Search

To date, we have access to enormous inventories of stellar spectra that allow the extraction of atmospheric parameters and chemical abundances essential in stellar studies. However, characterizing such a large amount of data is complex and requires a good understanding of the studied object to ensure reliable and homogeneous results. In this study, we present a methodology to measure homogenously the basic atmospheric parameters and detailed chemical abundances of over 1600 thin disk main-sequence stars in the 100 pc solar neighborhood, using APOGEE-2 infrared spectra. We employed the code tonalli to determine the atmospheric parameters using a prior on log g. The log g prior in tonalli implies an understanding of the treated population and helps to find physically coherent answers. Our atmospheric parameters agree within the typical uncertainties (100 K in Teff, 0.15 dex in log g and $[M/H]$) with previous estimations of ASPCAP and Gaia DR3. We use our temperatures to determine a new infrared color-temperature sequence, in good agreement with previous works, that can be used for any main-sequence star. Additionally, we used the BACCHUS code to determine the abundances of Mg, Al, Si, Ca, and Fe in our sample. The five elements (Mg, Al, Si, Ca, Fe) studied have an abundance distribution centered around slightly sub-solar values, in agreement with previous results for the solar neighborhood. The over 1600 main-sequence stars atmospheric parameters and chemical abundances presented here are useful in follow-up studies of the solar neighborhood or as a training set for data-driven methods., Comment: 20 pages, 15 figures, 6 tables, Accepted in MNRAS for publication

We developed a grid of stellar rotation models for low-mass and solar-type Classical T Tauri stars (CTTS) ($0.3M_{\odot}

We developed a tool that measures equivalent widths of various lines in low resolution optical spectra, and it was applied to stellar spectra obtained as part of SDSS-V and LAMOST programs. These lines, such as Li I which directly indicates stellar youth, or optical H I and Ca II which in emission indicate activity associated with stellar youth, are commonly seen in YSOs. We observe several notable differences in the properties of these lines between YSOs and the field stars. Using these data, we devise a set of criteria through which it is possible to confirm the youth of stars that have been observed by the ABYSS program, as well as to identify likely young stars that have serendipitously been observed by other programs. We examine the decrement of H lines seen in emission in CTTSs, and estimate the properties of the accretion stream that is responsible for the production of these lines. Finally, we examine the evolution of Li I as a function of age, and characterize the scatter in its abundance that appears to be intrinsic in young M dwarfs., Comment: 15 pages, 10 figures, accepted to AJ

The Sloan Digital Sky Survey IV (SDSS-IV) APOGEE-2 primary science goal was to observe red giant stars throughout the Galaxy to study its dynamics, morphology, and chemical evolution. The APOGEE instrument, a high-resolution 300 fiber H-band (1.55-1.71 micron) spectrograph, is also ideal to study other stellar populations in the Galaxy, among which are a number of star forming regions and young open clusters. We present the results of the determination of six stellar properties ($T_{eff}$, $\log{g}$, [Fe/H], $L/L_\odot$, $M/M_\odot$, and ages) for a sample that is composed of 3360 young stars, of sub-solar to super-solar types, in sixteen Galactic star formation and young open cluster regions. Those sources were selected by using a clustering method that removes most of the field contamination. Samples were also refined by removing targets affected by various systematic effects of the parameter determination. The final samples are presented in a comprehensive catalog that includes all six estimated parameters. This overview study also includes parameter spatial distribution maps for all regions and Hertzprung-Russell ($L/L_\odot$ vs. $T_{eff}$) diagrams. This study serves as a guide for detailed studies on individual regions, and paves the way for the future studies on the global properties of stars in the pre-main sequence phase of stellar evolution using more robust samples., Comment: 6 figures, 4 tables

Very young (t $\lesssim$ 10 Myrs) stars possess strong magnetic fields that channel ionized gas from the interiors of their circumstellar discs to the surface of the star. Upon impacting the stellar surface, the shocked gas recombines and emits hydrogen spectral lines. To characterize the density and temperature of the gas within these accretion streams, we measure equivalent widths of Brackett (Br) 11-20 emission lines detected in 1101 APOGEE spectra of 326 likely pre-main sequence accretors. For sources with multiple observations, we measure median epoch-to-epoch line strength variations of 10% in Br11 and 20% in Br20. We also fit the measured line ratios to predictions of radiative transfer models by Kwan & Fischer. We find characteristic best-fit electron densities of $n_e$ = 10$^{11} - 10^{12}$ cm$^{-3}$, and excitation temperatures that are inversely correlated with electron density (from T$\sim$5000 K for $n_e \sim 10^{12}$ cm$^{-3}$, to T$\sim$12500 K at $n_e \sim 10^{11}$ cm$^{-3}$). These physical parameters are in good agreement with predictions from modelling of accretion streams that account for the hydrodynamics and radiative transfer within the accretion stream. We also present a supplementary catalog of line measurements from 9733 spectra of 4255 Brackett emission line sources in the APOGEE DR17 dataset., Comment: 19 pages, 9 figures, accepted to AJ

