Pre-main Sequence Evolutionary Tracks Research Articles

APOGEE Net: Improving the Derived Spectral Parameters for Young Stars through Deep Learning

Abstract Machine learning allows for efficient extraction of physical properties from stellar spectra that have been obtained by large surveys. The viability of machine-learning approaches has been demonstrated for spectra covering a variety of wavelengths and spectral resolutions, but most often for main-sequence (MS) or evolved stars, where reliable synthetic spectra provide labels and data for training. Spectral models of young stellar objects (YSOs) and low-mass MS stars are less well-matched to their empirical counterparts, however, posing barriers to previous approaches to classify spectra of such stars. In this work, we generate labels for YSOs and low-mass MS stars through their photometry. We then use these labels to train a deep convolutional neural network to predict , T eff, and Fe/H for stars with Apache Point Observatory Galactic Evolution Experiment (APOGEE) spectra in the DR14 data set. This “APOGEE Net” has produced reliable predictions of for YSOs, with uncertainties of within 0.1 dex and a good agreement with the structure indicated by pre-MS evolutionary tracks, and it correlates well with independently derived stellar radii. These values will be useful for studying pre-MS stellar populations to accurately diagnose membership and ages.

Characterization of the Low-Mass Companion HD 142527 B

Context. The circumstellar disk of the Herbig Fe star HD 142527 is host to several remarkable features including a warped inner disk, a 120 au-wide annular gap, a prominent dust trap and several spiral arms. A low-mass companion, HD 142527 B, was also found orbiting the primary star at ~14 au. Aims. This study aims to better characterize this companion, which could help explain its impact on the peculiar geometry of the disk. Method. We observed the source with VLT/SINFONI in H + K band in pupil-tracking mode. Data were post-processed with several algorithms based on angular differential imaging (ADI). Results. HD 142527 B is conspicuously re-detected in most spectral channels, which enables us to extract the first medium-resolution spectrum of a low-mass companion within 0.″1 from its central star. Fitting our spectrum with both template and synthetic spectra suggests that the companion is a young M2.5 ± 1.0 star with an effective temperature of 3500 ± 100 K, possibly surrounded with a hot (1700 K) circum-secondary environment. Pre-main sequence evolutionary tracks provide a mass estimate of 0.34 ± 0.06 M⊙, independent of the presence of a hot environment. However, the estimated stellar radius and age do depend on that assumption; we find a radius of 1.37 ± 0.05 R⊙ (resp. 1.96 ± 0.10 R⊙) and an age of 1.8_(-0.5)^(+1.2) Myr (resp. 0.75 ± 0.25 Myr) in the case of the presence (resp. absence) of a hot environment contributing in H + K. Our new values for the mass and radius of the companion yield a mass accretion rate of 4.1–5.8×10^(−9) M⊙ yr^(−1) (2–3% that of the primary). Conclusions. We have constrained the physical properties of HD 142527 B, thereby illustrating the potential for SINFONI+ADI to characterize faint close-in companions. The new spectral type makes HD 142527 B a twin of the well-known TW Hya T Tauri star, and the revision of its mass to higher values further supports its role in shaping the disk.

Characterization of low-mass companion HD 142527 B

Context. The circumstellar disk of the Herbig Fe starHD 142527is host to several remarkable features including a warped inner disk, a 120 au-wide annular gap, a prominent dust trap and several spiral arms. A low-mass companion,HD 142527B, was also found orbiting the primary star at ~14 au.Aims. This study aims to better characterize this companion, which could help explain its impact on the peculiar geometry of the disk.Method. We observed the source with VLT/SINFONI inH + Kband in pupil-tracking mode. Data were post-processed with several algorithms based on angular differential imaging (ADI).Results.HD 142527B is conspicuously re-detected in most spectral channels, which enables us to extract the first medium-resolution spectrum of a low-mass companion within 0.″1 from its central star. Fitting our spectrum with both template and synthetic spectra suggests that the companion is a young M2.5 ± 1.0 star with an effective temperature of 3500 ± 100 K, possibly surrounded with a hot (1700 K) circum-secondary environment. Pre-main sequence evolutionary tracks provide a mass estimate of 0.34 ± 0.06M⊙, independent of the presence of a hot environment. However, the estimated stellar radius and age do depend on that assumption; we find a radius of 1.37 ± 0.05R⊙(resp. 1.96 ± 0.10R⊙) and an age of 1.8-0.5+1.2Myr (resp. 0.75 ± 0.25 Myr) in the case of the presence (resp. absence) of a hot environment contributing inH+K. Our new values for the mass and radius of the companion yield a mass accretion rate of 4.1–5.8 × 10−9M⊙yr−1(2–3% that of the primary).Conclusions. We have constrained the physical properties ofHD 142527B, thereby illustrating the potential for SINFONI+ADI to characterize faint close-in companions. The new spectral type makesHD 142527B a twin of the well-known TW Hya T Tauri star, and the revision of its mass to higher values further supports its role in shaping the disk.

