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Register a new userThe surprisingly carbon-rich environment of the S-type star W Aql
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W Aql is an asymptotic giant branch (AGB) star with an atmospheric elemental abundance ratio C/O$approx$0.98 and reported circumstellar molecular abundances intermediate between those of M-type (C/O$<$1) and C-type (C/O$>$1) AGB stars. This intermediate status is considered typical for S-type stars, although our understanding of the chemical content of their circumstellar envelopes (CSEs) is currently rather limited. We performed observations in the frequency range 159-268 GHz with the APEX telescope and make abundance estimates through comparison to available spectra towards some well-studied AGB stars and based on rotational diagram analysis in the case of SiC2. We conclude that W Aqls CSE appears considerably closer to that of a C-type AGB star than to that of an M-type AGB star. In particular, we detect emission from C2H, SiC2, SiN, and HC3N, molecules previously only detected towards the CSEs of C-type stars. This conclusion, based on the chemistry of the gaseous component of the CSE, is further supported by reports in the literature on the presence of atmospheric molecular bands and spectral features of dust species typical for C-type AGB stars. Although our observations mainly trace species in the outer regions of the CSE, our conclusion matches closely that based on recent chemical equilibrium models for the inner wind of S-type stars: the atmospheric and circumstellar chemistry of S-type stars likely resembles that of C-type AGB stars much more closely than that of M-type AGB stars. Further observational investigation of the gaseous circumstellar chemistry of S-type stars is required to characterise its dependence on the atmospheric C/O. Non-equilibrium chemical models of the CSEs of AGB stars need to address the particular class of S-type stars and the chemical variety that is induced by the range in atmospheric C/O.
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The CO(J=3-2) emission from the CSE of the binary S-type AGB star W Aql has been observed at subarcsecond resolution using ALMA. The aim of this paper is to investigate the wind properties of the AGB star and to analyse how the known companion has shaped the CSE. The average mass-loss rate during the creation of the detected CSE is estimated through modelling, using the ALMA brightness distribution and previously published single-dish measurements as observational constraints. The ALMA observations are presented and compared to the results from a 3D smoothed particle hydrodynamics (SPH) binary interaction model with the same properties as the W Aql system and with two different orbital eccentricities. Three-dimensional radiative transfer modelling is performed and the response of the interferometer is modelled and discussed. The estimated average mass-loss rate of W~Aql agrees with previous results. The size of the emitting region is consistent with photodissociation models. The CO(J=3-2) emission is dominated by a smooth component overlayed with two weak arc patterns with different separations. The larger pattern is predicted by the binary interaction model with separations of 10 and therefore likely due to the known companion. It is consistent with a binary orbit with low eccentricity. The smaller separation pattern is asymmetric and coincides with the dust distribution, but the separation timescale (200 yrs) is not consistent with any known process of the system. The separation of the known companions of the system is large enough to not have a very strong effect on the circumstellar morphology. The density contrast across the envelope of a binary with an even larger separation will not be easily detectable, even with ALMA, unless the orbit is strongly asymmetric or the AGB star has a much larger mass-loss rate.
S-type asymptotic giant branch (AGB) stars are thought to be intermediates in the evolution of oxygen- to carbon-rich AGB stars. The chemical compositions of their circumstellar envelopes are also intermediate, but have not been studied in as much detail as their carbon- and oxygen-rich counterparts. We aim to determine the abundances of AlCl and AlF from rotational lines, which have been observed for the first time towards an S-type AGB star, W Aql. In combination with models based on PACS observations, we aim to update our chemical kinetics network based on these results. We analyse ALMA observations towards W Aql of AlCl in the ground and first two vibrationally excited states and AlF in the ground vibrational state. Using radiative transfer models, we determine the abundances and spatial abundance distributions of Al$^{35}$Cl, Al$^{37}$Cl, and AlF. We also model HCl and HF emission and compare these models to PACS spectra to constrain the abundances of these species. AlCl is found in clumps very close to the star, with emission confined within 0.1$^{primeprime}$ of the star. AlF emission is more extended, with faint emission extending 0.2$^{primeprime}$ to 0.6$^{primeprime}$ from the continuum peak. We find peak abundances, relative to H$_2$, of $1.7times 10^{-7}$ for Al$^{35}$Cl, $7times 10^{-8}$ for Al$^{37}$Cl and $1times 10^{-7}$ for AlF. From the PACS spectra, we find abundances of $9.7times 10^{-8}$ and $leq 10^{-8}$, relative to H$_2$, for HCl and HF, respectively. The AlF abundance exceeds the solar F abundance, indicating that fluorine synthesised in the AGB star has already been dredged up to the surface of the star and ejected into the circumstellar envelope. From our analysis of chemical reactions in the wind, we conclude that AlF may participate in the dust formation process, but we cannot fully explain the rapid depletion of AlCl seen in the wind.
