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We report the relative abundances of the three stable isotopes of silicon, $^{28}$Si, $^{29}$Si and $^{30}$Si, across the Galaxy using the $v = 0, J = 1 to 0$ transition of silicon monoxide. The chosen sources represent a range in Galactocentric radii ($R_{rm GC}$) from 0 to 9.8 kpc. The high spectral resolution and sensitivity afforded by the GBT permit isotope ratios to be corrected for optical depths. The optical-depth-corrected data indicate that the secondary-to-primary silicon isotope ratios $^{29}{rm Si}/^{28}{rm Si}$ and $^{30}{rm Si}/^{28}{rm Si}$ vary much less than predicted on the basis of other stable isotope ratio gradients across the Galaxy. Indeed, there is no detectable variation in Si isotope ratios with $R_{rm GC}$. This lack of an isotope ratio gradient stands in stark contrast to the monotonically decreasing trend with $R_{rm GC}$ exhibited by published secondary-to-primary oxygen isotope ratios. These results, when considered in the context of the expectations for chemical evolution, suggest that the reported oxygen isotope ratio trends, and perhaps that for carbon as well, require further investigation. The methods developed in this study for SiO isotopologue ratio measurements are equally applicable to Galactic oxygen, carbon and nitrogen isotope ratio measurements, and should prove useful for future observations of these isotope systems.
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Chemically tagging groups of stars born in the same birth cluster is a major goal of spectroscopic surveys. To investigate the feasibility of such strong chemical tagging, we perform a blind chemical tagging experiment on abundances measured from APOGEE survey spectra. We apply a density-based clustering algorithm to the eight dimensional chemical space defined by [Mg/Fe], [Al/Fe], [Si/Fe], [K/Fe], [Ti/Fe], [Mn/Fe], [Fe/H], and [Ni/Fe], abundances ratios which together span multiple nucleosynthetic channels. In a high quality sample of 182,538 giant stars, we detect twenty-one candidate clusters with more than fifteen members. Our candidate clusters are more chemically homogeneous than a population of non-member stars with similar [Mg/Fe] and [Fe/H], even in abundances not used for tagging. Group members are consistent with having the same age and fall along a single stellar-population track in logg vs. Teff space. Each groups members are distributed over multiple kpc, and the spread in their radial and azimuthal actions increases with age. We qualitatively reproduce this increase using N-body simulations of cluster dissolution in Galactic potentials that include transient winding spiral arms. Observing our candidate birth clusters with high-resolution spectroscopy in other wavebands to investigate their chemical homogeneity in other nucleosynthetic groups will be essential to confirming the efficacy of strong chemical tagging. Our initially spatially-compact but now widely dispersed candidate clusters will provide novel limits on chemical evolution and orbital diffusion in the Galactic disc, and constraints on star formation in loosely-bound groups.
Gaia DR2 provides unprecedented precision in measurements of the distance and kinematics of stars in the solar neighborhood. Through applying unsupervised machine learning on DR2s 5-dimensional dataset (3d position + 2d velocity), we identify a number of clusters, associations, and co-moving groups within 1 kpc and $|b|<30^circ$ (many of which have not been previously known). We estimate their ages with the precision of $sim$0.15 dex. Many of these groups appear to be filamentary or string-like, oriented in parallel to the Galactic plane, and some span hundreds of pc in length. Most of these string lack a central cluster, indicating that their filamentary structure is primordial, rather than the result of tidal stripping or dynamical processing. The youngest strings ($<$100 Myr) are orthogonal to the Local Arm. The older ones appear to be remnants of several other arm-like structures that cannot be presently traced by dust and gas. The velocity dispersion measured from the ensemble of groups and strings increase with age, suggesting a timescale for dynamical heating of $sim$300 Myr. This timescale is also consistent with the age at which the population of strings begins to decline, while the population in more compact groups continues to increase, suggesting that dynamical processes are disrupting the weakly bound string populations, leaving only individual clusters to be identified at the oldest ages. These data shed a new light on the local galactic structure and a large scale cloud collapse.
The all-Galaxy CO survey of Dame, Hartmann, & Thaddeus (2001) is by far the most uniform, large-scale Galactic CO survey. Using a dendrogram-based decomposition of this survey, we present a catalog of 1064 massive molecular clouds throughout the Galactic plane. This catalog contains $2.5 times 10^8$ solar masses, or $25^{+10.7}_{-5.8} %$ of the Milky Ways estimated H$_2$ mass. We track clouds in some spiral arms through multiple quadrants. The power index of Larsons first law, the size-linewidth relation, is consistent with 0.5 in all regions - possibly due to an observational bias - but clouds in the inner Galaxy systematically have significantly (~ 30%) higher linewidths at a given size, indicating that their linewidths are set in part by Galactic environment. The mass functions of clouds in the inner Galaxy versus the outer Galaxy are both qualitatively and quantitatively distinct. The inner Galaxy mass spectrum is best described by a truncated power-law with a power index of $gamma=-1.6pm0.1$ and an upper truncation mass $M_0 = (1.0 pm 0.2) times 10^7 M_odot$, while the outer Galaxy mass spectrum is better described by a non-truncating power law with $gamma=-2.2pm0.1$ and an upper mass $M_0 = (1.5 pm 0.5) times 10^6 M_odot$, indicating that the inner Galaxy is able to form and host substantially more massive GMCs than the outer Galaxy. Additionally, we have simulated how the Milky Way would appear in CO from extragalactic perspectives, for comparison with CO maps of other galaxies.
Star formation takes place in the dense gas phase, and therefore a simple dense gas and star formation rate relation has been proposed. With the advent of multi-beam receivers, new observations show that the deviation from linear relations is possible. In addition, different dense gas tracers might also change significantly the measurement of dense gas mass and subsequently the relation between star formation rate and dense gas mass. We report the preliminary results the DEnse GAs in MAssive star-forming regions in the Milky Way (DEGAMA) survey that observed the dense gas toward a suit of well-characterized massive star forming regions in the Milky Way. Using the resulting maps of HCO$^{+}$ 1--0, HCN 1--0, CS 2--1, we discuss the current understanding of the dense gas phase where star formation takes place.
The standard cosmological model ($Lambda$-CDM) predicts that galaxies are built through hierarchical assembly on cosmological timescales$^{1,2}$. The Milky Way, like other disc galaxies, underwent violent mergers and accretion of small satellite galaxies in its early history. Thanks to Gaia-DR2$^3$ and spectroscopic surveys$^4$, the stellar remnants of such mergers have been identified$^{5-7}$. The chronological dating of such events is crucial to uncover the formation and evolution of the Galaxy at high redshift, but it has so far been challenging owing to difficulties in obtaining precise ages for these oldest stars. Here we combine asteroseismology -- the study of stellar oscillations -- with kinematics and chemical abundances, to estimate precise stellar ages ($sim$ 11%) for a sample of stars observed by the $mathit{Kepler}$ space mission$^8$. Crucially, this sample includes not only some of the oldest stars that were formed inside the Galaxy, but also stars formed externally and subsequently accreted onto the Milky Way. Leveraging this resolution in age, we provide compelling evidence in favour of models in which the Galaxy had already formed a substantial population of its stars (which now reside mainly in its thick disc) before the in-fall of the satellite galaxy Gaia-Enceladus/Sausage$^{5,6}$ around 10 billions years ago
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