The Star-formation Law in Galactic High-mass Star-forming Molecular Clouds

We study the star formation (SF) law in 12 Galactic molecular clouds with ongoing high-mass star formation (HMSF) activity, as traced by the presence of a bright IRAS source and other HMSF tracers. We define the molecular cloud (MC) associated to each IRAS source using 13CO line emission, and count the young stellar objects (YSOs) within these clouds using GLIMPSE and MIPSGAL 24 micron Spitzer databases.The masses for high luminosity YSOs (Lbol>10~Lsun) are determined individually using Pre Main Sequence evolutionary tracks and the evolutionary stages of the sources, whereas a mean mass of 0.5 Msun was adopted to determine the masses in the low luminosity YSO population. The star formation rate surface density (sigsfr) corresponding to a gas surface density (siggas) in each MC is obtained by counting the number of the YSOs within successive contours of 13CO line emission. We find a break in the relation between sigsfr and siggas, with the relation being power-law (sigsfr ~ siggas^N) with the index N varying between 1.4 and 3.6 above the break. The siggas at the break is between 150-360 Msun/pc^2 for the sample clouds, which compares well with the threshold gas density found in recent studies of Galactic star-forming regions. Our clouds treated as a whole lie between the Kennicutt (1998) relation and the linear relation for Galactic and extra-galactic dense star-forming regions. We find a tendency for the high-mass YSOs to be found preferentially in dense regions at densities higher than 1200 Msun/pc^2 (~0.25 g/cm^2).

A probable pre-main sequence chemically peculiar star in the open cluster Stock 16

We used the Ultraviolet and Visual Echelle Spectrograph of the ESO-Very Large Telescope to obtain a high resolution and high signal-to-noise ratio spectrum of Stock 16-12, an early-type star which previous Delta-a photometric observations suggest being a chemically peculiar (CP) star. We used spectral synthesis to perform a detailed abundance analysis obtaining an effective temperature of 8400 +/- 400 K, a surface gravity of 4.1 +/- 0.4, a microturbulence velocity of 3.4 +0.7/-0.3 km/s, and a projected rotational velocity of 68 +/- 4 km/s. We provide photometric and spectroscopic evidence showing the star is most likely a member of the young Stock 16 open cluster (age 3-8 Myr). The probable cluster membership, the star's position in the Hertzsprung-Russell diagram, and the found infrared excess strongly suggest the star is still in the pre-main-sequence (PMS) phase. We used PMS evolutionary tracks to determine the stellar mass, which ranges between 1.95 and 2.3 Msun, depending upon the adopted spectroscopic or photometric data results. Similarly, we obtained a stellar age ranging between 4 and 6 Myr, in agreement with that of the cluster. Because the star's chemical abundance pattern resembles well that known of main sequence CP metallic line (Am) stars, the object sets important constraints to the diffusion theory. Additional spectroscopic and spectropolarimetric data allowed us to conclude that the object is probably a single non-magnetic star.

DISTANCE AND KINEMATICS OF THE TW HYDRAE ASSOCIATION FROM PARALLAXES

From common proper motion and signatures of youth, researchers have identified about 30 members of a putative TW Hydrae Association. Only four of these had parallactic distances from Hipparcos. We have measured parallaxes and proper motions for 14 primary members. We combine these with literature values of radial velocities to show that the Galactic space motions of the stars, with the exception of TWA 9 and 22, are parallel and do not indicate convergence at a common formation point sometime in the last few million years. The space motions of TWA 9 and 22 do not agree with the others and indicate that they are not TWA members. The median parallax is 18 mas or 56 pc. We further analyze the stars' absolute magnitudes on pre-main sequence evolutionary tracks and find a range of ages with a median of 10.1 Myr and no correlation between age and Galactic location. The TWA stars may have formed from an extended and filamentary molecular cloud but are not necessarily precisely coeval.

