We review the wide range of observed properties of Herbig Ae/Be stars and try to combine this rich data set into a consistent picture of their circumstellar environment and evolutionary status. We discuss in some detail the geometry of the circumstellar environment. The presence of disks in Herbig Ae/Be stars is inferred from direct and indirect observational evidence. Envelopes can dominate the spectral energy distributions on large angular scales. In some stars, a fairly massive disk persists until the star has reached the main sequence. The evidence for an evolutionary link between isolated Herbig Ae/Be stars and fl Pictoris is summarized.

HERBIG Ae/Be STARS

We present a new classification method for quasar identification in the EROS-2 and MACHO data sets based on a boosted version of a random forest classifier. We use a set of variability features including parameters of a continuous autoregressive model. We prove that continuous autoregressive parameters are very important discriminators in the classification process. We create two training sets (one for EROS-2 and one for MACHO data sets) using known quasars found in the Large Magellanic Cloud (LMC). Our model's accuracy in both EROS-2 and MACHO training sets is about 90 per cent precision and 86 per cent recall, improving the state-of-the-art models, accuracy in quasar detection. We apply the model on the complete, including 28 million objects, EROS-2 and MACHO LMC data sets, finding 1160 and 2551 candidates, respectively. To further validate our list of candidates, we cross-matched our list with 663 previously known strong candidates, getting 74 per cent of matches for MACHO and 40 per cent in EROS.



The main difference on matching level is because EROS-2 is a slightly shallower survey which translates to significantly lower signal-to-noise ratio light curves.

An improved quasar detection method in EROS-2 and MACHO LMC data sets

We present a new classification method for quasar identification in the EROS-2 and MACHO datasets based on a boosted version of Random Forest classifier. We use a set of variability features including parameters of a continuous auto regressive model. We prove that continuous auto regressive parameters are very important discriminators in the classification process. We create two training sets (one for EROS-2 and one for MACHO datasets) using known quasars found in the LMC. Our model's accuracy in both EROS-2 and MACHO training sets is about 90% precision and 86% recall, improving the state of the art models accuracy in quasar detection. We apply the model on the complete, including 28 million objects, EROS-2 and MACHO LMC datasets, finding 1160 and 2551 candidates respectively. To further validate our list of candidates, we crossmatched our list with a previous 663 known strong candidates, getting 74% of matches for MACHO and 40% in EROS-2. The main difference on matching level is because EROS-2 is a slightly shallower survey which translates to significantly lower signal-to-noise ratio lightcurves.

An improved quasar detection method in EROS-2 and MACHO LMC datasets

We present optical spectra of pre-main-sequence (PMS) candidates around the H? region taken with the Southern African Large Telescope in the low metallicity (Z) Galactic region Sh 2-284, which includes the open cluster Dolidze 25 with an atypical low metallicity of Z ~ 1/5 Z ?. It has been suggested on the basis of both theory and observations that PMS mass-accretion rates, , are a function of Z. We present the first sample of spectroscopic estimates of mass-accretion rates for PMS stars in any low-Z star-forming region. Our data set was enlarged with literature data of H? emission in intermediate-resolution R-band spectroscopy. Our total sample includes 24 objects spanning a mass range between 1 and 2 M ? and with a median age of approximately 3.5?Myr. The vast majority (21 out of 24) show evidence for a circumstellar disk on the basis of Two Micron All Sky Survey and Spitzer infrared photometry. We find in the 1-2 M ? interval to depend quasi-quadratically on stellar mass, with ??, and inversely with stellar age, with ??. Furthermore, we compare our spectroscopic measurements with solar Z Galactic PMS stars in the same mass range, but, surprisingly find no evidence for a systematic change in with Z. We show that literature accretion-rate studies are influenced by detection limits, and we suggest that may be controlled by factors other than Z *, M *, and age.

http://iopscience.iop.org/article/10.1088/0004-637X/800/2/113/pdf

Pre-Main-Sequence Accretion in the Low Metallicity Galactic Star-forming Region Sh 2-284

We present optical spectra of pre-main sequence (PMS) candidates around the H$\alpha$ region taken with the Southern African Large Telescope, SALT, in the low metallicity ($Z$) Galactic region Sh 2-284, which includes the open cluster Dolidze 25 with an atypical low metallicity of $Z$ $\sim$ 1/5 $Z_{\odot}$. It has been suggested on the basis of both theory and observations that PMS mass-accretion rates, $\dot M_{\rm{acc}}$, are a function of $Z$. We present the first sample of spectroscopic estimates of mass-accretion rates for PMS stars in any low-$Z$ star-forming region. Our data-set was enlarged with literature data of H$\alpha$ emission in intermediate-resolution R-band spectroscopy. Our total sample includes 24 objects spanning a mass range between 1 - 2 $M_{\odot}$ and with a median age of approximately 3.5 Myr. The vast majority (21 out of 24) show evidence for a circumstellar disk on the basis of 2MASS and Spitzer infrared photometry. We find $\dot M_{\rm{acc}}$ in the 1 - 2 $M_{\odot}$ interval to depend quasi-quadratically on stellar mass, with $\dot M_{\rm{acc}}$ $\propto$ $M_{\ast}^{2.4\,\pm\,0.35}$, and inversely with stellar age $\dot M_{\rm{acc}}$ $\propto$ $t_{\ast}^{-\,0.7\,\pm\,0.4}$. Furthermore, we compare our spectroscopic $\dot M_{\rm{acc}}$ measurements with solar $Z$ Galactic PMS stars in the same mass range, but, surprisingly find no evidence for a systematic change in $\dot M_{\rm{acc}}$ with $Z$. We show that literature accretion-rate studies are influenced by detection limits, and we suggest that $\dot M_{\rm{acc}}$ may be controlled by factors other than $Z_{\ast}$, $M_{\ast}$, and age.

Pre-main sequence accretion in the low metallicity Galactic star-forming region Sh 2-284

Candidate pre-main-sequence stars were observed in the bar of the Large Magellanic Cloud during the search for dark matter in the galactic halo. Seven blue stars of apparent visual magnitude 15 to 17 had irregular photometric variations and hydrogen emission lines in their optical spectra, which suggested that these stars are pre-main-sequence stars of about 10 solar masses. These stars are slightly more massive and definitely more luminous than are Herbig AeBe pre-main-sequence stars in our own galaxy. Continued observations of these very young stars from another galaxy, which are probably at the pre-hydrogen-burning stage, should provide important clues about early stages of star formation.

https://science.sciencemag.org/content/sci/272/5264/995.full.pdf

Pre-main-sequence star candidates in the bar of the Large Magellanic Cloud

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Pre-main-sequence star candidates in the bar of the Large Magellanic Cloud