Low-mass stellar and substellar companions are indispensable objects for verifying evolutionary models that transition from low-mass stars to planets. Their formation is likely also affected by the surrounding stellar environment. The Fornax-Horologium (FH) association is a recently classified young association in the solar neighbourhood with a dissolving open cluster in its centre. It has not been searched widely for substellar companions and exoplanets with direct imaging. We search for companions of stars in the FH and investigate the formation and evolution of companions during the expansion and dissolution of a star cluster. We conduct a direct-imaging survey of 49 stars in FH with VLT/SPHERE. We present HD 24121B, a companion at the hydrogen-burning limit that orbits star HD 24121 in the core cluster of the FH. The companion is located at a projected angular separation of 2.082 $\pm$ 0.004 arcsec from the central star and has contrasts of $Δ$H2 = 5.07 $\pm$ 0.05 mag and $Δ$H3= 4.98 $\pm$ 0.05 mag. We estimate a photometric mass of 74.9 $\pm$ 7.5 $M_J$ for HD 24121B, which places it at the boundary of brown dwarfs and low-mass stars. It is a new benchmark object for evolutiona
In stars that lie on the main sequence in the Hertzsprung-Russel diagram, like our sun, hydrogen is fused to helium in a number of nuclear reaction chains and series, such as the proton-proton chain and the carbon-nitrogen-oxygen cycles. Precisely determined thermonuclear rates of these reactions lie at the foundation of the standard solar model. This review, the third decadal evaluation of the nuclear physics of hydrogen-burning stars, is motivated by the great advances made in recent years by solar neutrino observatories, putting experimental knowledge of the proton-proton chain neutrino fluxes in the few-percent precision range. The basis of the review is a one-week community meeting held in July 2022 in Berkeley, California, and many subsequent digital meetings and exchanges. The relevant reactions of solar and stellar hydrogen burning are reviewed here, from both theoretical and experimental perspectives. Recommendations for the state of the art of the astrophysical S-factor and its uncertainty are formulated for each of them. Several other topics of paramount importance for the solar model are reviewed, as well: recent and future neutrino experiments, electron screening, radi
Globular clusters contain vast repositories of metal-poor stars that represent some of the oldest stellar generations in the Universe. The archaeological footprint of early Galactic evolution may be retained in the measurable properties of globular clusters, such as their ages, mass functions and chemical abundances. Until recently, all photometric studies of globular clusters were restricted to stellar members. Now, the sensitivity of JWST can extend this analysis to the substellar regime. If detected in sufficient numbers, brown dwarf members can provide tight constraints on the properties of their parent population. We present SANDee - a new grid of stellar models that accurately represent the color-magnitude diagrams of globular clusters across the hydrogen-burning limit at a wide range of metallicities. Using JWST NIRCam photometry and the new models, we identify three brown dwarfs in the globular cluster NGC 6397 with effective temperatures of 1300-1800 K, confirmed by both proper motion and model fitting. We use the observed luminosities of discovered brown dwarfs to obtain the first age estimate of a globular cluster from its substellar cooling sequence: 13.4 +/- 3.3 Gyr. W
The TESS mission detected a companion orbiting TIC 71268730, categorized it as a planet candidate, and designated the system TOI-5375. Our follow-up analysis using radial velocity data from the Habitable-zone Planet Finder (HPF), photometric data from Red Buttes Observatory (RBO), and speckle imaging with NN-EXPLORE Exoplanet Stellar Speckle Imager (NESSI) determined that the companion is a very low mass star (VLMS) near the hydrogen-burning mass limit with a mass of 0.080$\pm{0.002} M_{\Sun}$ ($83.81\pm{2.10} M_{J}$), a radius of 0.1114$^{+0.0048}_{-0.0050} R_{\Sun}$ (1.0841$^{0.0467}_{0.0487} R_{J}$), and brightness temperature of $2600\pm{70}$ K. This object orbits with a period of 1.721553$\pm{0.000001}$ days around an early M dwarf star ($0.62\pm{0.016}M_{\Sun}$). TESS photometry shows regular variations in the host star's TESS light curve, which we interpreted as activity-induced variation of $\sim$2\%, and used this variability to measure the host star's stellar rotation period of 1.9716$^{+0.0080}_{-0.0083}$ days. The TOI-5375 system provides tight constraints on stellar models of low-mass stars at the hydrogen-burning limit and adds to the population in this important regi
