>1.1 M$_{\odot }$). Because of this, the mean age of habitable terrestrial planets in our Galaxy today may be younger than Earth's age. Our results suggest that the best targets for future surveys of biosphere signatures may be planets between 0.5 and 4.0 M$_{\oplus }$, in systems younger than the Solar System. These planets may present the widest and long-lived CHZ.

Interstellar Objects from Broken Dyson Spheres

Without extensive maintenance, Dyson spheres inevitably disintegrate by asteroid impacts over billions of years. The resulting fragments would appear as anomalous interstellar objects, potentially sharing the unusual shape and motion of 1I/‘Oumuamua or the unusual material strength of the first two interstellar meteors, IM1 and IM2. If the Dyson sphere's tiles are light sails, the number of fragments could exceed that of interstellar asteroids because of their reduced escape speed from the star and the increase in stellar luminosity during the red giant phase.

An empirical relation to estimate host galaxy stellar light from AGN spectra

ABSTRACT Measurement of black hole mass for low-z (z≤ 0.8) Active Galactic Nuclei (AGNs) is difficult due to the strong contribution from host galaxy stellar light necessitating detailed spectral decomposition to estimate the AGN luminosity. Here, we present an empirical relation to estimate host galaxy stellar luminosity from the optical spectra of AGNs at z ≤ 0.8. The spectral data were selected from the fourteenth data release of the Sloan Digital Sky Survey (SDSS-DR14) quasar catalogue having a signal-to-noise ratio at 5100 Å (SNR5100) >10 containing 11 415 quasars. The median total luminosity (log (Ltotal/[erg s−1])), stellar luminosity (log (Lstar/[erg s−1])), and AGN continuum luminosity ((log Lcont/[erg s−1])) in our sample are 44.52, 44.06, and 44.30, respectively. We fit the AGN power-law continuum, host galaxy, and iron blend contribution, simultaneously over the entire available spectrum. We found the host galaxy fraction to anticorrelate with continuum luminosity and can be well-represented by a polynomial function, which can be used to correct the stellar light contribution from AGN spectra. We also found anticorrelation between host galaxy fraction and iron strength, Eddington ratio, and redshift. The empirical relation gives comparable results of host-fraction with the image decomposition method.

Determining Dust Properties in Protoplanetary Disks: SED-derived Masses and Settling with ALMA

We present spectral energy distribution (SED) modeling of 338 disks around T Tauri stars from 11 star-forming regions, ranging from ∼0.5 to 10 Myr old. The disk masses we infer from our SED models are typically greater than those reported from (sub)millimeter surveys by a factor of 1.5–5, with the discrepancy being generally higher for the more massive disks. Masses derived from (sub)millimeter fluxes rely on the assumption that the disks are optically thin at all millimeter wavelengths, which may cause the disk masses to be underestimated since the observed flux is not sensitive to the whole mass in the disk; SED models do not make this assumption and thus yield higher masses. Disks with more absorbing material should be optically thicker at a given wavelength, which could lead to a larger discrepancy for disks around massive stars when the disk temperature is scaled by the stellar luminosity. We also compare the disk masses and degree of dust settling across the different star-forming regions and find that disks in younger regions are more massive than disks in older regions, but with a similar degree of dust settling. Together, these results offer potential partial solutions to the “missing” mass problem: disks around T Tauri stars may indeed have enough material to form planetary systems, though previous studies have underestimated the mass by assuming the disks to be optically thin; these planetary systems may also form earlier than previously theorized since significant dust evolution (i.e., settling) is already apparent in young disks.

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.

Potential effects of stellar winds on gas dynamics in debris disks leading to observable belt winds

