Star Counts Research Articles

A Comparison of Rotating and Binary Stellar Evolution Models: Effects on Massive Star Populations

Abstract Both rotation and interactions with binary companions can significantly affect massive star evolution, altering interior and surface abundances, mass loss rates and mechanisms, observed temperatures and luminosities, and their ultimate core-collapse fates. The Geneva and the Binary Population and Spectral Synthesis (BPASS) stellar evolution codes include detailed treatments of the effects of rotation and binary evolution, respectively, and can illustrate the impact of these phenomena on massive stars and stellar populations. However, a direct comparison of these two widely used codes is vital if we hope to use their predictions for interpreting observations. In particular, rotating and binary models will predict different young stellar populations, impacting the outputs of stellar population synthesis (SPS) and the resulting interpretation of large samples of massive stars based on commonly used tools such as star count ratios. Here we compare the Geneva and BPASS evolutionary models, using an interpolated SPS scheme introduced in our previous work and a novel Bayesian framework to present the first in-depth direct comparison of massive stellar populations produced from single, rotating, and binary nonrotating evolution models. We calculate both models’ predicted values of star count ratios and compare the results to observations of massive stars in Westerlund 1, h + χ Persei, and both Magellanic Clouds. We also consider the limitations of both the observations and the models, and how to quantitatively include observational completeness limits in SPS models. We demonstrate that the methods presented here, when combined with robust stellar evolutionary models, offer a potential means of estimating the physical properties of massive stars in large stellar populations.

Maximum entropy estimation of the Galactic bulge morphology via the VVV Red Clump

ABSTRACT The abundance and narrow magnitude dispersion of Red Clump (RC) stars make them a popular candidate for mapping the morphology of the bulge region of the Milky Way. Using an estimate of the RC’s intrinsic luminosity function, we extracted the three-dimensional density distribution of the RC from deep photometric catalogues of the VISTA Variables in the Via Lactea (VVV) survey. We used maximum entropy-based deconvolution to extract the spatial distribution of the bulge from Ks-band star counts. We obtained our extrapolated non-parametric model of the bulge over the inner 40° × 40° region of the Galactic centre. Our reconstruction also naturally matches on to a parametric fit to the bulge outside the VVV region and inpaints overcrowded and high extinction regions. We found a range of bulge properties consistent with other recent investigations based on the VVV data. In particular, we estimated the bulge mass to be in the range $[1.3,1.7]\times 10^{10} \, \mathrm{M}_\odot$, the X-component to be between 18 per cent and 25 per cent of the bulge mass, and the bulge angle with respect to the Sun–Galactic centre line to be between 18° and 32°. Studies of the FermiLarge Area Telescope (LAT) gamma-ray Galactic centre excess suggest that the excess may be traced by Galactic bulge distributed sources. We applied our deconvolved density in a template fitting analysis of this Fermi–LAT GeV excess and found an improvement in the fit compared to previous parametric-based templates.

Structure of the outer Galactic disc with Gaia DR2

Context. The structure of outer disc of our Galaxy is still not well described, and many features need to be better understood. The second Gaia data release (DR2) provides data in unprecedented quality that can be analysed to shed some light on the outermost parts of the Milky Way. Aims. We calculate the stellar density using star counts obtained from Gaia DR2 up to a Galactocentric distance R = 20 kpc with a deconvolution technique for the parallax errors. Then we analyse the density in order to study the structure of the outer Galactic disc, mainly the warp. Methods. In order to carry out the deconvolution, we used the Lucy inversion technique for recovering the corrected star counts. We also used the Gaia luminosity function of stars with MG < 10 to extract the stellar density from the star counts. Results. The stellar density maps can be fitted by an exponential disc in the radial direction hr = 2.07 ± 0.07 kpc, with a weak dependence on the azimuth, extended up to 20 kpc without any cut-off. The flare and warp are clearly visible. The best fit of a symmetrical S-shaped warp gives zw ≈ z⊙ + (37 ± 4.2(stat.) − 0.91(syst.))pc ⋅ (R/R⊙)2.42 ± 0.76(stat.) + 0.129(syst.)sin(ϕ + 9.3° ±7.37° (stat.) + 4.48° (syst.)) for the whole population. When we analyse the northern and southern warps separately, we obtain an asymmetry of an ∼25% larger amplitude in the north. This result may be influenced by extinction because the GaiaG band is quite prone to extinction biases. However, we tested the accuracy of the extinction map we used, which shows that the extinction is determined very well in the outer disc. Nevertheless, we recall that we do not know the full extinction error, and neither do we know the systematic error of the map, which may influence the final result. The analysis was also carried out for very luminous stars alone (MG < −2), which on average represents a younger population. We obtain similar scale-length values, while the maximum amplitude of the warp is 20 − 30% larger than with the whole population. The north-south asymmetry is maintained.