The Sloan Digital Sky Survey has recently initiated its 5th survey generation (SDSS-V), with a central focus on stellar spectroscopy. In particular, SDSS-V Milky Way Mapper program will deliver multi-epoch optical and near-infrared spectra for more than 5 million stars across the entire sky, covering a large range in stellar mass, surface temperature, evolutionary stage, and age. About 10% of those spectra will be of hot stars of OBAF spectral types, for whose analysis no established survey pipelines exist. Here we present the spectral analysis algorithm, Zeta-Payne, developed specifically to obtain stellar labels from SDSS-V spectra of stars with these spectral types and drawing on machine learning tools. We provide details of the algorithm training, its test on artificial spectra, and its validation on two control samples of real stars. Analysis with Zeta-Payne leads to only modest internal uncertainties in the near-IR with APOGEE (optical with BOSS): 3-10% (1-2%) for Teff, 5-30% (5-25%) for v*sin(i), 1.7-6.3 km/s(0.7-2.2 km/s) for RV, $<0.1$ dex ($<0.05$ dex) for log(g), and 0.4-0.5 dex (0.1 dex) for [M/H] of the star, respectively. We find a good agreement between atmospheric parameters of OBAF-type stars when inferred from their high- and low-resolution optical spectra. For most stellar labels the APOGEE spectra are (far) less informative than the BOSS spectra of these stars, while log(g), v*sin(i), and [M/H] are in most cases too uncertain for meaningful astrophysical interpretation. This makes BOSS low-resolution optical spectra better for stellar labels of OBAF-type stars, unless the latter are subject to high levels of extinction., Comment: 25 pages, 12 figures

We train a convolutional neural network, APOGEE Net, to predict $T_\mathrm{eff}$, $\log g$, and, for some stars, [Fe/H], based on the APOGEE spectra. This is the first pipeline adapted for these data that is capable of estimating these parameters in a self-consistent manner not only for low mass stars, (such as main sequence dwarfs, pre-main sequence stars, and red giants), but also high mass stars with $T_\mathrm{eff}$ in excess of 50,000 K, including hot dwarfs and blue supergiants. The catalog of ~650,000 stars presented in this paper allows for a detailed investigation of the star forming history of not just the Milky Way, but also of the Magellanic clouds, as different type of objects tracing different parts of these galaxies can be more cleanly selected through their distinct placement in $T_\mathrm{eff}$-$\log g$ parameter space than in previous APOGEE catalogs produced through different pipelines., Comment: 19 pages, 11 figures. Accepted to AJ. Data are available through https://www.sdss.org/dr17/data_access/value-added-catalogs/?vac_id=apogee-net:-a-unified-spectral-model Code is available through https://github.com/hutchresearch/ApogeeNet2

We present a large-scale study of stellar rotation for T Tauri stars in the Orion Star-Forming Complex. We use the projected rotational velocity ($v\sin(i)$) estimations reported by the APOGEE-2 collaboration as well as individual masses and ages derived from the position of the stars in the HR diagram, considering Gaia-EDR3 parallaxes and photometry plus diverse evolutionary models. We find an empirical trend for $v\sin(i)$ decreasing with age for low-mass stars ($0.4 M_{\odot} < M_{\ast} < 1.2 M_{\odot}$). Our results support the existence of a mechanism linking $v\sin(i)$ to the presence of accreting protoplanetary disks, responsible for regulating stellar rotation in timescales of about 6 Myr, which is the timescale in which most of the T Tauri stars lose their inner disk. Our results provide important constraints to models of rotation in the early phases of evolution of young stars and their disks., Comment: 24 pages, 10 figures, Accepted for publication in ApJ

We present the core mass function (CMF) of the massive star-forming clump G33.92+0.11 using 1.3 mm observations obtained with the Atacama Large Millimeter/submillimeter Array (ALMA). With a resolution of 1000 au, this is one of the highest resolution CMF measurements to date. The CMF is corrected by flux and number incompleteness to obtain a sample that is complete for gas masses $M\gtrsim2.0\ M_\odot$. The resulting CMF is well represented by a power-law function ($dN/d\log M\propto M^\Gamma$), whose slope is determined using two different approaches: $i)$ by least-squares fitting of power-law functions to the flux- and number-corrected CMF, and $ii)$ by comparing the observed CMF to simulated samples with similar incompleteness. We provide a prescription to quantify and correct a flattening bias affecting the slope fits in the first approach, which is caused by small-sample or edge effects when the data is represented by either classical histograms or a kernel density estimate, respectively. The resulting slopes from both approaches are in good agreement each other, with $\Gamma=-1.11_{-0.11}^{+0.12}$ being our adopted value. Although this slope appears to be slightly flatter than the Salpeter slope $\Gamma=-1.35$ for the stellar initial mass function (IMF), we find from Monte Carlo simulations that the CMF in G33.92+0.11 is statistically indistinguishable from the Salpeter representation of the stellar IMF. Our results are consistent with the idea that the form of the IMF is inherited from the CMF, at least at high masses and when the latter is observed at high-enough resolution., Comment: Accepted for publication in ApJ