Context. Gas has been successfully detected in many extrasolar systems around mature stars aged between 10 Myr and ∼1 Gyr that include planetesimal belts. Gas in these mature disks is thought to be released from planetesimals and has been modeled using a viscous disk approach where the gas expands inwards and outwards from the belt where it is produced. Therefore, the gas has so far been assumed to make up the circumstellar disk orbiting the star; however, at low densities, this may not be an adequate assumption, as the gas could be blown out by the stellar wind instead. Aims. In this paper, we aim to explore the timeframe in which a gas disk transitions to such a gas wind and whether this information can be used to determine the stellar wind properties around main sequence stars, which are otherwise difficult to obtain. Methods. We developed an analytical model for A to M stars that can follow the evolution of gas outflows and target the moment of transition between a disk or a wind in order to make a comparison with current observations. The crucial criterion here is the gas density for which gas particles are no longer protected from the impact of stellar wind protons at high velocities and on radial trajectories. Results. We find that: (1) belts with a radial width, ΔR, with gas densities <7 (ΔR/50 au)−1 cm−3, would create a wind rather than a disk, which would explain the recent outflowing gas detection in NO Lup; (2) the properties of this belt wind can be used to measure stellar wind properties such as their densities and velocities; (3) very early-type stars can also form gas winds due to the star’s radiation pressure, instead of a stellar wind; (4) debris disks with low fractional luminosities, f, are more likely to create gas winds, which could be observed with current facilities. Conclusions. Systems containing low gas masses, such as Fomalhaut or TWA 7, or more generally, debris disks with fractional luminosities of f ≲ 10−5(L⋆/L⊙)−0.37 or stellar luminosity ≳20 L⊙ (A0V or earlier) are more likely to create gas outflows (or belt winds) than gas disks. Gas that is observed to be outflowing at high velocity in the young system NO Lup could be an example of such belt winds. Future observing predictions in this wind region should account for the stellar wind in the attempt to detect the gas. The detection of these gas winds is possible with ALMA (CO and CO+ could serve as good wind tracers). This would allow us to constrain the stellar wind properties of main-sequence stars, as these properties are otherwise difficult to measure, since, for example, there are no successful measures around A stars at present.

Redshift–luminosity characterization of active galactic nucleus galaxies having obscuring dusty material using the spectral energy distribution from the X-ray to far-infrared

ABSTRACT The spectral energy distribution (SED) from the X-ray to far-IR is used to characterize active galactic nuclei (AGNs) having obscuring dusty material. A sample of 73 AGNs, with redshifts of z < 2.5, is subdivided based on the optical spectra into subgroups of type 1 and 2 AGNs and no-AGN. The x-cigale code is used to produce the SEDs of these galaxies, fitting them with their observed flux densities. Accordingly, this code provides estimates of the physical properties, including the stellar mass, dust-to-gas mass ratio, far-UV attenuation, star-formation rate, and luminosity. All these properties and luminosity components, namely stellar, AGN, X-ray, and dust luminosities, are characterized versus the redshift. With the caveat that the sample is flux-limited and thus is affected by Malmquist bias, the results show that these properties increase as a function of redshift with various variation rates for each galaxy group, except for the far-UV attenuation of type 1 AGNs, which shows a slight reduction with redshift. The stellar, AGN, and X-ray luminosities show tight correlations with the intrinsic AGN luminosity, which is, in principle, attributed to the intrinsic AGN accretion power acting as a fuelling source. This power has an impact on most other correlations versus the redshift when the intrinsic AGN luminosity increases with redshift. Owing to their different emission sources, these luminosities make different fractional contributions to the total luminosity. The stellar luminosity, where the dust luminosity is the most effective, has the highest contribution of all galaxy groups, while the AGN luminosity has the lowest.

Star-forming early-type galaxies and quiescent late-type galaxies in the local Universe