Probing Axial Symmetry Breaking in the Galaxy with Gaia Data Release 2

Abstract We study a set of solar neighborhood (d < 3 kpc) stars from Gaia Data Release 2 to determine azimuthal star count differences, i.e., left and right of the line from the Galactic Center (GC) through the Sun—and compare these differences north and south. In this companion paper to Gardner et al., we delineate our procedures to remove false asymmetries from sampling effects, incompleteness, and/or interloper populations, as this is crucial to tests of axisymmetry. Particularly, we have taken care to make appropriate selections of magnitude, color, in-plane Galactocentric radius, and Galactic and . We find that requiring parallax determinations of high precision induces sampling biases, so that we eschew such requirements and exclude, e.g., regions around the lines of sight to the Magellanic Clouds, along with their mirror-image lines of sight, to ensure well-matched data sets. After making conservative cuts, we demonstrate the existence of azimuthal asymmetries and find differences in those, north and south. These asymmetries give key insights into the nature and origins of the perturbations on Galactic matter, allowing us to assess the relative influence of the Magellanic Clouds (LMC and SMC), the Galactic bar, and other masses on the Galactic mass distribution, as described in Gardner et al. The asymmetry’s radial dependence reveals variations that we attribute to the Galactic bar, and it changes sign at a radius of (0.95 ± 0.03)R 0, with R 0 the Sun–GC distance, to give us the first direct assessment of the outer Lindblad resonant radius.

Stellar Density Profiles of Dwarf Spheroidal Galaxies

Abstract We apply a flexible parametric model, a combination of generalized Plummer profiles, to infer the shapes of the stellar density profiles of the Milky Way’s satellite dwarf spheroidal galaxies (dSphs). We apply this model to 40 dSphs using star counts from the Sloan Digital Sky Survey, PanStarrs-1 Survey, Dark Energy Survey, and Dark Energy Camera Legacy Survey. Using mock data, we examine systematic errors associated with modeling assumptions and identify conditions under which our model can identify “non-standard” stellar density profiles that have central cusps and/or steepened outer slopes. Applying our model to real dwarf spheroidals, we do not find evidence for centrally cusped density profiles among the fifteen Milky Way satellites for which our tests with mock data indicate there would be sufficient detectability. We do detect steepened (with respect to a standard Plummer model) outer profiles in several dSphs—Fornax, Leo I, Leo II, and Reticulum II—which may point to distinct evolutionary pathways for these objects. However, the outer slope of the stellar density profile does not yet obviously correlate with other observed galaxy properties.

VVV WIN 1733−3349: a low extinction window to probe the far side of the Milky Way bulge

ABSTRACT Windows of low extinction in the Milky Way (MW) have been used along the past decades for the study of the Galactic structure and the stellar population across the inner bulge and disc. Here, we report the analysis of another low extinction near-IR window discovered by the VISTA Variables in the Vía Láctea Survey (VVV). VVV WIN 1733−3349 is about half a degree in size and is conveniently located right in the MW plane, at Galactic coordinates (l, b) = (−5.2, −0.3). The mean extinction of VVV WIN 1733−3349 is $A_{K_{\mathrm{ s}}}$ = 0.61 ± 0.08 mag, which is much smaller than the extinction in the surrounding area. The excess in the star counts is consistent with the reduced extinction and complemented by studying the distribution of red clump (RC) stars. Thanks to the strategic low-latitude location of VVV WIN 1733−3349, we are able to interpret their RC density fluctuations with the expected overdensities due to the presence of the spiral arms beyond the bulge. In addition, we find a clear excess in the number of microlensing events within the window, which corroborates our interpretation that VVV WIN 1733−3349 is revealing the far side of the MW bulge.