APOGEE spectra offer $\lesssim$1 km s$^{-1}$ precision in the measurement of stellar radial velocities (RVs). This holds even when multiple stars are captured in the same spectrum, as happens most commonly with double-lined spectroscopic binaries (SB2s), although random line of sight alignments of unrelated stars can also occur. We develop a code that autonomously identifies SB2s and higher order multiples in the APOGEE spectra, resulting in 7273 candidate SB2s, 813 SB3s, and 19 SB4s. We estimate the mass ratios of binaries, and for a subset of these systems with sufficient number of measurements we perform a complete orbital fit, confirming that most systems with period $<$10 days have circularized. Overall, we find a SB2 fraction ($F_{SB2}$) $\sim$3\% among main sequence dwarfs, and that there is not a significant trend in $F_{SB2}$ with temperature of a star. We are also able to recover a higher $F_{SB2}$ in sources with lower metallicity, however there are some observational biases. We also examine light curves from TESS to determine which of these spectroscopic binaries are also eclipsing. Such systems, particularly those that are also pre- and post-main sequence, are good candidates for a follow-up analysis to determine their masses and temperatures., Comment: Accepted to AJ. 23 pages, 17 figures

We investigate a sample of 6 Herbig Ae/Be stars belonging to the Orion OB1 association, as well as 73 low mass objects, members of the $\sigma$ Orionis cluster, in order to explore the angular momentum evolution at early stages of evolution, and its possible connection with main-sequence Ap/Bp magnetic stars. Using FIES and HECTOCHELLE spectra, we obtain projected rotational velocities through two independent methods. Individual masses, radii, and ages were computed using evolutionary models, distance, and cluster extinction. Under the assumption that similar physical processes operate in both, T Tauri and Herbig Ae/Be stars, we construct snapshots of the protostar's rotation against mass during the first 10 Myr with the aid of a rotational model that includes a variable disc lifetime, changes in the stellar moment of inertia, a dipolar magnetic field with variable strength, and angular momentum loss through stellar winds powered by accretion. We use these snapshots, as well as the rotational data, to infer a plausible scenario for the angular momentum evolution. We find that magnetic field strengths of a few k$G$ at 3 Myr are required to match the rotational velocities of both groups of stars. Models with masses between 2-3 $M_{\odot}$ display larger angular momentum by a factor of $\sim 3$, in comparison to stars of similar spectral types on the main-sequence. Even though some quantitative estimates on this dramatic decrease with age, for Ap/Bp magnetic main-sequence stars are presented, the results obtained for the angular momentum evolution do not explain their low rotation., Comment: 39 pages, 18 figures, Accepted for publication in AJ

We present a catalog of 167 newly discovered, irregular variables spanning a $\sim$7 deg${^2}$ area that encompasses the G 305 star-forming complex, one of the most luminous giant H II regions in the Galaxy. We aim to unveil and characterize the young stellar object (YSO) population of the region by analyzing the $K_{\rm s}$-band variability and $JHK_{\rm s}$ infrared colors from the {\it VISTA Variables in the V\'ia L\'actea} (VVV) survey. Additionally, SDSS-IV APOGEE-2 infrared spectra of selected objects are analyzed. The sample show relatively high amplitudes ($0.661<\Delta K_{\rm S} <3.521$ mag). Most of them resemble sources with outbursts with amplitude $>1$ mag and duration longer than a few days, typically at least a year, known as {\it Eruptive Variables}. About 60% are likely to be Class II/Flat/I objects. This is also confirmed by the spectral index $\alpha$ when available. From the analysis of APOGEE-2 near-infrared spectra of sources in the region, another 122 stars are classified as YSOs, and displays some infrared variability. The measured effective temperature $T_{\rm eff}$ peak is around 4000K and they are slightly super-solar in metal abundance. The modal radial velocity is approximately $-$41 km/s. Combining available catalogs of YSOs in the region with our data, we investigate the spatial distributions of 700 YSOs. They are clearly concentrated within the central cavity formed by the massive clusters Danks 1 and 2. The calculated surface density for the entire catalog is 0.025 YSOs/pc$^{-2}$, while the central cavity contains 10 times more objects per area (0.238 YSOs/pc$^{-2}$)., Comment: 22 pages, 13 figures

(ABRIDGED) Disentangling line-of-sight alignments of young stellar populations is crucial for observational studies of star-forming complexes. This task is particularly problematic in a Cygnus-X subregion where several components, located at different distances, are overlapped: the Berkeley 87 young massive cluster, the poorly-known [DB2001] Cl05 embedded cluster, and the ON2 star-forming complex, in turn composed of several HII regions. We aim at providing a methodology for building an exhaustive census of young objects that can consistently deal with large differences in both extinction and distance. OMEGA2000 near-infrared observations of the Berkeley 87 / ON2 field are merged with archival data from Gaia, Chandra, Spitzer, and Herschel, as well as cross-identifications from the literature. To address the incompleteness effects and selection biases that arise from the line-of-sight overlap, we adapt existing methods for extinction estimation and young object classification, and we define the intrinsic reddening index, $R_\mathrm{int}$, a new tool to separate intrinsically red sources from those whose infrared color excess is caused by extinction. We also introduce a new method to find young stellar objects based on $R_\mathrm{int}$. The flexibility of our approach allows to overcome photometric biases in order to obtain homogeneous catalogs of young sources. As a result, we find 571 objects whose classification is related to recent or ongoing star formation. Together with other point sources with individual estimates of distance or extinction, we compile a catalog of 3005 objects to be used for further membership work. A new distance for Berkeley 87, ($1673 \pm 17$) pc, is estimated as a median of 13 spectroscopic members with accurate Gaia EDR3 parallaxes. Our multi-wavelength census will serve as a basis for disentangling the overlapped populations., Comment: Accepted for publication in Astronomy and Astrophysics. 19 pages, 10 figures. Full versions of Tables 2 and C.1 will be archived electronically at the CDS

