We present yields from stars of mass in the range M ≤ M ≤ 8 M of metallicities Z = 3 × 10−4 and 8 × 10−3, thus encompassing the chemistry of low- and high-Z globular clusters. The yields are based on ...full evolutionary computations, following the evolution of the stars from the pre-main sequence through the asymptotic giant branch phase, until the external envelope is lost.
Independent of metallicity, stars with M < 3 M are dominated by third dredge-up, thus ejecting into their surroundings gas enriched in carbon and nitrogen. Conversely, hot bottom burning is mainly responsible for the modification of the surface chemistry of more massive stars, whose mass exceeds 3 M: their gas shows traces of proton-capture nucleosynthesis.
The extent of hot bottom burning turns out to be strongly dependent on metallicity. Models with Z = 8 × 10−3 achieve a modest depletion of oxygen, barely reaching −0.3 dex, and do not activate the Mg-Al chain. Low-Z models with Z = 3 × 10−4 achieve a strong nucleosynthesis at the bottom of the envelope, with a strong destruction of the surface oxygen and magnesium; the most extreme chemistry is reached for models of mass ∼6 M, where δO/Fe ∼ −1.2 and δMg/Fe ∼ −0.6. Sodium is found to be produced in modest quantities at these low Zs, because the initial increase due to the combined effect of the second dredge-up and of 22Ne burning is compensated by the later destruction via proton capture. A great increase by a factor of ∼10 in the aluminium content of the envelope is also expected. These results can be used to understand the role played by intermediate-mass stars in the self-enrichment scenario of globular clusters: the results from spectroscopic investigations of stars belonging to the second generation of clusters with different metallicity will be used as an indirect test of the reliability of the present yields.
The treatment of mass loss and convection is confirmed as the main uncertainties affecting the results obtained in the context of the modelling of the thermal pulses phase. An indirect proof of this comes from the comparison with other investigations in the literature, based on a different prescription for the efficiency of convection in transporting energy and using a different recipe to determine the mass-loss rate.
ABSTRACT
We present the results for a sample of B stars in the Large Magellanic Cloud young double stellar cluster NGC 1850 A and NGC 1850 B, as observed with the integral-field spectrograph at the ...Very Large Telescope, the Multi Unit Spectroscopic Explorer (MUSE). We compare the observed equivalent widths (EWs) of four He lines (4922, 5015, 6678, and 7065 Å) with those determined from synthetic spectra computed with different He mass fractions (Y = 0.25, 0.27, 0.30, and 0.35) with the code synspec, which takes into account the non-local thermodynamic equilibrium effect. From this comparison, we determine the He mass fraction of the B stars, finding a distribution that is not homogeneous. The stars can be divided in three groups: He-weak (Y < 0.24) and He-normal (0.24 ≤ Y ≤ 0.26) stars, belonging to the main sequence of NGC 1850 A, and He-rich stars (0.33 ≤ Y ≤ 0.38), situated in the main sequence associated with NGC 1850 B. We analyse the stellar rotation as possibly being responsible for the anomalous features of the He lines in the He-rich stars. We provide a simple analysis of the differences between the observed EWs and those obtained from theoretical models with different rotation velocities (Vsini = 0 and 250 km s–1). The resolution of the MUSE spectra does not allow us to obtain a conclusive result; however, our analysis support the He-enhanced hypothesis.