New constraints on the structure of the nuclear stellar cluster of the Milky Way from star counts and MIR imaging

Context. The Milky Way nuclear star cluster (MWNSC) is a crucial laboratory for studying the galactic nuclei of other galaxies, but its properties have not been determined unambiguously until now. Aims. We aim to study the size and spatial structure of the MWNSC. Methods. This study uses data and methods that address potential shortcomings of previous studies on the topic. We use 0.2″ angular resolution Ks data to create a stellar density map in the central 86.4 pc × 21 pc at the Galactic center. We include data from selected adaptive-optics-assisted images obtained for the inner parsecs. In addition, we use Spitzer/IRAC mid-infrared (MIR) images. We model the Galactic bulge and the nuclear stellar disk in order to subtract them from the MWNSC. Finally, we fit a Sérsic model to the MWNSC and investigate its symmetry. Results. Our results are consistent with previous work. The MWNSC is flattened with an axis ratio of q = 0.71 ± 0.10, an effective radius of Re = (5.1 ± 1.0) pc, and a Sérsic index of n = 2.2 ± 0.7. Its major axis may be tilted out of the Galactic plane by up to −10°. The distribution of the giants brighter than the Red Clump (RC) is found to be significantly flatter than the distribution of the faint stars. We investigate the 3D structure of the central stellar cusp using our results on the MWNSC structure on large scales to constrain the deprojection of the measured stellar surface number density, obtaining a value of the 3D inner power law of γ = 1.38 ± 0.06sys ± 0.01stat. Conclusions. The MWNSC shares its main properties with other extragalactic NSCs found in spiral galaxies. The differences in the structure between bright giants and RC stars might be related to the existence of not completely mixed populations of different ages. This may hint at recent growth of the MWNSC through star formation or cluster accretion.

Comparing Observed Stellar Kinematics and Surface Densities in a Low-latitude Bulge Field to Galactic Population Synthesis Models

Abstract We present an analysis of Galactic bulge stars from Hubble Space Telescope Wide Field Camera 3 observations of the Stanek window (l, b = [0.25, −2.15]) from two epochs approximately two years apart. This data set is adjacent to the provisional Wide-field Infrared Survey Telescope (WFIRST) microlensing field. Proper motions are measured for approximately 115,000 stars down to 28th mag in V band and 25th mag in I band, with accuracies of 0.5 mas yr−1 (20 km s−1) at I ≈ 21. A cut on the longitudinal proper motion μ l allows us to separate disk and bulge populations and produce bulge-only star counts that are corrected for photometric completeness and efficiency of the proper-motion cut. The kinematic dispersions and surface density in the field are compared to the nearby SWEEPS sight line, finding a marginally larger-than-expected gradient in stellar density. The observed bulge star counts and kinematics are further compared to the Besançon, Galaxia, and GalMod Galactic population synthesis models. We find that most of the models underpredict low-mass bulge stars by ∼33% below the main-sequence turnoff, and upwards of ∼70% at redder J and H wavebands. While considering inaccuracies in the Galactic models, we give implications for the exoplanet yield from the WFIRST microlensing mission.

The Density Structure of Simulated Stellar Streams

Abstract Star particles in a set of dense clusters are self-consistently evolved within an LCDM dark matter distribution with an n-body code. The clusters are started on nearly circular orbits in the more massive sub-halos. Each cluster develops a stellar tidal stream, initially within its original sub-halo. When a sub-halo merges into the main halo the early time stream is dispersed as a somewhat chaotic thick stream, roughly the width of the orbit of the cluster in the sub-halo. Once the cluster orbits freely in the main halo the star stream forms a thin stream again, usually resulting in a thin stream surrounded by a wider distribution of star particles lost at earlier times. To examine the role of the lower-mass dark matter sub-halos in the creation of density variations along the thin tidal star streams two realizations of the simulation are run with and without a normal cold dark matter sub-halo population below 4 × 108 . About 70(40)% of thin streams show density variations that are 2(5) times the star count noise level, irrespective of the presence or absence of low-mass sub-halos. A counts-in-cells analysis (related to the two-point correlation function and power spectrum) of the density along nearly 8000° of streams in the two well matched models finds that the full sub-halo population leads to slightly larger, but statistically significant, density fluctuations on scales of 2°–6°.