We present a semi-empirical spectral classification scheme for normal B-type stars using near-infrared spectra (1.5-1.7 $\mu$m) from the SDSS APOGEE2-N DR14 database. The main motivation for working with B-type stars is their importance in the evolution of young stellar clusters, however we also take advantage of having a numerous sample (316 stars) of B-type star candidates in APOGEE2-N, for which we also have optical (3600-9100 \angstrom) counterparts from the LAMOST survey. By first obtaining an accurate spectral classification of the sources using the LAMOST DR3 spectra and the canonical spectral classification scheme Gray & Corbally 2009, we found a linear relation between optical spectral types and the equivalent widths of the hydrogen lines of the Brackett series in the APOGEE2-N NIR spectra. This relation extends smoothly from a similar relation for O and early-B stars found by Roman-Lopes et al. (2018). This way, we obtain a catalog of B-type sources with features in both the optical and NIR, and a classification scheme refined down to one spectral sub-class., Comment: 11 pages, 7 figures and 1 table

The early optical emission of gamma-ray bursts gives an opportunity to understand the central engine and first stages of these events. About 30\% of GRBs present flares whose origin is still a subject of discussion. We present optical photometry of GRB 180620A with the COATLI telescope and RATIR instrument. COATLI started to observe from the end of prompt emission at $T+39.3$~s and RATIR from $T+121.4$~s. We supplement the optical data with the X-ray light curve from \emph{Swift}/XRT. %The optical and X-ray light curves show very unusual behavior with features clearly beyond the standard fireball model. We observe an optical flare from $T+110$ to $T+550$~s, with a temporal index decay $\alpha_\mathrm{O,decay}=1.32\pm 0.01$, and a $\Delta t/t=1.63$, which we interpret as the signature of a reverse shock component. After the initial normal decay the light curves show a long plateau from $T+500$ to $T+7800$~s both in X-rays and the optical before decaying again after an achromatic jet break at $T+7800$~s. Fluctuations are seen during the plateau phase in the optical. Adding to the complexity of GRB afterglows, the plateau phase (typically associated with the coasting phase of the jet) is seen in this object after the ``normal'' decay phase (emitted during the deceleration phase of the jet) and the jet break phase occurs directly after the plateau. We suggest that this sequence of events can be explained by a rapid deceleration of the jet with $t_d\lesssim 40$ s due to the high density of the environment ($\approx 100$ cm$^{-3}$) followed by reactivation of the central engine which causes the flare and powers the plateau phase., Comment: 13 pages, 2 figures, 2 tables, submitted to ApJ

We present a comparative study of the physical properties and the spatial distribution of column density peaks in two Giant Molecular Clouds (GMC), the Pipe Nebula and Orion A, which exemplify opposite cases of star cluster formation stages. The density peaks were extracted from dust extinction maps constructed from Herschel/SPIRE farinfrared images. We compare the distribution functions for dust temperature, mass, equivalent radius and mean volume density of peaks in both clouds, and made a more fair comparison by isolating the less active Tail region in Orion A and by convolving the Pipe Nebula map to simulate placing it at a distance similar to that of the Orion Complex. The peak mass distributions for Orion A, the Tail, and the convolved Pipe, have similar ranges, sharing a maximum near 5 M$_\odot$, and a similar power law drop above 10 M$_\odot$. Despite the clearly distinct evolutive stage of the clouds, there are very important similarities in the physical and spatial distribution properties of the column density peaks, pointing to a scenario where they form as a result of uniform fragmentation of filamentary structures across the various scales of the cloud, with density being the parameter leading the fragmentation, and with clustering being a direct result of thermal fragmentation at different spatial scales. Our work strongly supports the idea that the formation of clusters in GMC could be the result of the primordial organization of pre-stellar material, Comment: 17 pages, 14 figures. Accepted for publication in Monthly Notices of the Royal Astronomical Society

