Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userSpatially resolved carbon and oxygen isotopic ratios in NGC 253 using optically thin tracers
59
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
To trace chemical evolution in a prototypical starburst environment, we spatially resolve the carbon and oxygen isotope ratios across the central molecular zone (full size ~$sim 600$ pc) in the nearby starburst galaxy NGC 253. We imaged the emission of the optically thin isotopologues $^{13}$CO, C$^{18}$O, C$^{17}$O, $^{13}$C$^{18}$O at a spatial resolution $sim50$ pc, comparable to the typical size of giant molecular associations. Optical depth effects and contamination of $^{13}$C$^{18}$O by C$_4$H is discussed and accounted for to derive column densities. This is the first extragalactic detection of the double isotopologue $^{13}$C$^{18}$O. Derived isotopic ratios $^{12}$C/$^{13}$C$sim21pm6$, $^{16}$O/$^{18}$O$sim130pm40$, and $^{18}$O/$^{17}$O$sim4.5pm0.8$ differ from the generally adopted values in the nuclei of galaxies. The molecular clouds in the central region of NGC 253 show similar rare isotope enrichment to those within the central molecular zone of the Milky way. This enrichment is attributed to stellar nucleosynthesis. Measured isotopic ratios suggest an enhancement of $^{18}$O as compared to our Galactic center, which we attribute to an extra $^{18}$O injection from massive stars. Our observations show evidence for mixing of distinct gas components with different degrees of processing. We observe an extra molecular component of highly processed gas on top of the already proposed less processed gas being transported to the central region of NGC 253. Such multicomponent nature and optical depth effects may hinder the use of isotopic ratios based on spatially unresolved line to infer the star formation history and/or initial stellar mass function properties in the nuclei of galaxies.
rate research
Read More
C and O isotopic ratios are reported for a sample of 46 Mira and SRa-type variable AGB stars. Vibration-rotation 1st and 2nd overtone CO lines in 1.5 to 2.5 $mu$m spectra were measured to derive isotopic ratios for 12C/13C, 16O/17O, and 16O/18O. Comparisons with previous measurements for individual stars and with various samples of evolved stars are discussed. Models for solar composition AGB stars of different initial masses are used to interpret our results. We find that the majority of the M stars had main sequence masses < 2 Msun and have not experienced sizable third dredge-up episodes. The progenitors of the four S-type stars in our sample are slightly more massive. Of the 6 C stars in the sample three have clear evidence relating their origin to the occurrence of the third dredge-up. Comparisons with O-rich presolar grains from AGB stars that lived before the formation of the solar system reveal variations in the interstellar medium chemical composition. The present generation of low-mass AGB stars show a large spread of 16O/17O ratios, similar to that of group 1 presolar grains and in agreement with theoretical expectations for the composition of mass 1.2 to 2 Msun stars after the 1st dredge up. On the contrary, the 16O/18O ratios of present-day LPVs are definitely smaller than those of group 1 grains. This occurrence is most probably a consequence of the the decrease with time of the 16O/18O ratio in the interstellar medium due to the chemical evolution of the Milky Way. One star in our sample has an O composition similar to that of group 2 presolar grains originating in an AGB star undergoing extramixing. This occurrence may indicate that the extramixing process is hampered at high metallicity or, equivalently, favored at low metallicity. Similar to O-rich grains no star in our sample shows evidence of HBB, expected for massive AGB stars.
Carbon and oxygen isotopic ratios are reported for a sample of 51 SRb- and Lb-type variable asymptotic giant branch stars. Vibration-rotation first- and second-overtone CO lines in 1.5-2.5 mum spectra were measured to derive isotopic ratios for 12C/13C, 16O/17O, and 16O/18O. Comparisons with previous measurements for individual stars and with various samples of evolved stars, as available in the extant literature, are discussed. Using the oxygen isotopic ratios, the masses of the SRb stars can be derived. Combining the masses with Gaia luminosities, the SRb stars are shown to be antecedents of the Mira variables. The limiting parameters where plane-parallel, hydrostatic equilibrium model atmospheres can be used for abundance analysis of M giants are explored.
