Star Research Articles

Jets from the Upper Scorpius Variable Young Star System 2MASS J16075796-2040087 via KECK/HIRES Spectro-astrometry

2MASS J16075796-2040087 is an ∼5 Myr young star in Upper Sco with evidence for accretion bursts on a timescale of about 15 days and, uncommonly for its age, outflows traced by multicomponent forbidden emission lines (FELs). The accretion bursts may be triggered by a companion at ∼4.6 au. We analyze HIRES spectra optimised for spectro-astrometry to better understand the origin of the several FEL velocity components and determine whether a magnetohydrodynamic (MHD) disk wind is present. The FEL high-velocity component (HVC) traces an asymmetric, bipolar jet ∼700 au long. The jet’s position angle ∼ 277° is not perpendicular to the disk. The lower-velocity emission, classified previously as a disk wind low-velocity component, is found to have more in common with the HVC and overall it is not possible to identify an MHD disk wind component. The spectro-astrometric signal of the low-velocity emission resembles those of jets and its density and ionisation fraction fall into the range of HVCs. We suggest a scenario where the accretion bursts due to the close companion power the jets past the age where such activity ends around most stars. The low-velocity emission here could come from a slow jet launched near the close companion and this emission would be blended with emission from the MHD wind.

Simulated analogues I: apparent and physical evolution of young binary protostellar systems

ABSTRACT Protostellar binaries harbour complex environment morphologies. Observations represent a snapshot in time, and projection and optical depth effects impair our ability to interpret them. Careful comparison with high-resolution models that include the larger star-forming region can help isolate the driving physical processes and give context in the time domain to the observations. We carry out four zoom-in simulations with au scale resolution that result in three binaries and a single star. For the first time ever, we follow the detailed evolution of a protobinary in a full molecular cloud context until a circumbinary disc forms. We investigate the gas dynamics around the young stars and extract disc sizes. Using radiative transfer, we obtain the evolutionary tracer Tbol of the binary systems. We find that the centrifugal radius in prestellar cores is a poor estimator of the resulting disc size due to angular momentum transport at all scales. For binaries, the disc sizes are regulated periodically by the binary orbit, having larger radii close to the apastron. The bolometric temperature differs systematically between edge-on and face-on views and shows a high-frequency time dependence correlated with the binary orbit and a low-frequency time dependence with larger episodic accretion events. These oscillations can cause the appearance of the system to change rapidly from class 0 to class I and, for short periods, even bring it to class II. The highly complex structure in early stages, as well as the binary orbit itself, affects the classical interpretation of protostellar classes, and the direct translation to evolutionary stages has to be done with caution and include other evolutionary indicators such as the extent of envelope material.

The JWST/NIRISS Deep Spectroscopic Survey for Young Brown Dwarfs and Free-floating Planets

The discovery and characterization of free-floating planetary-mass objects (FFPMOs) is fundamental to our understanding of star and planet formation. Here we report results from an extremely deep spectroscopic survey of the young star cluster NGC1333 using Near-InfraRed Imager and Slitless Spectrograph (NIRISS) wide field slitless spectroscopy on the James Webb Space Telescope. The survey is photometrically complete to K ∼ 21, and includes useful spectra for objects as faint as K ∼ 20.5. The observations cover 19 known brown dwarfs, for most of which we confirm spectral types using NIRISS spectra. We discover six new candidates with L-dwarf spectral types that are plausible planetary-mass members of NGC1333, with estimated masses between 5 and 15 M Jup. One, at ∼5 M Jup, shows clear infrared excess emission and is a good candidate to be the lowest-mass object known to have a disk. We do not find any objects later than mid-L spectral type (M ≲ 4 M Jup). The paucity of Jupiter-mass objects, despite the survey’s unprecedented sensitivity, suggests that our observations reach the lowest-mass objects that formed like stars in NGC1333. Our findings put the fraction of FFPMOs in NGC1333 at ∼10% of the number of cluster members, significantly more than expected from the typical log-normal stellar mass function. We also search for wide binaries in our images and report a young brown dwarf with a planetary-mass companion.

Is the young star association e Cha double?

Is the young star association e Cha double?