Aims. The general consensus is that late-type galaxies undergo intense star-formation, activity while early-type galaxies are mostly inactive. We question this general rule and investigate the existence of star-forming early-type and quiescent late-type galaxies in the local Universe. By computing the physical properties of these galaxies and by using information on their structural properties as well as the density of their local environment, we seek to understand the differences from their ‘typical’ counterparts. Methods. We made use of the multi-wavelength photometric data (from the ultraviolet to the sub-millimetre), for 2209 morphologically classified galaxies in the Galaxy And Mass Assembly survey. Furthermore, we separated the galaxies into subsets of star-forming and quiescent based on their dominant ionising process, making use of established criteria based on the WHα width and the [NII/Hα] ratio. Taking advantage of the spectral energy distribution fitting code CIGALE, we derived galaxy properties, such as the stellar mass, dust mass, and star-formation rate, and we also estimated the unattenuated and the dust-absorbed stellar emission, for both the young (≤200 Myr) and old (> 200 Myr) stellar populations. Results. We find that about 47% of E/S0 galaxies in our sample show ongoing star-formation activity and 8% of late-type galaxies are quiescent. The star-forming elliptical galaxies, together with the little blue spheroids, constitute a population that follows the star-forming main sequence of spiral galaxies very well. The fraction of the luminosity originating from young stars in the star-forming early-type galaxies is quite substantial (∼25%) and similar to that of the star-forming late-type galaxies. The stellar luminosity absorbed by the dust (and used to heat the dust grains) is highest in star-forming E/S0 galaxies (an average of 35%) followed by star-forming Sa-Scd galaxies (27%) with this fraction becoming significantly smaller for their quiescent analogues (6% and 16%, for E/S0 and Sa-Scd, respectively). Star-forming and quiescent E/S0 galaxies donate quite different fractions of their young stellar luminosities to heat up the dust grains (74% and 36%, respectively), while these fractions are very similar for star-forming and quiescent Sa-Scd galaxies (59% and 60%, respectively). Investigating possible differences between star-forming and quiescent galaxies, we find that the intrinsic (unattenuated) shape of the SED of the star-forming galaxies is, on average, very similar for all morphological types. Concerning their structural parameters, quiescent galaxies tend to show larger values of the r-band Sérsic index and larger effective radii (compared to star-forming galaxies). Finally, we find that star-forming galaxies preferably reside in lower density environments compared to the quiescent ones, which exhibit a higher percentage of sources being members of groups.

A Red Giants’ Toy Story

In spite of the spectacular progress accomplished by stellar evolution theory, some simple questions remain unanswered. One of these questions is “Why do stars become red giants?”. Here we present a relatively simple analytical answer to this question. We validate our analysis by constructing a quantitative toy model of a red giant and comparing its predictions to full stellar evolutionary models. We find that the envelope forces the value of at, and above, the burning shell into a very narrow range of possible values. Together with the fact that the stellar material at the burning shell both provides and transports most of the stellar luminosity, this leads to tight relations between the thermodynamic variables at the burning shell and the mass and radius of the core—T s (M c , R s ), P s (M c , R s ), and ρ s (M c , R s ). When complemented by typical mass–radius relations of the helium cores, this implies that for all stellar masses the evolution of the core dictates the values of T s , P s , and ρ s . We show that for all stellar masses evolution leads to an increase in the pressure and density contrasts between the shell and the core, forcing a huge expansion of the layers on top of the burning shell. Besides explaining why stars become red giants our analysis also offers a mathematical demonstration of the so-called shell homology relations, and provides simple quantitative answers to some properties of low-mass red giants.

Improved stellar limits on a light CP-even scalar

We derive improved stellar luminosity limits on a generic light CP-even scalar field S mixing with the Standard Model (SM) Higgs boson from the supernova SN1987A, the Sun, red giants (RGs) and white dwarfs (WDs). For the first time, we include the geometric effects for the decay and absorption of S particles in the stellar interior. For SN1987A and the Sun, we also take into account the detailed stellar profiles. We find that a broad range of the scalar mass and mixing angle can be excluded by our updated astrophysical constraints. For instance, SN1987A excludes 1.0 × 10-7 ≲ sinθ ≲ 3.0 × 10-5 and scalar mass up to 219 MeV, which covers the cosmological blind spot with a high reheating temperature. The updated solar limit excludes the mixing angle in the range of 1.5 × 10-12 < sinθ < 1, with scalar mass up to 45 keV. The RG and WD limits are updated to 5.3 × 10-13 < sinθ < 0.39 and 2.8 × 10-18 < sinθ < 1.8 × 10-4, with scalar mass up to 392 keV and 290 keV, respectively.

A Maximum Likelihood Calibration of the Tip of the Red Giant Branch Luminosity from High Latitude Field Giants Using Gaia Early Data Release 3 Parallaxes