Theory and simulations suggest that it is possible to form low-mass hydrogen-burning stars, brown dwarfs and planetary-mass objects via disc fragmentation. As disc fragmentation results in the formation of several bodies at comparable distances to the host star, their orbits are generally unstable. Here, we study the dynamical evolution of these objects. We set up the initial conditions based on the outcomes of the SPH simulations of Stamatellos & Whitworth, and for comparison we also study the evolution of systems resulting from lower-mass fragmenting discs. We refer to these two sets of simulations as set 1 and set 2. At 10 Myr, approximately half of the host stars have one companion left, and approximately 22% (set 1) to 9.8% (set 2) of the host stars are single. Systems with multiple secondaries in relatively stable configurations are common (about 30% and 44%, respectively). The majority of the companions are ejected within 1 Myr with velocities mostly below 5 km/s, with some runaway escapers with velocities over 30 km/s. About 6% (set 1) and 2% (set 2) of the companions pair up into very low-mass binary systems. The majority of these pairs escape as very low-mass binaries
Model predictions of the amount of the radioisotope 26Al produced in hydrogen-burning environments require reliable estimates of the thermonuclear rates for the 26gAl(p,γ)27Si and 26mAl(p,γ)27Si reactions. These rates depend upon the spectroscopic properties of states in 27Si within about 1 MeV of the 26gAl+p threshold (Sp = 7463 keV). We have studied the 28Si(3He,α)27Si reaction at 25 MeV using a high-resolution quadrupole-dipole-dipole-dipole magnetic spectrograph. For the first time with a transfer reaction, we have constrained Jπ values for states in 27Si over Ex = 7.0 - 8.1 MeV through angular distribution measurements. Aside from a few important cases, we generally confirm the energies and spin-parity assignments reported in a recent γ-ray spectroscopy study. The magnitudes of neutron spectroscopic factors determined from shell-model calculations are in reasonable agreement with our experimental values extracted using this reaction.
It is unknown whether or not low-mass stars can form at low metallicity. While theoretical simulations of Population III (Pop III) star formation show that protostellar disks can fragment, it is impossible for those simulations to discern if those fragments survive as low-mass stars. We report the discovery of a low-mass star on a circular orbit with orbital period P = 34.757 +/- 0.010 days in the ultra metal-poor (UMP) single-lined spectroscopic binary system 2MASS J18082002--5104378. The secondary star 2MASS J18082002--5104378 B has a mass M_2 = 0.14_{-0.01}^{+0.06} M_Sun, placing it near the hydrogen-burning limit for its composition. The 2MASS J18082002--5104378 system is on a thin disk orbit as well, making it the most metal-poor thin disk star system by a considerable margin. The discovery of 2MASS J18082002--5104378 B confirms the existence of low-mass UMP stars and its short orbital period shows that fragmentation in metal-poor protostellar disks can lead to the formation and survival of low-mass stars. We use scaling relations for the typical fragment mass and migration time along with published models of protostellar disks around both UMP and primordial composition stars
We present new models for low-mass stars down to the hydrogen-burning limit that consistently couple atmosphere and interior structures, thereby superseding the widely used BCAH98 models. The new models include updated molecular linelists and solar abundances, as well as atmospheric convection parameters calibrated on 2D/3D radiative hydrodynamics simulations. Comparison of these models with observations in various colour-magnitude diagrams for various ages shows significant improvement over previous generations of models. The new models can solve flaws that are present in the previous ones, such as the prediction of optical colours that are too blue compared to M dwarf observations. They can also reproduce the four components of the young quadruple system LkCa 3 in a colour-magnitude diagram with one single isochrone, in contrast to any presently existing model. In this paper we also highlight the need for consistency when comparing models and observations, with the necessity of using evolutionary models and colours based on the same atmospheric structures.
We suggest that a high proportion of brown dwarfs are formed by gravitational fragmentation of massive extended discs around Sun-like stars. Such discs should arise frequently, but should be observed infrequently, precisely because they fragment rapidly. By performing an ensemble of radiation-hydrodynamic simulations, we show that such discs fragment within a few thousand years, and produce mainlybrown dwarf (BDs) stars, but also planetary mass (PM) stars and very low-mass hydrogen-burning (HB) stars. Most of the the PM stars and BDs are ejected by mutual interactions. We analyse the statistical properties of these stars, and compare them with observations. After a few hundred thousand years the Sun-like primary is typically left with a close low-mass HB companion, and two much wider companions: a low-mass HB star and a BD star, or a BD-BD binary. There is a BD desert extending out to at least ~100 AU; this is because BDs tend to be formed further out than low-mass HB stars, and then they tend to be scattered even further out, or even into the field. BDs form with discs of a few Mj and radii of a few tens of AU, and they are more likely to retain these discs if they remain bound to