Context. The abundances of the three main isotopes of oxygen are altered in the course of the CNO-cycle. When the first dredge-up mixes the burning products to the surface, the nucleosynthesis processes can be probed by measuring oxygen isotopic ratios. Aims. By measuring 16O/17O and 16O/18O in red giants of known mass we compare the isotope ratios with predictions from stellar and galactic evolution modelling. Methods. Oxygen isotopic ratios were derived from the K-band spectra of six red giants. The sample red giants are open cluster members with known masses of between 1.8 and 4.5 Msun . The abundance determination employs synthetic spectra calculated with the COMARCS code. The effect of uncertainties in the nuclear reaction rates, the mixing length, and of a change in the initial abundance of the oxygen isotopes was determined by a set of nucleosynthesis and mixing models using the FUNS code. Results. The observed 16O/17O ratios are in good agreement with the model results, even if the measured values do not present clear evidence of a variation with the stellar mass. The observed 16O/18O ratios are clearly lower than the predictions from our reference model. Variations in nuclear reaction rates and mixing length parameter both have only a very weak effect on the predicted values. The 12C/13C ratios of the K giants studied implies the absence of extra-mixing in these objects. Conclusions. A comparison with galactic chemical evolution models indicates that the 16O/18O abundance ratio underwent a faster decrease than predicted. To explain the observed ratios, the most likely scenario is a higher initial 18O abundance combined with a lower initial 16 O abundance. Comparing the measured 18 O/17 O ratio with the corresponding value for the ISM points towards an initial enhancement of 17O as well. Limitations imposed by the observations prevent this from being a conclusive result.
Early star-forming galaxies produced copious ionizing photons. A fraction of these photons escaped gas within galaxies to reionize the entire Universe. This escape fraction is crucial for determining how the Universe became reionized, but the neutral intergalactic medium precludes direct measurement of the escape fraction at high-redshifts. Indirect estimates of the escape fraction must describe how the Universe was reionized. Here, we present new Keck Cosmic Web Imager spatially-resolved spectroscopy of the resonant Mg II 2800 doublet from a redshift 0.36 galaxy, J1503+3644, with a previously observed escape fraction of 6%. The Mg II emission has a similar spatial extent as the stellar continuum, and each of the Mg II doublet lines are well-fit by single Gaussians. The Mg II is optically thin. The intrinsic flux ratio of the red and blue Mg II emission line doublet, $R = F_{2796}/F_{2803}$, is set by atomic physics to be two, but Mg$^+$ gas along the line of sight decreases $R$ proportional to the Mg II optical depth. Combined with the metallicity, $R$ estimates the neutral gas column density. The observed $R$ ranges across the galaxy from 0.8-2.7, implying a factor of 2 spatial variation of the relative escape fraction. All of the ionizing photons that escape J1503+3644 pass through regions of high $R$. We combine the Mg II emission and dust attenuation to accurately estimate the absolute escape fractions for ten local Lyman Continuum emitting galaxies and suggest that Mg II can predict escape fraction within the Epoch of Reionization.
We re-analysed the carbon and oxygen isotopic ratios in the atmospheres of the two bright K giants Arcturus and Aldebaran. Previous determinations of their 16O/18O ratios showed a rough agreement with FDU expectations; however, the estimated 16O/17O and 12C/13C ratios were lower than in the canonical predictions. These anomalies are interpreted as signs of the occurrence of non-convective mixing episodes. We re-investigated this issue in order to verify whether the observed data can be reproduced in this hypothesis and if the well determined properties of the two stars can help us in fixing the uncertain parameters characterizing non-convective mixing and its physical nature. We used high-resolution infrared spectra to derive the 12C/13C and 16O/17O/18O ratios from CO molecular lines near 5 mu. We also reconsidered the determination of the stellar parameters to build the proper atmospheric and evolutionary models. We found that both the C and the O isotopic ratios for the two stars considered actually disagree with pure FDU predictions. This reinforces the idea that non-convective transport episodes occurred in them. By reproducing the observed elemental and isotopic abundances with the help of parametric models of nucleosynthesis and mass circulation, we derived constraints on the properties of non convective mixing. We find that very slow mixing is incapable of explaining the observed data, which require a fast transport. Circulation mechanisms with speeds intermediate between those typical of diffusive and of convective mixing should be at play. We however conclude with a word of caution on the conclusions possible at this stage, as the parameters for the mass transport are rather sensitive to the stellar mass and initial composition.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