The purpose of this study is the identification of young ($1< age < 100$ Myr), nearby ($d \leqslant100$ pc) moving groups (YNMGs) through their kinematic signature. YNMGs could be the result of the recent dispersal of young embedded clusters, such that they still represent kinematically cold groups, carrying the residual motion of their parental cloud. Using the fact that a large number ($\sim$ 14000) of the RAVE sources with evidence of chromospheric activity, also present signatures of stellar youth, we selected a sample of solar type sources with the highest probability of chromospheric activity to look for common kinematics. We made use of radial velocity information from RAVE and astrometric parameters from GAIA DR2 to construct a 6-dimension position-velocity vector catalog for our full sample. We developed a method based on the grouping of stars with similar orientation of their velocity vectors, which we call the Cone Method Sampling. Using this method, we detected 646 sources with high significance in the velocity space, with respect to the average orientation of artificial distributions made from a purely Gaussian velocity ellipsoid with null vertex deviation. We compared this sample of highly significant sources with a catalog of YNMGs reported in previous studies, which yield 75 confirmed members. From the remaining sample, about 50% of the sources have ages younger than 100 Myr, which indicate they are highly probable candidates to be new members of identified or even other YNMGs in the solar neighborhood., Comment: Complete tables (table 1 and table 2) will be available in the electronic version

We report new, $\sim$1000 AU spatial resolution observations of 225 GHz dust continuum emission towards the OB cluster-forming molecular clump G33.92+0.11. On parsec scales, this molecular clump presents a morphology with several arm-like dense gas structures surrounding the two central massive ($\gtrsim$100 $M_{\odot}$) cores. From the new, higher resolution observations, we identified 28 localized, spatially compact dust continuum emission sources, which may be candidates of young stellar objects. Only one of them is not embedded within known arm-like (or elongated) dense gas structures. The spatial separations of these compact sources can be very well explained by Jeans lengths. We found that G33.92+0.11 may be consistently described by a marginally centrifugally supported, Toomre unstable accretion flow which is approximately in a face-on projection. The arm-like overdensities are natural consequence of the Toomre instability, which can fragment to form young stellar objects in shorter time scales than the timescale of the global clump contraction. On our resolved spatial scales, there is not yet evidence that the fragmentation is halted by turbulence, magnetic field, or stellar feedback., Comment: 24 pages, 18 figures. Accepted to publish on December 04, 2018; updated to arXiv on December 05, 2018

Wide-field optical surveys have begun to uncover large samples of fast (t_rise < 5d), luminous (M_peak < -18), blue transients. While commonly attributed to the breakout of a supernova shock into a dense wind, the great distances to the transients of this class found so far have hampered detailed investigation of their properties. We present photometry and spectroscopy from a comprehensive worldwide campaign to observe AT2018cow (ATLAS18qqn), the first fast-luminous optical transient to be found in real time at low redshift. Our first spectra (<2 days after discovery) are entirely featureless. A very broad absorption feature suggestive of near-relativistic velocities develops between 3-8 days, then disappears. Broad emission features of H and He develop after >10 days. The spectrum remains extremely hot throughout its evolution, and the photospheric radius contracts with time (receding below R<10^14 cm after 1 month). This behaviour does not match that of any known supernova, although a relativistic jet within a fallback supernova could explain some of the observed features. Alternatively, the transient could originate from the disruption of a star by an intermediate-mass black hole, although this would require long-lasting emission of highly super-Eddington thermal radiation. In either case, AT2018cow suggests that the population of fast luminous transients represents a new class of astrophysical event. Intensive follow-up of this event in its late phases, and of any future events found at comparable distance, will be essential to better constrain their origins., Comment: Corrected Figure 8 / Table 4 to use final fits. Includes machine-readable photometry table (hopefully for real this time)

We present subarcsecond 1.3 mm continuum ALMA observations towards the Orion Molecular Cloud 1 South (OMC-1S) region, down to a spatial resolution of 74 AU, which reveal a total of 31 continuum sources. We also present subarcsecond 7 mm continuum VLA observations of the same region, which allow to further study fragmentation down to a spatial resolution of 40 AU. By applying a Mean Surface Density of Companions method we find a characteristic spatial scale at ~560 AU, and we use this spatial scale to define the boundary of 19 `cores' in OMC-1S as groupings of millimeter sources. We find an additional characteristic spatial scale at ~2900 AU, which is the typical scale of the filaments in OMC-1S, suggesting a two-level fragmentation process. We measured the fragmentation level within each core and find a higher fragmentation towards the southern filament. In addition, the cores of the southern filament are also the densest (within 1100 AU) cores in OMC-1S. This is fully consistent with previous studies of fragmentation at spatial scales one order of magnitude larger, and suggests that fragmentation down to 40 AU seems to be governed by thermal Jeans processes in OMC-1S., Comment: Accepted to ApJ

Recently we have witnessed the first multi-messenger detection of colliding neutron stars through Gravitational Waves (GWs) and Electromagnetic (EM) waves (GW170817), thanks to the joint efforts of LIGO/Virgo and Space/Ground-based telescopes. In this paper, we report on the RATIR followup observation strategies and show the results for the trigger G194575. This trigger is not of astrophysical interest; however, is of great interests to the robust design of a followup engine to explore large sky error regions. We discuss the development of an image-subtraction pipeline for the 6-color, optical/NIR imaging camera RATIR. Considering a two band ($i$ and $r$) campaign in the Fall of 2015, we find that the requirement of simultaneous detection in both bands leads to a factor $\sim$10 reduction in false alarm rate, which can be further reduced using additional bands. We also show that the performance of our proposed algorithm is robust to fluctuating observing conditions, maintaining a low false alarm rate with a modest decrease in system efficiency that can be overcome utilizing repeat visits. Expanding our pipeline to search for either optical or NIR detections (3 or more bands), considering separately the optical $riZ$ and NIR $YJH$ bands, should result in a false alarm rate $\approx 1\%$ and an efficiency $\approx 90\%$. RATIR's simultaneous optical/NIR observations are expected to yield about one candidate transient in the vast 100 $\mathrm{deg^2}$ LIGO error region for prioritized followup with larger aperture telescopes., Comment: 10 pages, 8 figures, 1 table, Accepted to AJ