Context. Finding the sources responsible for the hydrogen reionization is one of the most pressing issues in observational cosmology. Bright quasi-stellar objects (QSOs) are known to ionize their ...surrounding neighborhood, but they are too few to ensure the required HI ionizing background. A significant contribution by faint active galactic nuclei (AGNs), however, could solve the problem, as recently advocated on the basis of a relatively large space density of faint active nuclei at z > 4. Aims. This work is part of a long-term project aimed at measuring the Lyman Continuum escape fraction for a large sample of AGNs at z ~ 4 down to an absolute magnitude of M1450 ~ −23. We have carried out an exploratory spectroscopic program to measure the HI ionizing emission of 16 faint AGNs spanning a broad U − I color interval, with I ~ 21–23, and 3.6 < z < 4.2. These AGNs are three magnitudes fainter than the typical SDSS QSOs (M1450 ≲−26) which are known to ionize their surrounding IGM at z ≳ 4. Methods. We acquired deep spectra of these faint AGNs with spectrographs available at the VLT, LBT, and Magellan telescopes, that is, FORS2, MODS1-2, and LDSS3, respectively. The emission in the Lyman Continuum region, close to 900 Å rest frame, has been detected with a signal to noise ratio of ~10–120 for all 16 AGNs. The flux ratio between the 900 Å rest-frame region and 930 Å provides a robust estimate of the escape fraction of HI ionizing photons. Results. We have found that the Lyman Continuum escape fraction is between 44 and 100% for all the observed faint AGNs, with a mean value of 74% at 3.6 < z < 4.2 and − 25.1 ≲ M1450 ≲−23.3, in agreement with the value found in the literature for much brighter QSOs (M1450 ≲−26) at the same redshifts. The Lyman Continuum escape fraction of our faint AGNs does not show any dependence on the absolute luminosities or on the observed U − I colors of the objects. Assuming that the Lyman Continuum escape fraction remains close to ~75% down to M1450 ~ − 18, we find that the AGN population can provide between 16 and 73% (depending on the adopted luminosity function) of the whole ionizing UV background at z ~ 4, measured through the Lyman forest. This contribution increases to 25–100% if other determinations of the ionizing UV background are adopted from the recent literature. Conclusions. Extrapolating these results to z ~ 5–7, there are possible indications that bright QSOs and faint AGNs can provide a significant contribution to the reionization of the Universe, if their space density is high at M1450 ~ −23.
Abstract
We present the results of the spectroscopic and photometric follow-up of two field galaxies that were selected as possible stellar counterparts of local high-velocity clouds. Our analysis ...shows that the two systems are distant (D > 20 Mpc) dwarf irregular galaxies unrelated to the local H i clouds. However, the newly derived distance and structural parameters reveal that the two galaxies have luminosities and effective radii very similar to the recently identified ultra diffuse galaxies (UDGs). At odds with classical UDGs, they are remarkably isolated, having no known giant galaxy within ∼2.0 Mpc. Moreover, one of them has a very high gas content compared to galaxies of similar stellar mass, with a H i
to stellar mass ratio
$M_{\rm H\, {\small {I}}}/M_{\rm \star }\sim 90$
, typical of almost-dark dwarfs. Expanding on this finding, we show that extended dwarf irregulars overlap the distribution of UDGs in the MV
versus log r
e plane and that the sequence including dwarf spheroidals, dwarf irregulars and UDGs appears as continuously populated in this plane. This may suggest an evolutionary link between dwarf irregulars and UDGs.
Abstract
We present observations of the most bright main-sequence stars in the Small Magellanic Cloud stellar cluster NGC 330 obtained with the integral-field spectrograph, the Multi Unit ...Spectroscopic Explorer, at the Very Large Telescope. The use of this valuable instrument allows us to study both photometric and spectroscopic properties of stellar populations of this young star cluster. The photometric data provide us a precise color–magnitude diagram, which seems to support the presence of two stellar populations of ages of ≈18 Myr and ≈30 Myr assuming a metallicity of
Z
= 0.002. Thanks to the spectroscopic data, we derive a helium abundance of 10 main-sequence stars within the effective radius of
R
eff
= 20″ of NGC 330, thus leading to an estimation of
= 10.93 ± 0.05(1
σ
). The helium elemental abundances of stars likely belonging to the two possible stellar populations do not show differences or dichotomy within the uncertainties. Thus, our results suggest that the two stellar populations of NGC 330, if they exist, share similar original He abundances. If we consider stellar rotation velocity in our analysis, a coeval (30 Myr) stellar population, experiencing different values of rotation, cannot be excluded. In this case, the mean helium abundance
obtained in our analysis is 11.00 ± 0.05 dex. We also verified that possible non-LTE (NLTE) effects cannot be identified with our analysis because of the spectral resolution and they are within our derived abundance He uncertainties. Moreover, the analysis of the He abundance as a function of the distance from the cluster center of the observed stars does not show any correlation.