The Local Bubble Is a Local Chimney: A New Model from 3D Dust Mapping

Leveraging a high-resolution 3D dust map of the solar neighborhood from Edenhofer et al., we derive a new 3D model for the dust-traced surface of the Local Bubble, the supernova-driven cavity surrounding the Sun. We find that the surface of the Local Bubble is highly irregular in shape, with its peak extinction surface falling at an average distance of 170 pc from the Sun (spanning 70–600+ pc) with a typical thickness of 35 pc and a total dust-traced mass of (6.0 ± 0.7) × 105 M ⊙. The Local Bubble displays an extension in the Galactic northern hemisphere that is morphologically consistent with representing a “local chimney.” We argue this chimney was likely created by the “bursting” of this supernova-driven superbubble, leading to the funneling of interstellar medium (ISM) ejecta into the lower Galactic halo. We find that many well-known dust features and molecular clouds fall on the surface of the Local Bubble and that several tunnels to other adjacent cavities in the ISM may be present. Our new, parsec-resolution view of the Local Bubble may be used to inform future analysis of the evolution of nearby gas and young stars, the investigation of direct links between the solar neighborhood and the Milky Way’s lower halo, and numerous other applications.

ALMA-IMF

The stellar initial mass function (IMF) is critical to our understanding of star formation and the effects of young stars on their environment. On large scales, it enables us to use tracers such as UV or Hα emission to estimate the star formation rate of a system and interpret unresolved star clusters across the Universe. So far, there is little firm evidence of large-scale variations of the IMF, which is thus generally considered “universal”. Stars form from cores, and it is now possible to estimate core masses and compare the core mass function (CMF) with the IMF, which it presumably produces. The goal of the ALMA-IMF large programme is to measure the core mass function at high linear resolution (2700 au) in 15 typical Milky Way protoclusters spanning a mass range of 2.5 × 103 to 32.7 × 103 M⊙. In this work, we used two different core extraction algorithms to extract ≈680 gravitationally bound cores from these 15 protoclusters. We adopted a per core temperature using the temperature estimate from the point-process mapping Bayesian method (PPMAP). A power-law fit to the CMF of the sub-sample of cores above the 1.64 M⊙ completeness limit (330 cores) through the maximum likelihood estimate technique yields a slope of 1.97 ± 0.06, which is significantly flatter than the 2.35 Salpeter slope. Assuming a self-similar mapping between the CMF and the IMF, this result implies that these 15 high-mass protoclusters will generate atypical IMFs. This sample currently is the largest sample that was produced and analysed self-consistently, derived at matched physical resolution, with per core temperature estimates, and cores as massive as 150 M⊙. We provide both the raw source extraction catalogues and the catalogues listing the source size, temperature, mass, spectral indices, and so on in the 15 protoclusters.

Using 26Al to detect ongoing self-enrichment in young massive star clusters

Abstract Self-enrichment is one of the leading explanations for chemical anomalies in globular clusters. In this scenario, various candidate polluter stars have been proposed to eject gas with altered chemical composition during the self-enrichment process. Most of the proposed polluters will also eject radioactive 26Al into the surroundings. Hence, any detection of 26Al in young massive star clusters (YMCs) would support the self-enrichment scenario if YMCs were indeed the progenitors of globular clusters. Observations of gamma-ray data from COMPTEL and INTEGRAL, as well as detections of 26AlF molecules by the Atacama Large Millimeter-submillimeter Array (ALMA), indicate the maturing of 26Al detection methods. Detection possibilities will be enhanced in the short- to mid-term by the upcoming launch of the Compton Spectrometer and Imager (COSI). The Square Kilometer Array (SKA) could in principle also detect radio recombination lines of the positronium formed from the decay products of 26Al. Here, we show for a sample of YMCs in the nearby Universe, where self-enrichment could plausibly take place. For some nearby galaxies, this could enhance 26Al by an order of one magnitude. Detecting 26AlF with ALMA appears feasible for many candidate self-enrichment clusters, although significant challenges remain with other detection methods. The Large Magellanic Cloud, with its YMC R136, stands out as the most promising candidate. Detecting a 1.8 MeV radioactive decay line of 26Al here would require at least 15 months of targeted observation with COSI, assuming ongoing self-enrichment in R136.