Weighing the stellar constituents of the galactic halo with APOGEE red giant stars

ABSTRACT The stellar mass in the halo of the Milky Way is notoriously difficult to determine, owing to the paucity of its stars in the solar neighbourhood. With tentative evidence from Gaia that the nearby stellar halo is dominated by a massive accretion event – referred to as Gaia-Enceladus or Sausage – these constraints are now increasingly urgent. We measure the mass in kinematically selected mono-abundance populations (MAPs) of the stellar halo between −3 < [Fe/H] < −1 and 0.0 < [Mg/Fe] < 0.4 using red giant star counts from APOGEE DR14. We find that MAPs are well fit by single power laws on triaxial ellipsoidal surfaces, and we show that that the power-law slope α changes such that high [Mg/Fe] populations have α ∼ 4, whereas low [Mg/Fe] MAPs are more extended with shallow slopes, α ∼ 2. We estimate the total stellar mass to be $M_{*,\mathrm{tot}} = 1.3^{+0.3}_{-0.2}\times 10^{9}\ \mathrm{M_{\odot}}$, of which we estimate ${\sim}0.9^{+0.2}_{-0.1} \times 10^{9}\ \mathrm{M_{\odot}}$ to be accreted. We estimate that the mass of accreted stars with e > 0.7 is M*,accreted, e > 0.7 = 3 ± 1 (stat.) ± 1 (syst.) × 108 M⊙, or ${\sim}30{-}50{{\ \rm per\ cent}}$ of the accreted halo mass. If the majority of these stars are the progeny of a massive accreted dwarf, this places an upper limit on its stellar mass, and implies a halo mass for the progenitor of ∼1010.2 ± 0.2 M⊙. This constraint not only shows that the Gaia-Enceladus/Sausage progenitor may not be as massive as originally suggested, but that the majority of the Milky Way stellar halo was accreted. These measurements are an important step towards fully reconstructing the assembly history of the Milky Way.

Star-density Profiles of Six Old Star Clusters in the Large Magellanic Cloud

Abstract We used resolved star counts from Hubble Space Telescope (HST) images to determine the center of gravity and the projected density profiles of six old globular clusters (GCs) in the Large Magellanic Cloud (LMC), namely NGC 1466, NGC 1841, NGC 1898, NGC 2210, NGC 2257, and Hodge 11. For each system, the LMC field contribution was properly taken into account by making use, when needed, of parallel HST observations. The derived values of the center of gravity may differ by several arcseconds (corresponding to more dal 1 pc at the distance of the LMC) from previous determinations. The cluster density profiles are all well fit by King models, with structural parameters that may differ from the literature ones by even factors of two. Similar to what was observed for Galactic GCs, the ratio between the effective and the core radius has been found to anticorrelate with the cluster dynamical age.

Inferring Galactic Parameters from Chemical Abundances: A Multi-star Approach

Constraining parameters such as the initial mass function high-mass slope and the frequency of Type Ia supernovae is of critical importance in the ongoing quest to understand galactic physics and create realistic hydrodynamical simulations. In this paper, we demonstrate a method for precisely determining these using individual chemical abundances from a large set of stars, coupled with some estimate of their ages. Inference is performed via the simple chemical evolution model Chempy in a Bayesian framework, marginalizing over each star's specific interstellar medium parameters, including an element-specific "model error" parameter to account for inadequacies in our model. Hamiltonian Monte Carlo methods are used to sample the posterior function, which is made possible by replacing Chempy with a trained neural network at negligible error. The approach is tested using data from both Chempy and the IllustrisTNG simulation, showing subpercent agreement between inferred and true parameters using data from up to 1600 individual stellar abundances. For IllustrisTNG, the strongest constraints are obtained from metal ratios, competitive with those from other methods including star counts. Analysis using a different set of nucleosynthetic yields shows that incorrectly assumed yield models can give non-negligible bias in the derived parameters; this is reduced by our model errors, which further show how well the yield tables match the data. We also find a significant bias from analyzing only a small set of stars, as is often done in current analyses. The method can be easily applied to observational data, giving tight bounds on key galactic parameters from chemical abundances alone.