The stars in globular clusters are known to differ in their surface chemistry: spectroscopic investigations in recent decades outlined the presence of star-to-star differences in the abundances of ...the light elements, up to aluminium (and possibly silicon), suggesting that some stars were contaminated by an advanced proton-capture nucleosynthesis. The asymptotic giant branch (AGB) stars are one of the most promising candidates in producing the pollution of the intracluster medium, via the ejection of gas processed by hot bottom burning, from which new stellar generations are formed. This work is focused on the degree of nucleosynthesis involving magnesium, aluminium and silicon that these sources may experience. The key ingredient in determining the degree of magnesium depletion, and the amount of aluminium that can be produced, is the rate of proton capture on 25Mg, forming 26Al; an increase in this cross-section by a factor of 2 with respect to the highest value allowed by the NACRE compilation allows the reproduction of the extent of the Mg depletion observed, and is in qualitative agreement with the positive Al-Si correlation observed in a few clusters. The main uncertainties associated with the macro- and microphysics input are discussed and commented upon, and a comparison with recent spectroscopic results for globular clusters showing some degree of Mg-Al anticorrelation and Al-Si correlation is presented.
We compute the mass and composition of dust produced by stars with masses in the range
and with a metallicity of Z= 0.001 during their asymptotic giant branch (AGB) and super-AGB phases. Stellar ...evolution is followed from the pre-main-sequence phase using the code aton which provides, at each time-step, the thermodynamics and the chemical structure of the wind. We use a simple model to describe the growth of the dust grains under the hypothesis of a time-independent, spherically symmetric stellar wind. Although part of the modelling which describes the stellar outflow is not completely realistic, this approach allows a straight comparison with results based on similar assumptions present in the literature, and thus can be used as an indication of the uncertainties affecting the theoretical investigations focused on the dust formation process in the surroundings of AGB stars.
We find that the total mass of dust injected by AGB stars in the interstellar medium does not increase monotonically with stellar mass and ranges between a minimum of
for the 1.5-
stellar model up to
, for the 6-
case. Dust composition depends on the stellar mass: low-mass stars (
) produce carbon-rich dust, whereas more massive stars, experiencing Hot Bottom Burning, never reach the C-star stage, and produce silicates and iron. This is in partial disagreement with previous investigations in the literature, which are based on synthetic AGB models and predict that, when the initial metallicity is Z= 0.001, carbon-rich dust is formed at all stellar masses. The differences are due to the different modelling of turbulent convection in the super-adiabaticity regime. Also in this case, like for other physical features of the AGB, the treatment of super-adiabatic convection shows up as the most relevant issue affecting the dust formation process.
We also investigate super-AGB stars with masses in the range
that evolve over an ONe core. Due to a favourable combination of mass-loss and Hot Bottom Burning, these stars are predicted to be the most efficient silicate-dust producers, releasing
masses of dust.
We discuss the robustness of these predictions and their relevance for the nature and evolution of dust at early cosmic times.
