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Register a new userComplex cyanides as chemical clocks in hot cores
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In the high-mass star-forming region G35.20-0.74N, small scale (about 800 AU) chemical segregation has been observed in which complex organic molecules containing the CN group are located in a small location. We aim to determine the physical origin of the large abundance difference (about 4 orders of magnitude) in complex cyanides within G35.20-0.74 B, and we explore variations in age, gas and dust temperature, and gas density. We performed gas-grain astrochemical modeling experiments with exponentially increasing (coupled) gas and dust temperature rising from 10 to 500 K at constant H$_2$ densities of 10$^7$, 10$^8$, and 10$^9$ cm$^{-3}$. We tested the effect of varying the initial ice composition, cosmic-ray ionization rate, warm-up time (over 50, 200, and 1000 kyr), and initial (10, 15, and 25 K) and final temperatures (300 and 500 K). Varying the initial ice compositions within the observed and expected ranges does not noticeably affect the modeled abundances indicating that the chemical make-up of hot cores is determined in the warm-up stage. Complex cyanides vinyl and ethyl cyanide (CH$_2$CHCN and C$_2$H$_5$CN, respectively) cannot be produced in abundances (versus H$_2$) greater than 5x10$^{-10}$ for CH$_2$CHCN and 2x10$^{-10}$ for C$_2$H$_5$CN with a fast warm-up time (52 kyr), while the lower limit for the observed abundance of C$_2$H$_5$CN toward source B3 is 3.4x10$^{-10}$. Complex cyanide abundances are reduced at higher initial temperatures and increased at higher cosmic-ray ionization rates. Reproducing the observed abundances toward G35.20-0.74 Core B3 requires a fast warm-up at a high cosmic-ray ionization rate (1x10$^{-16}$ s$^{-1}$) at a high gas density (>10$^9$ cm$^{-3}$). G35.20-0.74 source B3 only needs to be about 2000 years older than B1/B2 for the observed chemical difference to be present. (This abstract has been shortened)
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Investigations of the origin and evolution of the Milky Way disk have long relied on chemical and kinematic identification of its components to reconstruct our Galactic past. Difficulties in determining precise stellar ages have restricted most studies to small samples, normally confined to the solar neighbourhood. Here we break this impasse with the help of asteroseismic inference and perform a chronology of the evolution of the disk throughout the age of the Galaxy. We chemically dissect the Milky Way disk population using a sample of red giant stars spanning out to 2~kpc in the solar annulus observed by the {it Kepler} satellite, with the added dimension of asteroseismic ages. Our results reveal a clear difference in age between the low- and high-$alpha$ populations, which also show distinct velocity dispersions in the $V$ and $W$ components. We find no tight correlation between age and metallicity nor [$alpha$/Fe] for the high-$alpha$ disk stars. Our results indicate that this component formed over a period of more than 2~Gyr with a wide range of [M/H] and [$alpha$/Fe] independent of time. Our findings show that the kinematic properties of young $alpha$-rich stars are consistent with the rest of the high-$alpha$ population and different from the low-$alpha$ stars of similar age, rendering support to their origin being old stars that went through a mass transfer or stellar merger event, making them appear younger, instead of migration of truly young stars formed close to the Galactic bar.
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We aim to reveal the physical properties and chemical composition of the cores in the California molecular cloud (CMC), so as to better understand the initial conditions of star formation. We made a high-resolution column density map (18.2) with Herschel data, and extracted a complete sample of the cores in the CMC with the textsl{fellwalker} algorithm. We performed new single-pointing observations of molecular lines near 90 GHz with the IRAM 30m telescope along the main filament of the CMC. In addition, we also performed a numerical modeling of chemical evolution for the cores under the physical conditions. We extracted 300 cores, of which 33 are protostellar and 267 are starless cores. About 51% (137 of 267) of the starless cores are prestellar cores. Three cores have the potential to evolve into high-mass stars. The prestellar core mass function (CMF) can be well fit by a log-normal form. The high-mass end of the prestellar CMF shows a power-law form with an index $alpha=-0.9pm 0.1$ that is shallower than that of the Galactic field stellar mass function. Combining the mass transformation efficiency ($varepsilon$) from the prestellar core to the star of $15pm 1%$ and the core formation efficiency (CFE) of 5.5%, we suggest an overall star formation efficiency of about 1% in the CMC. In the single-pointing observations with the IRAM 30m telescope, we find that 6 cores show blue-skewed profile, while 4 cores show red-skewed profile. [$rm {HCO}^{+}$]/[HNC] and [$rm {HCO}^{+}$]/$rm [N_{2}H^{+}]$ in protostellar cores are higher than those in prestellar cores; this can be used as chemical clocks. The best-fit chemical age of the cores with line observations is $sim 5times 10^4$~years.
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