Benchmark cool companions: ages and abundances for the PZ Telescopii system

We present new ages and abundance measurements for the pre-main sequence star PZ Tel. PZ Tel was recently found to host a young and low-mass companion. Using FEROS spectra we have measured atomic abundances (e.g. Fe and Li) and chromospheric activity for PZ Tel and used these to obtain metallicity and age estimates for the companion. We find PZ Tel to be a rapidly rotating (vsini=73\pm5km/s), ~solar metallicity star (logN(Fe)=-4.37 dex or [Fe/H]=0.05 dex) with a measured mean logR'HK of -4.12. We measure a NLTE lithium abundance of logN(Li)=3.1\pm0.1dex, which from depletion models gives rise to an age of 7+4-2 Myrs for the system. The measured chromospheric activity returns an age of 26\pm2Myrs, as does fitting pre-main sequence evolutionary tracks (Tau_evol=22\pm3Myrs), both of which are in disagreement with the lithium age. We speculate on reasons for this difference and introduce new models for lithium depletion that incorporates both rotation and magnetic field affects. We also synthesize solar, metal-poor and metal-rich substellar evolutionary models to better determine the bulk properties of PZ Tel B, showing that PZ Tel B is probably more massive than previous estimates, meaning the companion is not a giant exoplanet. We show how PZ Tel B compares to other currently known age and metallicity benchmark systems and try to empirically test the effects of dust opacity as a function of metallicity on the near infrared colours of brown dwarfs. Current models suggest that in the near infrared observations are more sensitive to low-mass companions orbiting more metal-rich stars. We also look for trends between infrared photometry and metallicity amongst a growing population of substellar benchmark objects, and identify the need for more data in mass-age-metallicity parameter space. [Abridged]

STAR FORMATION IN 30 DORADUS

Using observations obtained with the Wide Field Camera 3 (WFC3) on board the Hubble Space Telescope (HST), we have studied the properties of the stellar populations in the central regions of 30 Dor, in the Large Magellanic Cloud. The observations clearly reveal the presence of considerable differential extinction across the field. We characterise and quantify this effect using young massive main sequence stars to derive a statistical reddening correction for most objects in the field. We then search for pre-main sequence (PMS) stars by looking for objects with a strong (> 4 sigma) Halpha excess emission and find about 1150 of them over the entire field. Comparison of their location in the Hertzsprung-Russell diagram with theoretical PMS evolutionary tracks for the appropriate metallicity reveals that about one third of these objects are younger than ~4Myr, compatible with the age of the massive stars in the central ionising cluster R136, whereas the rest have ages up to ~30Myr, with a median age of ~12Myr. This indicates that star formation has proceeded over an extended period of time, although we cannot discriminate between an extended episode and a series of short and frequent bursts that are not resolved in time. While the younger PMS population preferentially occupies the central regions of the cluster, older PMS objects are more uniformly distributed across the field and are remarkably few at the very centre of the cluster. We attribute this latter effect to photoevaporation of the older circumstellar discs caused by the massive ionising members of R136.

Pre-main-sequence stars in the star-forming complex Sh 2-284

Located at the galactic anticenter, Sh 2-284 is a HII region which harbors several young open clusters; Dolidze 25, a rare metal poor (Z~0.004) young cluster, is one of these. Given its association with Sh 2-284, it is reasonable to assume the low metallicity for the whole HII region. Sh~2-284 is expected to host a significant population of Pre-Main Sequence (PMS) stars of both low and intermediate mass stars (Herbig Ae stars). We aim at characterizing these stars by means of a spectroscopic and photometric survey conducted with VIMOS@VLT and complemented with additional optical and infrared observations. In this survey we selected and characterized 23 PMS objects. We derived the effective temperature, the spectral energy distribution and luminosity of these objects; using theoretical PMS evolutionary tracks, with the appropriate metallicity, we estimated the mass and the age of the studied objects. We also estimated a distance of 4 Kpc for Sh 2-284 by using spectroscopic parallax of 3 OB stars. From the age determination we concluded that triggered star formation is in act in this region. Our results show that a significant fraction of the young stellar objects (YSOs) may have preserved their disk/envelopes, in contrast with what is found in other recent studies of low-metallicity star forming regions in the Galaxy. Finally, among the 23 bona fide PMS stars, we identified 8 stars which are good candidates to pulsators of the delta Scuti type.