The total stellar halo mass of the Milky Way

ABSTRACT We measure the total stellar halo luminosity using red giant branch (RGB) stars selected from Gaia data release 2. Using slices in magnitude, colour, and location on the sky, we decompose RGB stars belonging to the disc and halo by fitting two-dimensional Gaussians to the Galactic proper motion distributions. The number counts of RGB stars are converted to total stellar halo luminosity using a suite of isochrones weighted by age and metallicity, and by applying a volume correction based on the stellar halo density profile. Our method is tested and calibrated using Galaxia and N-body models. We find a total luminosity (out to 100 kpc) of $L_{\rm halo} = 7.9 \pm 2.0 \times 10^8\, \mathrm{L}_\odot$ excluding Sgr, and $L_{\rm halo} = 9.4 \pm 2.4 \times 10^8\, \mathrm{L}_\odot$ including Sgr. These values are appropriate for our adopted stellar halo density profile and metallicity distribution, but additional systematics related to these assumptions are quantified and discussed. Assuming a stellar mass-to-light ratio appropriate for a Kroupa initial mass function (M⋆/L = 1.5), we estimate a stellar halo mass of $M^\star _{\rm halo} = 1.4 \pm 0.4\times 10^9 \, \mathrm{M}_\odot$. This mass is larger than previous estimates in the literature, but is in good agreement with the emerging picture that the (inner) stellar halo is dominated by one massive dwarf progenitor. Finally, we argue that the combination of a ${\sim}10^9\, \mathrm{M}_\odot$ mass and an average metallicity of 〈[Fe/H]〉 ∼ −1.5 for the Galactic halo points to an ancient (∼10 Gyr) merger event.

The evolution of CNO isotopes: the impact of massive stellar rotators

ABSTRACT Chemical abundances and abundance ratios measured in galaxies provide precious information about the mechanisms, modes, and time-scales of the assembly of cosmic structures. Yet, the nucleogenesis and chemical evolution of elements heavier than helium are dictated mostly by the physics of the stars and the shape of the stellar mass spectrum. In particular, estimates of CNO isotopic abundances in the hot, dusty media of high-redshift starburst galaxies offer a unique glimpse into the shape of the stellar initial mass function (IMF) in extreme environments that cannot be accessed with direct observations (star counts). Underlying uncertainties in stellar evolution and nucleosynthesis theory, however, may hurt our chances of getting a firm grasp of the IMF in these galaxies. In this work, we adopt new yields for massive stars, covering different initial rotational velocities. First, we implement the new yield set in a well-tested chemical evolution model for the Milky Way. The calibrated model is then adapted to the specific case of a prototype submillimetre galaxy (SMG). We show that, if the formation of fast-rotating stars is favoured in the turbulent medium of violently star-forming galaxies irrespective of metallicity, the IMF needs to be skewed towards high-mass stars in order to explain the CNO isotopic ratios observed in SMGs. If, instead, stellar rotation becomes negligible beyond a given metallicity threshold, as is the case for our own Galaxy, there is no need to invoke a top-heavy IMF in starbursts.