We have studied the environment of the FU Ori type star V582 Aur. Our aim is to explore the star-forming region associated with this young eruptive star. Using slitless spectroscopy we searched for H alpha emission stars within a field of 11.5arcmin times 11.5arcmin, centred on V582 Aur. Based on UKIDSS and Spitzer Space Telescope data we further selected infrared-excess young stellar object candidates. In all, we identified 68 candidate low-mass young stars, 16 of which exhibited H alpha emission in the slitless spectroscopic images. The colour-magnitude diagram of the selected objects, based on IPHAS data, suggests that they are low-mass pre-main-sequence stars associated with the Aur OB 1 association, located at a distance of 1.3 kpc from the Sun. The bright-rimmed globules in the local environment of V582 Aur probably belong to the dark cloud LDN~1516. Our results suggest that star formation in these globules might have been triggered by the radiation field of a few hot members of Aur OB 1. The bolometric luminosity of V582 Aur, based on archival photometric data and on the adopted distance, is 150-320 Lsun.
We present the optical (UBVRI) and ultraviolet (Swift-UVOT) photometry, and optical spectroscopy of Type Ia supernova SN 2017hpa. We study broadband UV+optical light curves and low resolution spectroscopy spanning from $-13.8$ to $+108$~d from the maximum light in $B$-band. The photometric analysis indicates that SN 2017hpa is a normal type Ia with $Delta m_{B}(15) = 0.98pm0.16$ mag and $M_{B}=-19.45pm0.15$ mag at a distance modulus of $mu = 34.08pm0.09$ mag. The $(uvw1-uvv)$ colour evolution shows that SN 2017hpa falls in the NUV-blue group. The $(B-V)$ colour at maximum is bluer in comparison to normal type Ia supernovae. Spectroscopic analysis shows that the Si II 6355 absorption feature evolves rapidly with a velocity gradient, $dot{v}=128pm 7$ km s$^{-1}$ d$^{-1}$. The pre-maximum phase spectra show prominent C II 6580 {AA} absorption feature. The C II 6580 {AA} line velocity measured from the observed spectra is lower than the velocity of Si II 6355 {AA}, which could be due to a line of sight effect. The synthetic spectral fits to the pre-maximum spectra using syn++ indicate the presence of a high velocity component in the Si II absorption, in addition to a photospheric component. Fitting the observed spectrum with the spectral synthesis code TARDIS, the mass of unburned C in the ejecta is estimated to be $sim 0.019$~$M_{odot}$. The peak bolometric luminosity is $L^{bol}_{peak} = 1.43times10^{43}$ erg s$^{-1}$. The radiation diffusion model fit to the bolometric light curve indicates $0.61pm0.02$ $M_odot$ of $^{56}$Ni is synthesized in the explosion.
We present a dust spectral energy distribution (SED) and binary stellar population analysis revisiting the dust production rates (DPRs) in the winds of carbon-rich Wolf-Rayet (WC) binaries and their impact on galactic dust budgets. DustEM SED models of 19 Galactic WC ``dustars reveal DPRs of $dot{M}_dsim10^{-10}-10^{-6}$ M$_odot$ yr$^{-1}$ and carbon dust condensation fractions, $chi_C$, between $0.002 - 40%$. A large ($0.1 - 1.0$ $mu$m) dust grain size composition is favored for efficient dustars where $chi_Cgtrsim1%$. Results for dustars with known orbital periods verify a power-law relation between $chi_C$, orbital period, WC mass-loss rate, and wind velocity consistent with predictions from theoretical models of dust formation in colliding-wind binaries. We incorporated dust production into Binary Population and Spectral Synthesis (BPASS) models to analyze dust production rates from WC dustars, asymptotic giant branch stars (AGBs), red supergiants (RSGs), and core-collapse supernovae (SNe). BPASS models assuming constant star formation (SF) and a co-eval $10^6$ M$_odot$ stellar population were performed at low, Large Magellanic Cloud (LMC)-like, and solar metallicities (Z = 0.001, 0.008, and 0.020). Both constant SF and co-eval models show that SNe are net dust destroyers at all metallicities. Constant SF models at LMC-like metallicities show that AGB stars slightly outproduce WC binaries and RSGs by factors of $2-3$, whereas at solar metallicites WC binaries are the dominant source of dust for $sim60$ Myr until the onset of AGBs, which match the dust input of WC binaries. Co-eval population models show that for bursty SF, AGB stars dominate dust production at late times ($tgtrsim 70$ Myr).
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