THE AGE, STELLAR CONTENT, AND STAR FORMATION TIMESCALE OF THE B59 DENSE CORE

We have used moderate resolution, near-infrared spectra from the SpeX spectrograph on the NASA Infrared Telescope facility to characterize the stellar content of Barnard 59 (B59), the most active star-forming core in the Pipe Nebula. Measuring luminosity and temperature sensitive features in the spectra of 20 candidate YSOs, we identified likely background giant stars and measured each star's spectral type, extinction, and NIR continuum excess. We find that B59 is composed of late type (K4-M6) low-mass (0.9--0.1 M_sun) YSOs whose median stellar age is comparable to, if not slightly older than, that of YSOs within the Rho Oph, Taurus, and Chameleon star forming regions. Deriving absolute age estimates from pre-main sequence models computed by D'Antona et al., and accounting only for statistical uncertainties, we measure B59's median stellar age to be 2.6+/-0.8 Myrs. Including potential systematic effects increases the error budget for B59's median (DM98) stellar age to 2.6+4.1/-2.6 Myrs. We also find that the relative age orderings implied by pre-main sequence evolutionary tracks depend on the range of stellar masses sampled, as model isochrones possess significantly different mass dependencies. The maximum likelihood median stellar age we measure for B59, and the region's observed gas properties, suggest that the B59 dense core has been stable against global collapse for roughly 6 dynamical timescales, and is actively forming stars with a star formation efficiency per dynamical time of ~6%. This maximum likelihood value agrees well with recent star formation simulations that incorporate various forms of support against collapse, such as sub-critical magnetic fields, outflows, and radiative feedback from protostellar heating. [abridged]

Star formation history of Canis Major R1

The CMa R1 star-forming region contains several compact clusters as well as many young early-B stars. It is associated with a well-known bright rimmed nebula, the nature of which is unclear (fossil HII region or supernova remnant). To help elucidate the nature of the nebula, our goal was to reconstruct the star-formation history of the CMa R1 region, including the previously unknown older, fainter low-mass stellar population, using X-rays. We analyzed images obtained with the ROSAT satellite, covering ~5 sq. deg. Complementary VRI photometry was performed with the Gemini South telescope. Colour-magnitude and colour-colour diagrams were used in conjunction with pre-main sequence evolutionary tracks to derive the masses and ages of the X-ray sources. The ROSAT images show two distinct clusters. One is associated with the known optical clusters near Z CMa, to which ~40 members are added. The other, which we name the "GU CMa" cluster, is new, and contains ~60 members. The ROSAT sources are young stars with masses down to M_star ~0.5 M_sun, and ages up to 10 Myr. The mass functions of the two clusters are similar, but the GU CMa cluster is older than the cluster around Z CMa by at least a few Myr. Also, the GU CMa cluster is away from any molecular cloud, implying that star formation must have ceased; on the contrary (as already known), star formation is very active in the Z CMa region.

THE METALLICITY OF THE HD 98800 SYSTEM

Pre-main sequence (PMS) binaries and multiples enable critical tests of stellar models if masses, metallicities, and luminosities of the component stars are known. We have analyzed high-resolution, high signal-to-noise echelle spectra of the quadruple-star system HD 98800 and using spectrum synthesis, computed fits to the composite spectrum for a full range of plausible stellar parameters for the components. We consistently find that sub-solar metallicity yields fits with lower $\chi^2$ values, with an overall best-fit of $[M/H] = -0.20\pm0.10$. This metallicity appears to be consistent with PMS evolutionary tracks for the measured masses and luminosities of the components of HD 98800 but additional constraints on the system and modelling are needed.

