We revise the scenario of the formation of Galactic globular clusters (GCs) by adding the observed detailed chemical composition of their different stellar generations to the set of their global parameters. We exploit the unprecedented set of homogeneous abundances of more than 1200 red giants in 19 clusters, as well as additional data from literature, to give a new definition of bona fide GCs, as the stellar aggregates showing the Na-O anticorrelation. We propose a classification of GCs according to their kinematics and location in the Galaxy in three populations: disk/bulge, inner halo, and outer halo. We find that the luminosity function of GCs is fairly independent of their population, suggesting that it is imprinted by the formation mechanism only marginally affected by the ensuing evolution. We show that a large fraction of the primordial population should have been lost by the proto-GCs. The extremely low Al abundances found for the primordial population of massive GCs indicate a very fast enrichment process before the formation of the primordial population. We suggest a scenario for the formation of GCs that includes at least three main phases: i) the formation of a precursor population (likely due to the interaction of cosmological structures similar to those that led to the formation of dwarf spheroidals, but residing at smaller Galactocentric distances, with the early Galaxy or with other structures); ii) the triggering of a long episode of star formation (the primordial population) from the precursor population; and iii) the formation of the current GC, mainly within a cooling flow formed by the slow winds of a fraction of the primordial population. The precursor population is very effective in raising the metal content in massive and/or metal-poor (mainly halo) clusters, while its role is minor in small and/or metal-rich (mainly disk) ones. Finally, we use principal component analysis and multivariate relations to study the phase of metal enrichment from first to second generation. We conclude that most of the chemical signatures of GCs may be ascribed to a few parameters, the most important being metallicity, mass, and cluster age. Location within the Galaxy (as described by the kinematics) also plays some role, while additional parameters are required to describe their dynamical status.

/pdf/properties-of-stellar-generations-in-globular-clusters-and-4fbs6ufdn7.pdf

Properties of stellar generations in globular clusters and relations with global parameters

We present λ 1.3 mm Combined Array for Research in Millimeter-wave Astronomy observations of dust polarization toward 30 star-forming cores and eight star-forming regions from the TADPOL survey. We show maps of all sources, and compare the ~2".5 resolution TADPOL maps with ~20" resolution polarization maps from single-dish submillimeter telescopes. Here we do not attempt to interpret the detailed B-field morphology of each object. Rather, we use average B-field orientations to derive conclusions in a statistical sense from the ensemble of sources, bearing in mind that these average orientations can be quite uncertain. We discuss three main findings. (1) A subset of the sources have consistent magnetic field (B-field) orientations between large (~20") and small (~2".5) scales. Those same sources also tend to have higher fractional polarizations than the sources with inconsistent large-to-small-scale fields. We interpret this to mean that in at least some cases B-fields play a role in regulating the infall of material all the way down to the ~1000 AU scales of protostellar envelopes. (2) Outflows appear to be randomly aligned with B-fields; although, in sources with low polarization fractions there is a hint that outflows are preferentially perpendicular to small-scale B-fields, which suggests that in these sources the fields have been wrapped up by envelope rotation. (3) Finally, even at ~2".5 resolution we see the so-called polarization hole effect, where the fractional polarization drops significantly near the total intensity peak. All data are publicly available in the electronic edition of this article.

/pdf/tadpol-a-1-3-mm-survey-of-dust-polarization-in-star-forming-1lyuf9bs01.pdf

TADPOL: A 1.3 mm SURVEY OF DUST POLARIZATION IN STAR-FORMING CORES AND REGIONS

We present the results of a high angular resolution Very Large Array (VLA) Class I 44 GHz and Class II 6.7 GHz CH{sub 3}OH maser survey of a sample of {approx}20 massive young stellar object (MYSO) outflow candidates selected on the basis of extended 4.5 {mu}m emission in Spitzer Galactic Legacy Infrared Mid-Plane Survey Extraordinaire images. These 4.5 {mu}m selected candidates are referred to as extended green objects (EGOs), for the common coding of this band as green in three-color Infrared Array Camera images. The detection rate of 6.7 GHz Class II CH{sub 3}OH masers, which are associated exclusively with massive YSOs, toward EGOs is {approx}>64%-nearly double the detection rate of surveys using other MYSO selection criteria. The detection rate of Class I 44 GHz CH{sub 3}OH masers, which trace molecular outflows, is {approx}89% toward EGOs associated with 6.7 GHz CH{sub 3}OH masers. The two types of CH{sub 3}OH masers exhibit different spatial distributions: 6.7 GHz masers are centrally concentrated and usually coincide with 24 {mu}m emission, while 44 GHz masers are widely distributed and generally trace diffuse 4.5 {mu}m features. We also present results of a complementary James Clerk Maxwell Telescope (JCMT) single-pointing molecular line survey of EGOs inmore » the outflow tracers HCO{sup +}(3-2) and SiO(5-4). The HCO{sup +} line profiles and high SiO detection rate (90%) are indicative of the presence of active outflows. No 44 GHz continuum emission is detected at the 5 mJy beam{sup -1} (5{sigma}) level toward 95% of EGOs surveyed, excluding bright ultracompact H II regions as powering sources for the 4.5 {mu}m outflows. The results of our surveys constitute strong evidence that EGOs are young, massive YSOs, with active outflows, presumably powered by ongoing accretion.« less