DDOTI will be a wide-field robotic imager consisting of six 28-cm telescopes with prime focus CCDs mounted on a common equatorial mount. Each telescope will have a field of view of 12 square degrees, will have 2 arcsec pixels, and will reach a 10-sigma limiting magnitude in 60 seconds of r = 18.7 in dark time and r = 18.0 in bright time. The set of six will provide an instantaneous field of view of about 72 square degrees. DDOTI uses commercial components almost entirely. The first DDOTI will be installed at the Observatorio Astron\'omico Nacional in Sierra San Pedro Mart\'ir, Baja California, M\'exico in early 2017. The main science goals of DDOTI are the localization of the optical transients associated with GRBs detected by the GBM instrument on the Fermi satellite and with gravitational-wave transients. DDOTI will also be used for studies of AGN and YSO variability and to determine the occurrence of hot Jupiters. The principal advantage of DDOTI compared to other similar projects is cost: a single DDOTI installation costs only about US$500,000. This makes it possible to contemplate a global network of DDOTI installations. Such geographic diversity would give earlier access and a higher localization rate. We are actively exploring this option., Comment: To appear in the proceedings of SPIE conference 9910 "Observatory Operations: Strategies, Processes, and Systems VI". 12 pages

COATLI will provide 0.3 arcsec FWHM images from 550 to 900 nm over a large fraction of the sky. It consists of a robotic 50-cm telescope with a diffraction-limited fast-guiding imager. Since the telescope is small, fast guiding will provide diffraction-limited image quality over a field of at least 1 arcmin and with coverage of a large fraction of the sky, even in relatively poor seeing. The COATLI telescope will be installed at the at the Observatorio Astron\'omico Nacional in Sierra San Pedro M\'artir, M\'exico, during 2016 and the diffraction-limited imager will follow in 2017., Comment: To appear in the proceedings of SPIE conference 9908 "Ground-based and Airborne Instrumentation for Astronomy VI". 9 pages

Orion A hosts the nearest massive star factory, thus offering a unique opportunity to resolve the processes connected with the formation of both low- and high-mass stars. Here we present the most detailed and sensitive near-infrared (NIR) observations of the entire molecular cloud to date. With the unique combination of high image quality, survey coverage, and sensitivity, our NIR survey of Orion A aims at establishing a solid empirical foundation for further studies of this important cloud. In this first paper we present the observations, data reduction, and source catalog generation. To demonstrate the data quality, we present a first application of our catalog to estimate the number of stars currently forming inside Orion A and to verify the existence of a more evolved young foreground population. We used the European Southern Observatory's (ESO) Visible and Infrared Survey Telescope for Astronomy (VISTA) to survey the entire Orion A molecular cloud in the NIR $J, H$, and $K_S$ bands, covering a total of $\sim$18.3 deg$^2$. We implemented all data reduction recipes independently of the ESO pipeline. Estimates of the young populations toward Orion A are derived via the $K_S$-band luminosity function. Our catalog (799995 sources) increases the source counts compared to the Two Micron All Sky Survey by about an order of magnitude. The 90% completeness limits are 20.4, 19.9, and 19.0 mag in $J, H$, and $K_S$, respectively. The reduced images have 20% better resolution on average compared to pipeline products. We find between 2300 and 3000 embedded objects in Orion A and confirm that there is an extended foreground population above the Galactic field, in agreement with previous work. The Orion A VISTA catalog represents the most detailed NIR view of the nearest massive star-forming region and provides a fundamental basis for future studies of star formation processes toward Orion., Comment: Accepted for publication in A&A, high-quality version available at http://homepage.univie.ac.at/stefan.meingast/vision.pdf

We present a multi-wavelength study of the young stellar population in the Cygnus-X DR15 region. We studied young stars forming or recently formed at and around the tip of a prominent molecular pillar and an infrared dark cloud. Using a combination of ground based near-infrared, space based infrared and X-ray data, we constructed a point source catalog from which we identified 226 young stellar sources, which we classified into evolutionary classes. We studied their spatial distribution across the molecular gas structures and identified several groups possibly belonging to distinct young star clusters. We obtained samples of these groups and constructed K-band luminosity functions that we compared with those of artificial clusters, allowing us to make first order estimates of the mean ages and age spreads of the groups. We used a $^{13}$CO(1-0) map to investigate the gas kinematics at the prominent gaseous envelope of the central cluster in DR15, and we infer that the removal of this envelope is relatively slow compared to other cluster regions, in which gas dispersal timescale could be similar or shorter than the circumstellar disk dissipation timescale. The presence of other groups with slightly older ages, associated with much less prominent gaseous structures may imply that the evolution of young clusters in this part of the complex proceeds in periods that last 3 to 5 Myr, perhaps after a slow dissipation of their dense molecular cloud birthplaces., Comment: 35 pages, 10 figures. Accepted for Publication in The Astronomical Journal