Abstract
We calculate the dust formed around asymptotic giant branch (AGB) and super-AGB stars of metallicity Z = 0.008 by following the evolution of models with masses in the range 1 M⊙ ≤ M ≤ 8 M⊙ ...through the thermal pulses phase, assuming that dust forms via condensation of molecules within a wind expanding isotropically from the stellar surface. We find that, because of the strong hot bottom burning (HBB) experienced, high-mass models produce silicates, whereas lower mass objects are predicted to be surrounded by carbonaceous grains; the transition between the two regimes occurs at a threshold mass of 3.5 M⊙. These findings are consistent with the results presented in a previous investigation, for Z = 0.001. However, in the present higher metallicity case, the production of silicates in the more massive stars continues for the whole AGB phase, because the HBB experienced is softer at Z = 0.008 than at Z = 0.001; thus, the oxygen in the envelope, essential for the formation of water molecules, is never consumed completely. The total amount of dust formed for a given mass experiencing HBB increases with metallicity, because of the higher abundance of silicon, and the softer HBB, both factors favouring a higher rate of silicates production. This behaviour is not found in low-mass stars, because the carbon enrichment of the stellar surface layers, due to repeated third dredge-up episodes, is almost independent of the metallicity. Regarding cosmic dust enrichment by intermediate-mass stars, we find that the cosmic yield at Z = 0.008 is a factor of ∼5 larger than at Z = 0.001. In the lower metallicity case carbon dust dominates after ∼300 Myr, but at Z = 0.008 the dust mass is dominated by silicates at all times, with a prompt enrichment occurring after ∼40 Myr, associated with the evolution of stars with masses M ∼ 7.5-8 M⊙. These conclusions are partly dependent on the assumptions concerning the two important macrophysics inputs needed to describe the AGB phase, and still unknown from first principles: the treatment of convection, which determines the extent of the HBB experienced and of the third dredge-up following each thermal pulse, and mass-loss, essential in fixing the time-scale on which the stellar envelope is lost from the star.
ABSTRACT
We present the results from a dense multwavelength optical/UV, near-infrared (IR), and X-ray follow-up campaign of the nuclear transient AT 2017gge, covering a total of 1698 d from the ...transient’s discovery. The bolometric light curve, the blackbody temperature and radius, the broad H and He i λ5876 emission lines and their evolution with time, are all consistent with a tidal disruption event (TDE) nature. A soft X-ray flare is detected with a delay of ∼200 d with respect to the optical/UV peak and it is rapidly followed by the emergence of a broad He ii λ4686 and by a number of long-lasting high ionization coronal emission lines. This indicate a clear connection between a TDE flare and the appearance of extreme coronal line emission (ECLEs). An IR echo, resulting from dust re-radiation of the optical/UV TDE light is observed after the X-ray flare and the associated near-IR spectra show a transient broad feature in correspondence of the He i λ10830 and, for the first time in a TDE, a transient high-ionization coronal NIR line (the Fe xiii λ10798) is also detected. The data are well explained by a scenario in which a TDE occurs in a gas-and-dust rich environment and its optical/UV, soft X-ray, and IR emission have different origins and locations. The optical emission may be produced by stellar debris stream collisions prior to the accretion disc formation, which is instead responsible for the soft X-ray flare, emitted after the end of the circularization process.
Abstract
We present an optical/near-IR survey of 11 variable young stars (EXors and EXor candidates) aimed at deriving and monitoring their accretion properties. About 30 optical and near-infrared ...spectra (
R
∼
1500
–
2000
) were collected between 2014 and 2019 with the Large Binocular Telescope (LBT). From the spectral analysis we have derived the accretion luminosity (
L
acc
) and mass accretion rate (
M
̇
acc
), the visual extinction (
A
V
), the temperature and density of the permitted line formation region (
T
,
n
H
), and the signature of the outflowing matter. Two sources (ASASSN-13db and iPTF15afq) have been observed in outburst and quiescence, three during a high level of brightness (XZ Tau, PV Cep, and NY Ori), and the others in quiescence. These latter have
L
acc
and
M
̇
acc
in line with the values measured in classical T Tauri stars of similar mass. All sources observed more than once present
L
acc
and
M
̇
acc
variability. The most extreme case is ASASSN-13db, for which
M
̇
acc
decreases by two orders of magnitude from the outburst peak in 2015 to quiescence in 2017. Also, in NY Ori
L
acc
decreases by a factor 25 in one year. In 80% of the sample we detect the O
i
6300 Å line, a tracer of mass loss. From the variability of the H
α
/O
i
6300 Å ratio, we conclude that mass accretion variations are larger than mass loss variations. From the analysis of the H
i
recombination lines, a correlation is suggested between the density of the line formation region, and the level of accretion activity of the source.
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