http://iopscience.iop.org/article/10.1088/0004-637X/702/2/1615/pdf

A Class I and Class II CH3OH Maser Survey of EGOs from the GLIMPSE Survey

We present Submillimeter Array (SMA) λ = 0.88 and 1.3 mm broad-band observations, and Very Large Array (VLA) observations in NH_3 (J, K) = (1,1) up to (5,5), H_2O and CH_3OH maser lines towards the two most massive molecular clumps in infrared dark cloud (IRDC) G11.11−0.12. Sensitive high-resolution images reveal hierarchical fragmentation in dense molecular gas from the ∼1 pc clump scale down to ∼0.01 pc condensation scale. At each scale, the mass of the fragments is orders of magnitude larger than the Jeans mass. This is common to all four IRDC clumps we studied, suggesting that turbulence plays an important role in the early stages of clustered star formation. Masers, shock heated NH_3 gas, and outflows indicate intense ongoing star formation in some cores while no such signatures are found in others. Furthermore, chemical differentiation may reflect the difference in evolutionary stages among these star formation seeds. We find NH_3 ortho/para ratios of 1.1 ± 0.4, 2.0 ± 0.4, and 3.0 ± 0.7 associated with three outflows, and the ratio tends to increase along the outflows downstream. Our combined SMA and VLA observations of several IRDC clumps present the most in-depth view so far of the early stages prior to the hot core phase, revealing snapshots of physical and chemical properties at various stages along an apparent evolutionary sequence.

/pdf/hierarchical-fragmentation-and-differential-star-formation-16ti3cqkwr.pdf

Hierarchical fragmentation and differential star formation in the Galactic ‘Snake’: infrared dark cloud G11.11−0.12

We report on interferometric observations at 3.2 and 1.3 mm of the massive young stellar object IRAS 20126+4104 obtained in the C 34 Sa nd CH 3OH lines and in the continuum emission. The C 34 S data confirm the existence of a Keplerian disk, as already suggested by various authors. However, the mass of the central object is ∼7 M� , significantly less than previous estimates. We believe that such a discrepancy is due to the fact that the rotation curve is affected not only by the star but also by the mass in the innermost regions of the disk itself: this leads to an overestimate of the stellar mass when low-density tracers are used to study the velocity field over regions larger than a few seconds of arc (i.e. a few 0.01 pc). On the basis of the line profiles we speculate that accretion onto the star might be still occurring through the disk. This seems consistent with current models of high-mass star formation which predict an accretion luminosity equal to that of IRAS 20126+4104 for a 7 Mprotostar. The CH3OH lines trace both the disk and the bipolar outflow previously detected in other molecules such as HCO + ,S iO, and H 2. New H2 images obtained at 2.2 µm confirm that the outflow axis is undergoing precession. We elaborate a simple model that suitably fits the data thus allowing derivation of a few basic parameters of the precession.

/pdf/a-study-of-the-keplerian-accretion-disk-and-precessing-33fl3m6474.pdf

A study of the Keplerian accretion disk and precessing outflow in the massive protostar IRAS 20126+4104

We present a Very Large Array survey of 44 massive star-forming regions in the 44 GHz 70-61 A+ methanol transition; 37 fields showed maser emission. Thirty-one sources were also observed in the 23 GHz 92-101 A+ methanol line; two fields showed maser emission. Although the 44 GHz line is a class I maser, we find a large number of these masers in relatively close association with H II regions and water masers. Several sources show strong evidence for a correlation between 44 GHz masers and shocked molecular gas, supporting the interpretation that molecular outflows may give rise to class I maser emission. We provide maser positions with arcsecond accuracy that not only locate the masers with respect to other star formation phenomena, but also provide, for the stronger masers, phase referencing sources that can be used to calibrate future 7 mm (44 GHz) observations of these regions.

https://iopscience.iop.org/article/10.1086/423956/pdf

A Catalog of CH 3 OH 7 0 -6 1 A + Maser Sources in Massive Star-forming Regions

We have compiled the asymptotic giant, horizontal, and upper red giant branch (AGB, HB, and RGB) stars in the globular cluster M55 (NGC 6809). Using the star counts and the R-parameter we compute the initial helium abundance Y = 0.274 ± 0.016. The R2 = NAGB/NHB ratio (0.156 ± 0.023) is unusually high for a globular cluster, being almost 2 σ away from the predicted values, and the R1 = NAGB/NRGB ratio (0.272 ± 0.047) is the highest recorded for a massive globular cluster. We argue that M55's particular HB morphology and metallicity have produced long-lived HB stars that are not too blue to avoid producing AGB stars. This result hints that we are able to map evolutionary effects on the HB. Finally, although we find no evidence of variations in HB morphology with distance from the center of the cluster, the red HB stars are significantly less concentrated than the majority of HB stars, and the bluest HB stars are more centrally concentrated.

Carlos A. Vargas Álvarez

Papers

A Catalog of CH 3 OH 7 0 -6 1 A + Maser Sources in Massive Star-forming Regions

Evolved Stars in the Galactic Globular Cluster M55 (NGC 6809)