Context. The Pipe nebula is a molecular cloud that lacks star formation feedback and has a relatively simple morphology and velocity structure. This makes it an ideal target to test cloud evolution through collisions. Aims. We aim at drawing a comprehensive picture of this relatively simple cloud to better understand the formation and evolution of molecular clouds on large scales. Methods. We use archival data to compare the optical polarization properties, the visual extinction, and the 13CO velocities and linewidths of the entire cloud in order to identify trends among the observables. Results. The Pipe nebula can be roughly divided in two filaments with different orientations and gas velocity ranges: E-W at 2-4 km s-1 and N-S at 6-7 km s-1. The two filaments overlap at the bowl, where the gas shows a velocity gradient spanning from 2 to 7 km s-1. Compared to the rest of the Pipe nebula, the bowl gas appears to be denser and exhibits larger linewidths. In addition, the polarization data at the bowl shows lower angular dispersion and higher polarization degree. Cores in the bowl tend to cluster in space and tend to follow the 13CO velocity gradient. In the stem, cores tend to cluster in regions with properties similar to those of the bowl. Conclusions. The velocity pattern points to a collision between the filaments in the bowl region. The magnetic field seems to be compressed and strengthened in the shocked region. The proportional increase of density and magnetic field strength by a factor similar to the Alfv\'enic Mach number suggests a continuous shock at low Alfv\'enic Mach number under flux-freezing. Shocked regions seem to enhance the formation and clustering of dense cores., Comment: 5 pages, 2 figures. Accepted for publication in A&A

The infrared data from the Spitzer Space Telescope has provided an invaluable tool for identifying physical processes in star formation. In this study we calculate the IRAC color space of UV fluorescent molecular hydrogen (H$_2$) and Polycyclic Aromatic Hydrocarbon (PAH) emission in photodissociation regions (PDRs) using the Cloudy code with PAH opacities from Draine & Li 2007. We create a set of color diagnostics that can be applied to study the structure of PDRs and to distinguish between FUV excited and shock excited H$_2$ emission. To test this method we apply these diagnostics to Spitzer IRAC data of NGC 2316. Our analysis of the structure of the PDR is consistent with previous studies of the region. In addition to UV excited emission, we identify shocked gas that may be part of an outflow originating from the cluster., Comment: 5 pages, Accepted for publication in the Astrophysical Journal Letters

Sh 2-29 is a conspicuous star-forming region marked by the presence of massive embedded stars as well as several notable interstellar structures. In this research, our goals were to determine the role of magnetic fields and to study the size distribution of interstellar dust particles within this turbulent environment. We have used a set of optical and near-infrared polarimetric data obtained at OPD/LNA (Brazil) and CTIO (Chile), correlated with extinction maps, 2MASS data and images from DSS and Spitzer. The region's most striking feature is a swept out interstellar cavity whose polarimetric maps indicate that magnetic field lines were dragged outwards, pilling up along its borders. This led to a higher magnetic strength value ($\approx400\,\mu$G) and an abrupt increase in polarization degree, probably due to an enhancement in alignment efficiency. Furthermore, dense cloud fragmentations with peak $A_{V}$ between 20 and 37 mag were probably triggered by its expansion. The presence of $24\,\mu$m point-like sources indicates possible newborn stars inside this dense environment. A statistical analysis of the angular dispersion function revealed areas where field lines are aligned in a well-ordered pattern, seemingly due to compression effects from the HII region expansion. Finally, Serkowski function fits were used to study the ratio of the total-to-selective extinction, reveling a dual population of anomalous grain particles sizes. This trend suggests that both effects of coagulation and fragmentation of interstellar grains are present in the region., Comment: 20 pages, 12 figures, 2 tables; Accepted for The Astrophysical Journal

Using Spitzer Space Telescope and Chandra X-ray Observatory data, we identify YSOs in the Rosette Molecular Cloud (RMC). By being able to select cluster members and classify them into YSO types, we are able to track the progression of star formation locally within the cluster environments and globally within the cloud. We employ nearest neighbor method (NNM) analysis to explore the density structure of the clusters and YSO ratio mapping to study age progressions in the cloud. We find a relationship between the YSO ratios and extinction which suggests star formation occurs preferentially in the densest parts of the cloud and that the column density of gas rapidly decreases as the region evolves. This suggests rapid removal of gas may account for the low star formation efficiencies observed in molecular clouds. We find that the overall age spread across the RMC is small. Our analysis suggests that star formation started throughout the complex around the same time. Age gradients in the cloud appear to be localized and any effect the HII region has on the star formation history is secondary to that of the primordial collapse of the cloud., Comment: Accepted for publication in The Astrophysical Journal