Intermediate to low-mass stellar content of Westerlund 1

We have analysed near-infrared NTT/SofI observations of the starburst cluster Westerlund 1, which is among the most massive young clusters in the Milky Way. A comparison of colour-magnitude diagrams with theoretical main-sequence and pre-main sequence evolutionary tracks yields improved extinction and distance estimates of AKs = 1.13 ± 0.03 mag and d = 3.55 ± 0.17 kpc (DM = 12.75 ± 0.10 mag). The pre-main sequence population is best fit by a Palla & Stahler isochrone for an age of 3.2 Myr, while the main sequence population is in agreement with a cluster age of 3 to 5 Myr. An analysis of the structural parameters of the cluster yields that the half-mass radius of the cluster population increases towards lower mass, indicative of the presence of mass segregation. The cluster is clearly elongated with an eccentricity of 0.20 for stars with masses between 10 and 32 M� , and 0.15 for stars with masses in the range 3 to 10 M� . We derive the slope of the stellar mass function for stars with masses between 3.4 and 27 M� . In an annulus with radii between 0.75 and 1.5 pc from the cluster centre, we obtain a slope of Γ= −1.3. Closer in, the mass function of Westerlund #[

Measuring the Mass of a Pre-Main-Sequence Binary Star through the Orbit of TWA 5A

We present the results of a five year monitoring campaign of the close binary TWA 5Aab in the TW Hydrae association, using speckle and adaptive optics on the W.M. Keck 10 m telescopes. These measurements were taken as part of our ongoing monitoring of pre-main sequence (PMS) binaries in an effort to increase the number of dynamically determined PMS masses and thereby calibrate the theoretical PMS evolutionary tracks. Our observations have allowed us to obtain the first determination of this system's astrometric orbit. We find an orbital period of 5.94 +- 0.09 years and a semi-major axis of 0.066 +- 0.005. Combining these results with a kinematic distance, we calculate a total mass of 0.71 +- 0.14 M_sun (D/44 pc)^3. for this system. This mass measurement, as well as the estimated age of this system, are consistent to within 2$\sigma$ of all theoretical models considered. In this analysis, we properly account for correlated uncertainties, and show that while these correlations are generally ignored, they increase the formal uncertainties by up to a factor of five and therefore are important to incorporate. With only a few more years of observation, this type of measurement will allow the theoretical models to be distinguished.

PRE-MAIN SEQUENCE EVOLUTIONS OF SOLAR ABUNDANCE LOW MASS STARS

We present the Pre-Main Sequence (PMS) evolutionary tracks of stars with <TEX>$0.065{\sim}5.0M_{\odot}$</TEX>. The models were evolved from the PMS stellar birthline to the onset of hydrogen burning in the core. The convective turnover timescales which enables an observational test of theoretical model, particulary in the stellar dynamic activity, are also calculated. All models have Sun-like metal abundance, typically considered as the stars in the Galactic disk and the star formation region of Population I star. The convection phenomenon is treated by the usual mixing length approximation. All evolutionary tracks are available upon request.

Improved age constraints for the AB Doradus quadruple system

We present resolved NACO photometry of the close binary AB Dor B in H- and Ks-band. AB Dor B is itself known to be a wide binary companion to AB Dor A, which in turn has a very low-mass close companion named AB Dor C. These four known components make up the young and dynamically interesting system AB Dor, which will likely become a benchmark system for calibrating theoretical pre-main sequence evolutionary mass tracks for low-mass stars. However, for this purpose the actual age has to be known, and this subject has been a matter of discussion in the recent scientific literature. We compare our resolved photometry of AB Dor Ba and Bb with theoretical and empirical isochrones in order to constrain the age of the system. This leads to an age estimate of about 50 to 100 Myr. We discuss the implications of such an age range for the case of AB Dor C, and compare with other results in the literature.

Young Binaries as a Test for Pre-Main Sequence Evolutionary Tracks

AbstractObservations of young low-mass binaries (t ≲ 107 yr, M ≲ 3 M_⊙) can be used to calibrate pre-Main Sequence (pre-MS) evolutionary tracks. Recent high angular resolution HST/FGS, speckle, and long-baseline interferometry have resolved the astrometric orbits of a few SB2 pre-MS binaries and have provided the individual dynamical masses of their components as well as the system orbital parallaxes. Spectroscopic fits and filter photometry have permitted to determine SpT (temperatures) and a good estimate of the absolute magnitude (bolometric luminosity) of the components, which in turn allows one to place the components on a theoretical HR-diagram. In this way, one can check (a) whether the measured dynamical masses agree with the predicted masses on the tracks and (b) whether both components lie on an isochrone, as they should for a coeval physical pair of stars.With a sufficiently large sample of different masses and ages of resolved SB2 systems, most of the parameter space of pre-MS tracks can be tested, even for very low stellar masses (M &lt; 0.5 M_⊙) and very young ages (&lt; 2 Myr). This is a prerequisite in order to derive the IMF and star formation history in very young clusters and associations.