Measuring the Spot Variability of T Tauri Stars Using Near-infrared Atomic Fe and Molecular OH Lines

As part of the Young Exoplanets Spectroscopic Survey, this study explores the spot variability of 13 T Tauri Stars (TTSs) in the near-infrared H band, using spectra from the Immersion GRating INfrared Spectrometer. By analyzing effective temperature (T eff) sensitive lines of atomic Fe i at ∼1.56259 μm and ∼1.56362 μm, and molecular OH at ∼1.56310 and ∼1.56317 μm, we develop an empirical equivalent width ratio (EWR) relationship for T eff in the range of 3400–5000 K. This relationship allows for precise relative T eff estimates to within tens of Kelvin and demonstrates compatibility with solar metallicity target models. However, discrepancies between observational data and model predictions limit the extension of the T eff–EWR relationship to a broader parameter space. Our study reveals that both classical and weak-line TTSs can exhibit T eff variations exceeding 150 K over a span of 2 yr. The detection of a quarter-phase delay between the EWR and radial velocity phase curves in TTSs indicates spot-driven signals. A phase delay of 0.06 ± 0.13 for CI Tau, however, suggests additional dynamics, potentially caused by planetary interaction, inferred from a posited 1:1 commensurability between the rotation period and orbital period. Moreover, a positive correlation between T eff variation amplitude and stellar inclination angle supports the existence of high-latitude spots on TTSs, further enriching our understanding of stellar surface activity in young stars.

Spectroscopic Observations of the GALEX Nearby Young Star Survey Sample. I. Nearby Moving Group Candidates

The GALEX Nearby Young Star Search (GALNYSS) yielded the identification of more than 2000 late-type stars that, based on their ultraviolet and infrared colors and pre-Gaia proper motions, are potentially of age<200 Myr and lie within ~120 pc of Earth. We present the results of a campaign of medium- and high-resolution optical spectroscopy of 471 GALNYSS stars aimed at confirming their youth and their potential membership in nearby young stellar moving groups. We present radial velocity (RV), Li absorption, and H-alpha emission measurements for these spectroscopically observed GALNYSS stars, and assess their Li absorption and optical emission-line properties and infrared excesses. Our RV measurements are combined with literature and Gaia DR3 RV measurements and Gaia DR3 astrometry and photometry to obtain the spatiokinematics and color-magnitude positions of GALNYSS stars. We use these results to assess membership in the TW Hya, Tuc-Hor, Carina, Columba, and Argus Associations and the β Pic and AB Dor moving groups. We have identified 132 stars as candidate members of these seven groups; roughly half of these candidates are newly identified on the basis of data presented here. At least one-third of the 132 candidates are spectroscopic and/or photometric binaries and/or have comoving (visual) binary companions in Gaia DR3. The contingent of young, low-mass stars in the solar vicinity we identify here should provide excellent subjects for future direct imaging exoplanet surveys and studies of the early evolution of low-mass stars and their planetary progeny.

A Far-ultraviolet-detected Accretion Shock at the Star–Disk Boundary of FU Ori

FU Ori objects are the most extreme eruptive young stars known. Their 4–5 mag photometric outbursts last for decades and are attributed to a factor of up to 10,000 increase in the stellar accretion rate. The nature of the accretion disk-to-star interface in FU Ori objects has remained a mystery for decades. To date, attempts to directly observe a shock or boundary layer have been thwarted by the apparent lack of emission in excess of the accretion disk photosphere down to λ = 2300 Å. We present a new near-ultraviolet and the first high-sensitivity far-ultraviolet (FUV) spectrum of FU Ori. The FUV continuum is detected for the first time and, at λ = 1400 Å, is more than 104 times brighter than predicted by a viscous accretion disk. We interpret the excess as arising from a shock at the boundary between the disk and the stellar surface. We model the shock emission as a blackbody and find that the temperature of the shocked material is T FUV ≈ 16,000 ± 2000 K. The shock temperature corresponds to an accretion flow along the surface of the disk that reaches a velocity of 40 km s−1 at the boundary, consistent with predictions from simulations.

Chemical evolution of a young super star cluster at the Sunburst Arc

ABSTRACT Recent observations of high-redshift galaxies have revealed starburst galaxies with excessive amounts of nitrogen, well above that expected in standard evolutionary models. The Sunburst Arc galaxy, particularly its young and massive star cluster, represents the closest ($z=2.4$) and brightest of these as a strongly lensed object. In this work, we study the chemical history of this star cluster to determine the origin of the elevated gas-phase nitrogen using a chemical evolution model. Our model includes the enrichment of OB stars through stellar winds and core-collapse supernovae assuming that massive stars ($M\gt 25$ $\mathrm{ M}_\odot$) collapse directly into black holes at the end of their lives. We fit the model parameters to the observed chemical abundances of the Sunburst Arc cluster: O/H, C/O, and N/O. We find that the observed chemical abundances can be explained by models featuring intense star formation events, characterized by rapid gas accretion and high star formation efficiencies. Additionally, the stellar population contributing to the gas enrichment must exclude Wolf–Rayet stars. These conditions might be present in other nitrogen-rich objects as their similar chemical abundances suggest a common history. As previous studies have proposed the presence of Wolf–Rayet stars in the new nitrogen-rich objects, further research using chemodynamic modeling is necessary to ascertain the true nature of these objects.