We report the discovery of five transiting companions near the hydrogen-burning mass limit in close orbits around main sequence stars originally identified by the Transiting Exoplanet Survey Satellite (TESS) as TESS Objects of Interest (TOIs): TOI-148, TOI-587, TOI-681, TOI-746, and TOI-1213. Using TESS and ground-based photometry as well as radial velocities from the CORALIE, CHIRON, TRES, and FEROS spectrographs, we found the companions have orbital periods between 4.8 and 27.2 days, masses between 77 and 98 $\mathrm{M_{Jup}}$, and radii between 0.81 and 1.66 $\mathrm{R_{Jup}}$. These targets have masses near the uncertain lower limit of hydrogen core fusion ($\sim$73-96 $\mathrm{M_{Jup}}$), which separates brown dwarfs and low-mass stars. We constrained young ages for TOI-587 (0.2 $\pm$ 0.1 Gyr) and TOI-681 (0.17 $\pm$ 0.03 Gyr) and found them to have relatively larger radii compared to other transiting companions of a similar mass. Conversely we estimated older ages for TOI-148 and TOI-746 and found them to have relatively smaller companion radii. With an effective temperature of 9800 $\pm$ 200 K, TOI-587 is the hottest known main-sequence star to host a transiting brown dwarf
JCB has invested heavily in hydrogen to power its diggers and other machinery
A proper-motion separation of M4 members from field stars, using deep HST observations separated by a time base-line of 5 years, allows us to study a pure sample of cluster main-sequence stars almost to the minimum mass for hydrogen burning. High-precision photometry shows how badly current theoretical models fail to reproduce the color-magnitude diagram of low-mass stars of moderate metallicity ([M/H] ~ -1). This inability of theory to reproduce the luminosity-radius relation casts doubt on the theoretical mass-luminosity relation, which is needed to convert the observed luminosity function (LF) into a mass function (MF), as well as to convert our locally determined LF into a global MF. To the extent that we trust theoretical M-L relations for such transformations, we obtain a flat MF from the LF, and some indication that theoretical masses might be too low at a given luminosity, near the H-burning limit.
Brown dwarfs that gain mass through binary interactions may be pushed above the boundary that divides brown dwarfs from low-mass stars: the hydrogen burning limit (HBL). Some of these objects will make their way to the main sequence and may eventually be indistinguishable from ordinary low-mass stars, while others will remain brown dwarf-like, unable to burn hydrogen at a high enough rate to power their surface luminosity. We study the evolution of both types of object to provide a taxonomy and testable observational predictions for these objects depending on their evolutionary path. Using MESA simulations, we find that a subset of the objects that will eventually become stars experience an extended luminosity plateau, where their surface luminosity remains nearly constant on 100 Myr - Gyr timescales. We find that the plateau timescale is set by the amount of energy required to re-heat the cores of these objects to a level sufficient to sustain convection. The timescales required for the cores of these objects to "unfreeze" and arrive at the main sequence is long enough that surveys may be able to find objects in this evolutionary stage. These objects, along with those that never r
Millinovae are a new class of transient supersoft X-ray sources with no clear signature of mass ejection. They show similar triangle shapes of $V/I$ band light curves with thousand times fainter peaks than typical classical novae. Maccarone et al. regarded the prototype millinova, ASASSN-16oh, as a dwarf nova and interpreted the supersoft X-rays to originate from an accretion belt on a white dwarf (WD). Kato et al. proposed a nova model induced by a high-rate mass-accretion during a dwarf nova outburst; the X-rays originate from the photosphere of a hydrogen-burning hot WD whereas the $V/I$ band photons are from the irradiated accretion disk. Because each peak brightness differs largely from millinova to millinova, we suspect that not all the millinova candidates host a hydrogen burning WD. Based on the light curve analysis of the classical nova KT Eri that has a bright disk, we find that the disk is more than two magnitudes brighter when the disk is irradiated by the hydrogen burning WD than when not irradiated. We present the demarcation criterion for hydrogen burning to be $I_{\rm q} - I_{\rm max} > 2.2$, where $I_q$ and $I_{\rm max}$ are the $I$ magnitudes in quiescence and
We propose that the drastic photometric and spectroscopic changes affecting the symbiotic star V694 Mon since 2018, are due to its transition from the accreting-only state to steady hydrogen-burning on the surface of the white dwarf, closely mimicking the pattern followed by V4368 Sgr. The phase of peak optical brightness and weakest emission lines has probably been reached in early 2024. The high-velocity absorptions powered by jet-ejection and the wild flickering, which dominated the century-long quiescence, should not reappear as long has nuclear burning will hold (time-scale of decades). The 3500 Lsun burning luminosity suggests a mass of 0.60 Msun for the WD.