Mapping the stellar age of the Milky Way bulge with the VVV

Context. The bulge represents the best compromise between old and massive Galactic components, and as such its study is a valuable opportunity to understand how the bulk of the Milky Way formed and evolved. In addition, being the only bulge in which we can individually resolve stars in all evolutionary sequences, the properties of its stellar content provide crucial insights into the formation of bulges.Aims. We are providing a detailed and comprehensive census of the Milky Way bulge stellar populations by producing deep and accurate photometric catalogs of the inner ∼300 deg2of the Galaxy.Methods. We performed DAOPHOT/ALLFRAME point spread function (PSF) fitting photometry of multi-epochsJandKsimages provided by the VISTA Variables in the Vía Láctea (VVV) survey to obtain deep photometric catalogs. Artificial star experiments have been conducted on all images to properly assess the completeness and the accuracy of the photometric measurements.Results. We present a photometric database containing nearly 600 million stars across the bulge area surveyed by the VVV. Through the comparison of derived color-magnitude diagrams of selected fields representative of different levels of extinction and crowding, we show the quality, completeness and depth of the new catalogs. With the exception of the fields located along the plane, this new photometry samples stars down to ∼1–2 mag below the old main sequence turnoff with unprecedented accuracy. To demonstrate the tremendous potential inherent to this new dataset, we give a few examples of possible applications, including (i) star count studies through the dataset completeness map; (ii) surface brightness map; and (iii) cross-correlation withGaiaDR2.Conclusions. The database presented here represents an invaluable collection for the whole community, and we encourage its exploitation. The photometric catalogs including completeness information are publicly available through the ESO Science Archive as part of the MW-BULGE-PSPHOT release.

Abstract 3393: Altered immune response in the transcriptome of patients with lung cancer

Abstract Introduction The immune system is critical to surveying and eradicating abnormal cells, but tumor cells develop ways to escape immunosurveillance and induce an immunosuppressive state. We previously developed and validated gene expression (GE) signatures measured in the normal airway-epithelial brushings in patients undergoing bronchoscopy for suspicion of lung cancer (LC). In this study, we seek to understand if the immunosuppressive environment extends to the airway field of injury via profiling of endobronchial biopsies from the central airway containing a broader range of cell types, including immune cells. Methods Endobronchial biopsies from normal-appearing regions of the central airway were collected from ever smokers undergoing workup of indeterminate pulmonary nodules (7-30 mm in diameter) suspicious for LC at military and VA hospitals within the DECAMP consortium. Initially, total RNA from the biopsies (n=44, discovery-set) were isolated and sequenced. Reads were aligned to hg19 using STAR and gene level counts were quantified with RSEM. Poor quality samples were removed using FASTQC and RSEQC. Differential GE associated with cancer status was identified using edgeR, adjusting for smoking-status, COPD and sample quality. RNA from additional endobronchial biopsies (n=49, validation-set) were isolated and preprocessed similarly. Genes differentially expressed with LC status in the discovery-set were tested in the validation-set using gene set variation analysis (GSVA). Functional enrichment of cancer associated genes was explored using Enrichr. Comparison of cancer signatures identified in previously published LC studies was investigated using GSEA. CIBERSORT, xCell, TIMER software and single-cell RNA sequencing data generated from airway brushings were used to deconvolute the immune cell content of the bulk biopsy samples. Results We identified a GE signature associated with LC which was significantly and concordantly enriched in the validation set of biopsies and two previously published studies of LC-associated GE in airway brushings. Genes decreased in LC patient biopsies were enriched for genes involved in immune-related pathways, including cytokine interactions, the inflammatory response and neutrophil degranulation. Computational deconvolution and comparison with single-cell RNAseq data predicts a decrease in neutrophils in the airway of LC patients. Conclusion We identified LC-associated GE alterations in smokers presenting with indeterminate pulmonary nodules. Down-regulated genes in LC subjects are strongly associated with immune system function, specifically neutrophil biology. Subjects with LC appear to have an immunosuppressive environment directed towards myeloid cell populations, and this could have implications for the future development of immunoprevention therapies. Citation Format: Kahkeshan Hijazi, Julian Lel, Ehab Billatos, Elizabeth Moses, Christopher S. Stevenson, Matthew V. Lorenzi, Gang Liu, Joshua D. Campbell, Yusuke Koga, Jiarui Zhang, Fenghai Duan, Helga Marques, Marc E. Lenburg, Avrum E. Spira, Jennifer Beane. Altered immune response in the transcriptome of patients with lung cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 3393.