Explaining the oblate morphology of dwarf spheroidals with wave dark matter perturbations

ABSTRACT We investigate whether the oblate, spheroidal morphology of common dwarf spheroidal galaxies (dSph) may result from the slow relaxation of stellar orbits within a halo of wave dark matter ($\psi$DM) when starting from an initial disc of stars. Stellar orbits randomly walk over a Hubble time, perturbed by the pervasive ‘granular’ interference pattern of $\psi$DM, that fully modulates the dark matter density on the de Broglie scale. Our simulations quantify the level of stellar disc thickening over the Hubble time, showing that distribution of stars is predicted to become an oblate spheroid of increasing radius, that plausibly accounts for the morphology of dSph galaxies. We predict a low level of residual rotation remains after a Hubble time at the 1–3 km/s level, depending on orientation, that compares with recent claims of rotation for some well-studied local dSph galaxies. This steady internal dynamical evolution may be witnessed directly with JWST for well-resolved dwarf galaxies, appearing more oblate with look back time and tending to small discs of young stars at high redshift.

New AGN diagnostic diagrams based on the [OIII]lambda4363 auroral line

The James Webb Space Telescope (JWST) is revolutionizing our understanding of black hole formation and growth in the early Universe. However, JWST has also revealed that some of the classical diagnostics, such as the Baldwin, Phillips Terlevich (BPT) diagrams and X-ray emission, often fail to identify active galactic nuclei (AGN) at high redshift and low metallicity. Here we present three new rest-frame optical diagnostic diagrams to identify narrow-line Type II AGN, leveraging the auroral line, which has been detected in several JWST spectra. Specifically, we show that high values of the ratio provide a sufficient (but not necessary) condition to identify the presence of an AGN, based on empirical calibrations (using local and high-redshift sources) and on a broad range of photoionization models. These diagnostics are able to separate much of the AGN population from star-forming galaxies (SFGs): the average energy of an AGN's ionizing photons is higher than that of young stars in SFGs, and hence AGN can more efficiently heat the gas, thus boosting the line. We also found independent indications of AGN activity in some high-redshift sources (z>4) that were not previously identified as AGN with the traditional diagnostics diagrams, but that are placed in the AGN region of the diagnostic presented in this work. We note, conversely, that low values of can be associated either with SFGs or AGN excitation. We note that the fact that strong auroral lines are often associated with AGN does not imply that they cannot be used for direct metallicity measurements (provided that proper ionization corrections are applied), but it does affect the calibration of strong line metallicity diagnostics.

Early Signs of Structure in Protostellar Disks

Protostellar disks—rotating disks of gas and dust around pre-fusion ignition young stars—are divided into four main classes: Class 0: the youngest class, still deeply embedded in their parent molecular clouds and accumulating mass, with a lifespan on the order of 100k years. Class I: still embedded in molecular clouds, characterized by strong infrared and radio emissions generated by the heat from the collapsing central protostar, with a typical lifespan of a few hundred thousand years. Class II: more evolved with decreased accretion rates and active planet formation. Lifespan on the order of 2-3 million years. Class III: low accretion rate debris (mostly dust) disks. Remnants of planetary formation process. Typical lifespan of several million years. Focusing on the youngest disks, Classes 0 and I, we are investigating how early structure begins to show in the form of gaps and rings, indicating potential future planet formation. We use high resolution and high sensitivity (sub)millimeter wavelength observations from the ALMA (Atacama Large Millimeter/submillimeter Array) interferometer. Having identified approximately 10,700 potential Class 0 or I disks in the ALMA archive, we make use of several specialized code packages designed to fit or examine individual disks from their frequency space visibility data rather than from images to build a step-by-step process taking the initial ALMA data and providing the decision-making ability to categorize the disks into four primary categories: Smooth, Structured, Further Investigation, Not Usable. Having used the summer term to build, streamline, and document the needed process, we report building an initial catalog of usable Class 0 and Class I sources that contains 36 individual objects, with 7 categorized as Smooth, 8 as having Structure, and 6 needing Further Investigation.