Breathing pulses are mixing episodes that could develop during the core-helium burning phase of low- and intermediate-mass stars. The occurrence of breathing pulses is expected to bear consequences on the formation and evolution of white dwarfs, particularly on the core chemical structure, which can be probed by asteroseismology. We aim to explore the consequences of breathing pulses on the chemical profiles and pulsational properties of variable white-dwarf stars with hydrogen-rich envelopes, known as ZZ Ceti stars. We compute stellar models with masses of $1.0 M_{\odot}$ and $2.5 M_{\odot}$ in the zero-age main sequence, and evolve them through the core-helium burning phase to the thermal pulses on the asymptotic giant branch, and finally to advanced stages of white-dwarf cooling. We compare the chemical structure of the core of white dwarfs whose progenitors have experienced breathing pulses during the core-helium burning phase with the case in which breathing pulses have not occurred. We find that, when breathing pulses occur, the white-dwarf cores are larger and the central abundances of oxygen are higher than for the case in which the breathing pulses are suppressed, in line
We present a study of weak, thermonuclear X-ray bursts from the accreting millisecond X-ray pulsar SAX J1808.4-3658. We focus on a burst observed with the Neutron Star Interior Composition Explorer on 2019 August 9, and describe a similar burst observed with the Rossi X-ray Timing Explorer in 2005 June. These bursts occurred soon after outburst onset, $2.9$ and $1.1$ days, after the first indications of fresh accretion. We measure peak burst bolometric fluxes of $6.98 \pm 0.50 \times 10^{-9}$ and $1.54 \pm 0.10 \times 10^{-8}$ erg cm$^{-2}$ s$^{-1}$, respectively, which are factors of $\approx 30$ and $15$ less than the peak flux of the brightest, helium-powered bursts observed from this source. From spectral modeling we estimate accretion rates and accreted columns at the time of each burst. For the 2019 burst we estimate an accretion rate of $\dot M \approx 1.4-1.6 \times 10^{-10}$ $M_{\odot}$ yr$^{-1}$, and a column in the range $3.9-5.1 \times 10^7$ g cm$^{-2}$. For the 2005 event the accretion rate was similar, but the accreted column was half of that estimated for the 2019 burst. The low accretion rates, modest columns, and evidence for a cool neutron star in quiescence, sugg
The rate of the final step in the astrophysical $α$p-process, the $^{34}$Ar($α$,\textit{p})$^{37}$K reaction, suffers from large uncertainties due to lack of experimental data, despite having a considerable impact on the observable light curves of x-ray bursts and the composition of the ashes of hydrogen and helium burning on accreting neutron stars. We present the first direct measurement constraining the $^{34}$Ar($α$,p)$^{37}$K reaction cross section, using the Jet Experiments in Nuclear Structure and Astrophysics (JENSA) gas jet target. The combined cross section for the $^{34}$Ar,Cl($α$,p)$^{37}$K,Ar reaction is found to agree well with Hauser-Feshbach predictions. The $^{34}$Ar($α$,2p)$^{36}$Ar cross section, which can be exclusively attributed to the $^{34}$Ar beam component, also agrees to within the typical uncertainties quoted for statistical models. This indicates the applicability of the statistical model for predicting astrophysical ($α$,p) reaction rates in this part of the $α$p process, in contrast to earlier findings from indirect reaction studies indicating orders-of-magnitude discrepancies. This removes a significant uncertainty in models of hydrogen and helium bu
We have explored the impact of the latest equation of state (EOS) for dense hydrogen-helium mixtures (Chabrier \& Debras 2021), which takes into account the interactions between hydrogen and helium species, upon the evolution of very low mass stars and brown dwarfs (BD). These interactions modify the thermodynamic properties of the H/He mixture, notably the entropy, a quantity of prime importance for these fully convective bodies, but also the onset and the development of degeneracy throughout the body. This translates into a faster cooling rate, i.e. cooler isentropes for a given mass and age, and thus larger brown dwarf masses and smaller radii for given effective temperature and luminosity than the models based on previous EOSs. This means that objects of a given mass and age, in the range $M\lesssim 0.1\,\msol$, $τ\gtrsim 10^8$ yr, will have cooler effective temperatures and fainter luminosities. Confronting these new models with several observationally determined BD dynamical masses, we show that this improves the agreement between evolutionary models and observations and resolves at least part of the observed discrepancies between the properties of dynamical mass determin
We discuss the impact of residual nuclear burning in the cooling sequences of hydrogen-rich DA white dwarfs with very low metallicity progenitors ($Z=0.0001$). These cooling sequences are appropriate for the study of very old stellar populations. The results presented here are the product of self-consistent, fully evolutionary calculations. Specifically, we follow the evolution of white dwarf progenitors from the zero-age main sequence through all the evolutionary phases, namely the core hydrogen-burning phase, the helium-burning phase, and the thermally pulsing asymptotic giant branch phase to the white dwarf stage. This is done for the most relevant range of main sequence masses, covering the most usual interval of white dwarf masses --- from $0.53\, M_{\sun}$ to $0.83\, M_{\sun}$. Due to the low metallicity of the progenitor stars, white dwarfs are born with thicker hydrogen envelopes, leading to more intense hydrogen burning shells as compared with their solar metallicity counterparts. We study the phase in which nuclear reactions are still important and find that nuclear energy sources play a key role during long periods of time, considerably increasing the cooling times from