Pre-main Sequence models with convection described by 2D-RHD numerical simulations

AbstractThe Teff location of Pre-Main Sequence (PMS) evolutionary tracks depends on the treatment of over-adiabaticity. We present here the PMS evolutionary tracks computed by using the mixing length theory of convection (MLT) in which the αMLT = l/Hp parameter calibration is based on 2D–hydrodynamical models (Ludwig et al. 1999). These MLT–α2D stellar models and tracks are very similar to those computed with non–grey ATLAS9 atmospheric boundary conditions and Full Spectrum of Turbulence (FST) convection model both in the atmosphere and in the interior. As for the FST models, the comparison of the new tracks with the location on the HR diagram of pre–MS binaries is not completely satisfactory; and the pre–MS lithium depletion in the MLT–α2D tracks is still much larger than that expected from the observations of lithium in young open clusters. Thus, in spite of the fact that 2D RHD models should provide a better convection description than any local model, their introduction is not sufficient to reconcile theory and observations. Lithium depletion in young clusters points towards a convection efficiency which, in pre–MS, should be smaller than in the MS.

Rotational properties of the Orion nebular cluster revised

AbstractThe observational data of the Orion Nebula Cluster (ONC) is reanalyzed by means of new sets of pre-main sequence (PMS) evolutionary tracks including rotation, non-gray boundary conditions (BC's) and either low (LCE) or high convection efficiency (HCE), aiming better understanding of the appropriate physical constraints for the rotational evolution of the stars within the ONC. The role played by convection is a key aspect of our analysis, since there are conflicting results from theory and observations. We derived stellar masses and ages for the ONC by using both LCE and HCE and considered was the role of non-gray atmospheres. Our results show that the resulting mass distribution for the bulk of the ONC population is in the range 0.2-0.4M⊙ for our non-gray models, and in the range 0.1-0.3M⊙ for gray models. In agreement with previous works, we found that a large percentage (∼70%) of low-mass stars (M≤Mtr, where Mtr is a transition mass) in the ONC appears to be fast rotators (P&lt;4days). Mtr depends on the model choosen, being Mtr=0.5 for LCE, Mtr=0.35 for HCE and, as found in previous works, Mtr=0.25 for gray models. Finally, our analysis indicates that a second parameter is needed for a proper description of convection in the PMS phase.

New light on the old problem of lithium pre-main sequence depletion: models with 2D radiative-hydrodynamical convection

The T eff location of pre-main sequence (PMS) evolutionary tracks depends on the treatment of overadiabaticity. We present here the PMS evolutionary tracks computed by using the mixing length theory (MLT) of convection in which the α MLT = l/H p parameter calibration is based on 2D hydrodynamical models by Ludwig et al. These MLT-α 2D stellar models and tracks are very similar to those computed with non-grey ATLAS9 atmospheric boundary conditions and full spectrum of turbulence (FST) convection model both in the atmosphere and in the interior. The comparison of the new tracks with the location on the Hertzsprung-Russell (HR) diagram of PMS binaries is not completely satisfactory, as some binary components are located at too low T eff . Besides, the PMS lithium depletion in the MLT-α 2D tracks is still much larger than that expected from the observations of lithium in young open clusters. This result is similar to that of FST models. Thus, in spite of the fact that 2D radiative-hydrodynamical models should provide a better convection description than any local model, their introduction is not sufficient to reconcile theory and observations. Lithium depletion in young clusters points towards a convection efficiency which, in PMS, should be smaller than in the MS. The PMS lithium depletion decreases significantly in FST models if we reduce the solar metallicity down to the value suggested by Asplund et al., but the corresponding solar model does not reproduce the depth of the convective zone as determined by helioseismology.