Photometric classification of stars around the Milky Way’s central black hole

Context. The presence of young massive stars in the Galactic Centre (GC) raises questions about how such stars could form near the massive black hole Sagittarius A* (Sgr A*). Furthermore, the shape of the initial mass function (IMF) in this region seems to differ from its standard Salpeter/Kroupa law. Due to observational challenges such as extreme extinction and crowding, our understanding of the stellar population in this region remains limited, with spectroscopic data available only for selected small and comparably bright sources. Aims. We aim to improve our knowledge about the distribution and the IMF of young, massive, stars in the vicinity of Sgr A*. Methods. We used intermediate band (IB) photometry to identify candidates for massive young stars. To ensure robust classification, we applied three different, but complementary methods: Bayesian inference, a basic neural network, and a fast gradient-boosted trees algorithm. Results. We obtain spectral energy distributions for 6590 stars, 1181 of which have been previously classified spectroscopically. We identify 351 stars that are classified as early types by all three classification methods, with 155 of them being newly identified candidates. The radial density profiles for late and early-type stars fit well with broken power laws, revealing a break radius of 9.2 ± 0.6″ for early-type stars. The late-type stars show a core-like distribution around Sgr A* while the density of the early-type stars increases steeply towards the black hole, consistent with previous work. We infer a top-heavy IMF of the young stars near Sgr A* (R < 9″), with a power-law of 1.6 ± 0.1. At greater distances from Sgr A* a standard Salpeter/Kroupa IMF can explain the data. Additionally, we demonstrate that IB photometry can also constrain the metallicities of late-type stars, estimating metallicities for over 600 late-type stars. Conclusions. The variation of the IMF with radial distance from Sgr A* suggests that different mechanisms of star formation may have been at work in this region. The top-heavy IMF in the innermost region is consistent with star formation in a disc around Sgr A*.

Physical Properties of Embedded Clusters in ATLASGAL Clumps with H ii Regions

Using the optimal sampling model, we synthesized the embedded clusters of ATLASGAL clumps with H ii regions (H ii-clumps). The 0.1 Myr isochrone was used to estimate the bolometric luminosity of each star in an embedded cluster, we also added the accretion luminosity of each star in the embedded cluster. The total bolometric luminosity of synthetic embedded clusters can well fit the observed bolometric luminosity of H ii-clumps. More realistically, we considered the age spread in the young star and protostar populations in embedded clusters of H ii-clumps by modeling both constant and time-varying star formation histories (SFHs). According to the age distribution of the stellar population, we distributed the appropriate isochrones to each star, and sorted out the fraction of stellar objects that are still protostars (Class 0 and Class I phases) to properly add their accretion luminosities. Compared to a constant SFH, burst-like and time-dependent SFHs can better fit the observational data. We found that as long as 20% of the stars within the embedded cluster are still accreting, the contribution of accretion luminosity will be significant to the total bolometric luminosity of low-mass H ii-clumps with mass log10(M cl/M ⊙) < 2.8. Variations in the accretion rate, the SFE and the initial mass function and more physical processes like the external heating from H ii regions and the flaring from pre-main sequence stars and protostars need to be investigated to further explain the excess luminosity of low-mass H ii-clumps.

Hubble Space Telescope proper motions of Large Magellanic Cloud star clusters

We present proper motion (PM) measurements for a sample of 23 massive star clusters within the Large Magellanic Cloud using multi-epoch data from the Hubble Space Telescope (HST). We combined archival data from the ACS/WFC and WFC3/UVIS instruments with observations from a dedicated HST programme, resulting in time baselines between 4.7 and 18.2 yr available for PM determinations. For bright well-measured stars, we achieved nominal PM precisions of 55 μas yr−1 down to 11 μas yr−1. To demonstrate the potential and limitations of our PM data set, we analysed the cluster NGC 1850 and showcase a selection of different science applications. The precision of the PM measurements allows us to disentangle the kinematics of the various stellar populations that are present in the HST field. The cluster has a centre-of-mass motion that is different from the surrounding old field stars and also differs from the mean motion of a close-by group of very young stars. We determined the velocity dispersion of field stars to be 0.128 ± 0.003 mas yr−1 (corresponding to 30.3 ± 0.7 km s−1). The velocity dispersion of the cluster inferred from the PM data set most probably overestimates the true value, suggesting that the precision of the measurements at this stage is not sufficient for a reliable analysis of the internal kinematics of extra-galactic star clusters. Finally, we exploit the PM-cleaned catalogue of likely cluster members to determine any radial segregation between fast and slowly-rotating stars, finding that the former are more centrally concentrated. With this paper, we also release the astro-photometric catalogues for each cluster.