We present a study of dense structures in the L1495 filament in the Taurus Molecular Cloud and examine its star-forming properties. In particular we construct a dust extinction map of the filament using deep near-infrared observations, exposing its small-scale structure in unprecedented detail. The filament shows highly fragmented substructures and a high mass-per-length value of M_line=17 Msun/pc, reflecting star-forming potential in all parts of it. However, a part of the filament, namely B211, is remarkably devoid of young stellar objects. We argue that in this region the initial filament collapse and fragmentation is still taking place and star formation is yet to occur. In the star-forming part of the filament, we identify 39 cores with masses from 0.4...10 Msun and preferred separations in agreement with the local Jeans length. Most of these cores exceed the Bonnor-Ebert critical mass, and are therefore likely to collapse and form stars. The Dense Core Mass Function follows a power law with exponent Gamma=1.2, a form commonly observed in star-forming regions., Comment: 11 pages, 13 figures, accepted for publication in ApJ

We present Chandra X-ray images of the NGC 2237 young star cluster on the periphery of the Rosette Nebula. We detect 168 X-ray sources, 80% of which have counterparts in USNO, 2MASS, and deep FLAMINGOS images. These constitute the first census of the cluster members with 0.2<~M<~2 Msun. Star locations in near-infrared color-magnitude diagrams indicate a cluster age around 2 Myr with a visual extinction of 1

We present our analysis of a fully sampled, high resolution dust extinction map of the Barnard 59 complex in the Pipe Nebula. The map was constructed with the infrared color excess technique applied to a photometric catalog that combines data from both ground and space based observations. The map resolves for the first time the high density center of the main core in the complex, that is associated with the formation of a small cluster of stars. We found that the central core in Barnard 59 shows an unexpected lack of significant substructure consisting of only two significant fragments. Overall, the material appears to be consistent with being a single, large core with a density profile that can be well fit by a King model. A series of NH$_3$ pointed observations towards the high column density center of the core appear to show that the core is still thermally dominated, with sub-sonic non-thermal motions. The stars in the cluster could be providing feedback to support the core against collapse, but the relatively narrow radio lines suggest that an additional source of support, for example a magnetic field, may be required to stabilize the core. Outside the central core our observations reveal the structure of peripheral cores and resolve an extended filament into a handful of significant substructures whose spacing and masses appear to be consistent with Jeans fragmentation., Comment: Accepted to ApJ, DOP October 2009. 14 pages, 11 figures, 3 tables

We present arcsecond resolution mid-infrared and millimeter observations of the center of the young stellar cluster AFGL961 in the Rosette molecular cloud. Within 0.2 pc of each other, we find an early B star embedded in a dense core, a neighboring star of similar luminosity with no millimeter counterpart, a protostar that has cleared out a cavity in the circumcluster envelope, and two massive, dense cores with no infrared counterparts. An outflow emanates from one of these cores, indicating a deeply embedded protostar, but the other is starless, bound, and appears to be collapsing. The diversity of states implies either that protostellar evolution is faster in clusters than in isolation or that clusters form via quasi-static rather than dynamic collapse. The existence of a pre-stellar core at the cluster center shows that that some star formation continues after and in close proximity to massive, ionizing stars., Comment: 22 pages, 7 figures, accepted for publication in ApJ

We explore here the young stellar populations in the Rosette Molecular Cloud (RMC) region with high spatial resolution X-ray images from the Chandra X-ray Observatory, which are effective in locating weak-lined T Tauri stars as well as disk-bearing young stars. A total of 395 X-ray point sources are detected, 299 of which (76%) have an optical or near-infrared (NIR) counterpart identified from deep FLAMINGOS images. From X-ray and mass sensitivity limits, we infer a total population of about 1700 young stars in the survey region. Based on smoothed stellar surface density maps, we investigate the spatial distribution of the X-ray sources and define three distinctive structures and substructures within them. Structures B and C are associated with previously known embedded IR clusters, while structure A is a new X-ray-identified unobscured cluster. A high mass protostar RMCX #89 = IRAS 06306+0437 and its associated sparse cluster is studied. The different subregions are not coeval but do not show a simple spatial-age pattern. Disk fractions vary between subregions and are generally 20% of the total stellar population inferred from the X-ray survey. The data are consistent with speculations that triggered star formation around the HII region is present in the RMC, but do not support a simple sequential triggering process through the cloud interior. While a significant fraction of young stars are located in a distributed population throughout the RMC region, it is not clear they originated in clustered environments., Comment: Accepted to the Astrophysical Journal. 49 pages, 12 figures. For a version with high-quality figures, see http://www.astro.psu.edu/users/edf/RMC_accepted.pdf

The Rosette Complex in the constellation of Monoceros is a magnificent laboratory for the study of star formation. The region presents an interesting scenario, in which an expanding HII region generated by the large OB association NGC 2244 is interacting with a giant molecular cloud. Inside the cloud a number of stellar clusters have formed recently. In this chapter we present a review of past and present research on the region, and discuss investigations relevant to the physics of the nebula and the molecular cloud. We also review recent work on the younger embedded clusters and individual nebulous objects located across this important star forming region., Comment: 38 pages, 21 figures. Chapter for the book "Handbook of Star Forming Regions, Vol. I: The Northern Hemisphere. Ed. Bo Reipurth, ASP 2008"