Distribution of red clump stars does not support the X-shaped Galactic bulge

Context. Claims of an X-shaped Galactic bulge were based on the assumption of red clump stars as standard candles in some lines of sight crossing the off-plane bulge. However, some doubts have been cast on whether the two peaks in star counts along the line of sight really represent a double peak in the density distribution, or whether there is something wrong with the assumption of a unique constant absolute magnitude for all of these stars. Aims. With the advent of Gaia-DR2 parallaxes in combination with near-infrared VISTA-VVV data, we are able to check which of the hypotheses is correct. Methods. We calculated the median absolute magnitude MK corresponding to both peaks of putative red clumps in seven lines of sight with the lowest extinction in the interesting coordinates’ range. Results. The difference between the absolute magnitude of the bright and the faint peak is ΔMK ≈ 0.4. The selected stars in both peaks cannot be represented by the same red clump giants with constant MK ≈ −1.6. Conclusions. The hypothesis that the bulge contains an X-shape is based on the assumption that the faint and bright peaks of the density distribution towards the bulge are dominated by standard red clump stars. However, we show that both the faint and bright peaks cannot be dominated by standard red clump stars simultaneously.

An Initial Overview of the Extent and Structure of Recent Star Formation within the Serpens Molecular Cloud Using Gaia Data Release 2

Abstract The dense clusters within the Serpens Molecular Cloud are among the most active regions of nearby star formation. In this paper, we use Gaia DR2 parallaxes and proper motions to statistically measure ∼1167 kinematic members of Serpens, few of which have been previously identified, to evaluate the star formation history of the complex. The optical members of Serpens are concentrated in three distinct groups located at 380–480 pc; the densest clusters are still highly obscured by optically thick dust and have few optical members. The total population of young stars and protostars in Serpens is at least 2000 stars, including past surveys that were most sensitive to protostars and disks, and may be much higher. Distances to dark clouds measured from deficits in star counts are consistent with the distances to the optical star clusters. The Serpens Molecular Cloud is seen in the foreground of the Aquila Rift, dark clouds located at 600–700 pc, and behind patchy extinction, here called the Serpens Cirrus, located at ∼250 pc. Based on the lack of a distributed population of older stars, the star formation rate throughout the Serpens Molecular Cloud increased by at least a factor of 20 within the past ∼5 Myr. The optically bright stars in Serpens Northeast are visible because their natal molecular cloud has been eroded, not because they were flung outwards from a central factory of star formation. The separation between subclusters of 20–100 pc and the absence of an older population together lead to speculation that an external forcing was needed to trigger the active star formation.

Dynamics of the Milky Way Bar/Bulge

AbstractStellar surveys and dynamical models have recently led to important progress on understanding the dynamical structure of the Milky Way’s bar and central box/peanut bulge. This talk briefly reviews the density structure of the bulge and bar from star count tomography, the cylindrical rotation of bulge stars, and the measurements of their stellar masses and pattern speed that have been obtained by fitting dynamical models to the combined star count and line-of-sight velocity data. Recent work deriving absolute proper motions throughout the bulge from the VIRAC survey and Gaia has led to a new 3D measurement of the barred bulge kinematics which is expected to greatly improve the dynamical models, and has already confirmed the relatively slow pattern speed (∼40 kms−1 kpc−1) obtained from the previous dynamical and gas-dynamical modelling.

Unresolved Binaries and Galactic Clusters’ Mass Estimates

Abstract Binary stars are present in all stellar systems, yet their role is far from being fully understood. We investigate the effect of unresolved binaries in the derivation of open clusters’ mass by star counts. We start from the luminosity functions of five open clusters: IC 2714, NGC 1912, NGC 2099, NGC 6834, and NGC 7142. Luminosity functions are obtained via star counts extracted from the 2MASS database. The fraction of binaries is considered to be independent of stellar magnitude. We take into account different assumptions for the binary mass ratio distribution and assign binary masses using the so-called luminosity-limited pairing method and Monte-Carlo simulations. We show that cluster masses increase when binary stars are appropriately taken into account.