Competitive Entrapment of Hypervolatiles in Interstellar and Cometary Water Ice Analogs

The distribution of chemical species in protoplanetary disks around young stars, especially their division between gas and solid phases, fundamentally shapes the composition of future planets and planetesimals. This distribution is likely affected by entrapment, a mechanism whereby volatile species are mechanically or chemically bound within a less volatile ice. In this study, we investigate the entrapment efficiencies of four hypervolatiles (CH4, CO, N2, and Ar) in multicomponent water ice mixtures deposited at different temperatures and mixture ratios. At low ice deposition temperatures, we observe small differences in entrapment efficiency (CH4>CO>N2∼Ar) up to a factor of two across species. The differences in entrapment between species increase by up to an order of magnitude with increasing deposition temperature. The relative entrapment efficiencies are also impacted by changes in the overall hypervolatile concentration of the ice mixtures. Collectively, these experiments suggest that relative entrapment efficiencies are mainly regulated by small differences in binding energies to the ice matrix, though competition for the best sites also influences entrapment in more concentrated ices. We use these results to better inform interpretations of hypervolatile observations in comets and related objects.

On the Correlation between Young Massive Star Clusters and Gamma-Ray Unassociated Sources

Star clusters (SCs) are potential cosmic-ray accelerators and therefore are expected to emit high-energy radiation. However, a clear detection of gamma-ray emission from this source class has only been possible for a handful of cases. This could in principle result from two different reasons: either detectable SCs are limited to a small fraction of the total number of Galactic SCs, or gamma-ray-emitting SCs are not recognized as such and therefore are listed in the ensemble of unidentified sources. In this Letter we investigate this latter scenario by comparing available catalogs of SCs and H ii regions, obtained from Gaia and Wide-field Infrared Survey Explorer observations, to the gamma-ray GeV and TeV catalogs built from Fermi Large Area Telescope (LAT), H.E.S.S., and LHAASO data. The significance of the correlation between catalogs is evaluated by comparing the results with simulations of synthetic populations. A strong correlation emerges between Fermi-LAT-unidentified sources and H ii regions that trace massive SCs in the earliest (≲1–2 Myr) phase of their life, where no supernova explosions have happened yet, confirming that winds of massive stars can alone accelerate particles and produce gamma-ray emission at least up to GeV energies. The association with TeV energy sources is less evident. Similarly, no significant association is found between Gaia SCs and GeV nor TeV sources. We ascribe this fact to the larger extension of these objects but also to an intrinsic bias in the Gaia selection toward SCs surrounded by a lower target gas density, which would otherwise hinder the detection in the optical wave band.

The Kraft Break Sharply Divides Low-mass and Intermediate-mass Stars

Main-sequence stars transition at mid-F spectral types from slowly rotating (cooler stars) to rapidly rotating (hotter stars), a transition known as the Kraft Break and attributed to the disappearance of the outer convective envelope, causing magnetic braking to become ineffective. To define this Break more precisely, we assembled spectroscopic measurements of 405 F stars within 33.33 pc. Once young, evolved, and candidate binary stars are removed, the distribution of projected rotational velocities shows the Break to be well defined and relatively sharp. Nearly all stars redder than G BP − G RP = 0.60 mag are slowly rotating (vsini ≲20 km s−1), while only 4 of 32 stars bluer than G BP − G RP = 0.54 mag are slowly rotating, consistent with that expected for a random distribution of inclinations. The Break is centered at an effective temperature of 6550 K and has a width of about 200 K, corresponding to a mass range of 1.32–1.41 M ⊙. The Break is ∼450 K hotter than the stellar temperature at which hot Jupiters show a change in their obliquity distribution, often attributed to tidal realignment. The Break, as defined above, is nearly but not fully established in the ∼650 Myr Hyades cluster; it should be established in populations older than 1 Gyr. We propose that the Kraft Break provides a more useful division, for both professional and pedagogical purposes, between what are called low-mass stars and intermediate-mass stars; the Kraft Break is observationally well defined and is linked to a change in stellar structure.