The calibration of the tip of the red giant branch (TRGB) in the I band has a direct role in determinations of the Hubble constant, a subject of recent interest due to the discrepancy between direct and indirect estimates of its value. We present a maximum likelihood method designed to obtain an independent calibration of the brightness of TRGB using Gaia parallaxes from the Early Data Release 3 (EDR3) of Milky Way field giants at high Galactic latitude. We adopt simple parameterizations for the Milky Way stellar luminosity function and density law and optimize the likelihood of the observed sample as a function of those parameters. Using parameters to partially constrain the luminosity function from other galaxies similar to the Milky Way for which high-quality TRGB data are available, we find values of the TRGB magnitude of (stat) ± 0.09 (sys) mag, where the systematic uncertainty covers the range of shape parameters found in our Milky Way sample and in reference galaxies. While APASS Data Release 9 all-sky photometry is insufficient to provide a reliable constraint on the shape of the Milky Way luminosity function, we estimate that the photometry from Gaia Data Release 3 (mid-2022) will allow better constraints on the shape, and lower statistical uncertainties on the tip by a factor of 3. With the expected releases of improved parallax measurements from Gaia, the method of calibrating the TRGB luminosity from field giants is expected to reach ∼0.01 mag uncertainty, which is an important step toward a precise TRGB-based determination of the Hubble constant.

Ionizing feedback effects on star formation in globular clusters with multiple stellar populations

ABSTRACT Using 3D radiation-hydrodynamical simulations, we study the effects of ionizing radiation on the formation of second-generation (SG) stars in globular clusters (GCs) with multiple stellar populations. In particular, we focus on massive ($10^7 \, \mathrm{M}_{\odot }$) and young (40-Myr old) GCs. We consider stellar winds from asymptotic giant branch (AGB) stars, ram pressure, gas accretion on to the cluster, and photo-ionization feedback of binary stars. We find that the stellar luminosity is strong enough to warm and ionize the intracluster medium, but it does not lead to a significant gas expulsion. The cluster can thus retain the ejecta of AGB stars and the accreted pristine gas. In addition, efficient cooling occurs in the central region of the cluster within $50\, \mathrm{Myr}$ from the formation of first generation stars, leading to the formation of SG stars. Our results indicate that the inclusion of photo-ionization does not suppress SG formation, but rather delays it by about $\sim 10\, \mathrm{Myr}$. The time delay depends on the density of the pristine gas, so that a denser medium exhibits a shorter delay in star formation. Moreover, photo-ionization leads to a modest decrease in the total SG mass, compared to a model without it.

A Silent Revolution in Fundamental Astrophysics

Arbitrariness in the zeropoint of bolometric corrections is a paradigm that is nearly a century old and leads to two more paradigms. “Bolometric corrections must always be negative,” and “the bolometric magnitude of a star ought to be brighter than its V magnitude”. Both were considered valid before the IAU 2015 General Assembly Resolution B2, a revolutionary document that supersedes all three aforementioned paradigms. The purpose of this article is to initiate new insight into and a new understanding of the fundamental astrophysics and present new capabilities to obtain standard and more accurate stellar luminosities and gain more from accurate observations in the era after Gaia. The accuracy gained will aid in advancing stellar structure and evolution theories and also Galactic and extragalactic research, observational cosmology, and searches for dark matter and dark energy.

Identification and Parameter Determination of F-type Herbig Stars from LAMOST DR8

We identify 20 F-type Herbig stars and provide a list of 22 pre-main-sequence candidates from LAMOST DR8. The effective temperature, distance, extinction, stellar luminosity, mass, age, and radius are derived for each Herbig star based on optical spectra, photometry, Gaia EDR3 parallaxes, and pre-main-sequence evolutionary tracks. According to spectral energy distribution, 19 F-type Herbig stars belong to Class II YSOs, and one belongs to the flat-spectrum class. Four have Spitzer IRS spectra, of which three show extremely weak polycyclic aromatic hydrocarbon emissions, and three with both amorphous and crystalline silicate emissions share similar parameters and are at the same evolutionary stage. We detect a solar-nearby outbursting EXor Herbig star J034344.48+314309.3, a possible precursor of a Herbig Ae star. Intense emission lines of H i, He i, O i, Na i, and Ca ii originating from the rapid accretion during the outbursts are detected in its optical spectra, and silicate emission features are detected in its infrared spectrum. We also conduct a statistical analysis of the disk properties of all known Herbig stars using the defined infrared spectral indices ( and ). The proportion of Herbig stars with moderate infrared excesses decreases as effective temperature increases. The majority of the precursors (F, G, or K type) have moderate infrared excesses. Hotter Herbig stars tend to have a larger proportion with large infrared excesses. The trends may be due to the fact that hotter stars have larger areas of re-emitting dust, although there is some scatter due to the particularities of each disk.