Oxygen-rich Circumstellar Envelopes Research Articles

A deep search for C2H in the oxygen-rich post-AGB star OH 231.8+4.2

In carbon-rich environment, C2H emission is seen to be associated with HC3N, but whether it can trace the HC3N molecule in oxygen-rich environment is unknown. Here, we have searched for rotational emission from ethynyl radical (C2H) in the oxygen-rich circumstellar envelope (CSE) of the post-asymptotic giant branch (post-AGB) star OH 231.8+4.2, a renowned galactic proto-planetary nebula (PPN) known to display cyanoacetylene (HC3N) emission. Our observations were conducted using the James Clerk Maxwell Telescope (JCMT), and the total on-source time is 11.9 h. Base on local thermodynamic equilibrium (LTE) excitation analysis, we have calculated the column density and the abundance relative to H2 for C2H. The calculated column density for C2H is less than 1.4 × 1013 cm-2, which corresponds to a fractional abundance, f (C2H), less than 4.5 × 10–9. This inference suggests that the typical transformation pathway from C2H and C2H2 to HC3N may not play a significant role in O-rich environments. It indicates the presence of alternative, unidentified pathways that contribute to the formation of HC3N in O-rich circumstellar envelopes.

Molecules and Outflows in NML Cygni: New Insights from a 1 mm Spectral Line Survey

Abstract A sensitive (1σ rms ≤ 3 mK) 1 mm spectral survey (214.5–285.5 GHz) of the oxygen-rich circumstellar envelope of the red hypergiant NML Cygni (NML Cyg) has been conducted using the Sub-millimeter Telescope (SMT) of the Arizona Radio Observatory (ARO). Over 100 spectral lines were detected, arising from 17 different molecules, including the carbon-bearing species CO, HCN, HCO+, CN, and HNC; sulfur- and silicon-containing compounds H2S, SO, SO2, SiO, and SiS; and more exotic NaCl and AlO. The 1 mm spectrum of NML Cyg closely resembles that of VY Canis Majoris (VY CMa) suggesting that the chemistries of hypergiant stars are similar. The line profiles in NML Cyg consist of multiple velocity features, particularly evident in SO2 and SO. In addition to a spherical wind at the star’s systemic velocity, the spectra suggest an asymmetric, blueshifted component near V LSR = −21 ± 3 km s−1 and a collimated, redshifted component near 15 ± 3 km s−1, positioned ∼34° and ∼12°, respectively, from the line of sight. The red- and blueshifted flows appear to be randomly oriented, and likely trace sporadic mass loss events. Their LSR velocities align closely with those of 22 GHz water masers, suggesting an NE–SE orientation. The winds may also be associated with the asymmetric nebula in F555W HST images but extending to 5″ (∼600R *). NML Cyg appears to be another example of rare, massive stars with collimated, episodic ejections, analogous to Betelgeuse and VY CMa, lending support for an important new mass loss mechanism—surface activity.

Circumstellar ammonia in oxygen-rich evolved stars

Context. The circumstellar ammonia (NH3) chemistry in evolved stars is poorly understood. Previous observations and modelling showed that NH3 abundance in oxygen-rich stars is several orders of magnitude above that predicted by equilibrium chemistry.Aims. We would like to characterise the spatial distribution and excitation of NH3 in the oxygen-rich circumstellar envelopes (CSEs) of four diverse targets: IK Tau, VY CMa, OH 231.8+4.2, and IRC +10420. Methods. We observed NH3 emission from the ground state in the inversion transitions near 1.3 cm with the Very Large Array (VLA) and submillimetre rotational transitions with the Heterodyne Instrument for the Far-Infrared (HIFI) aboard Herschel Space Observatory from all four targets. For IK Tau and VY CMa, we observed NH3 rovibrational absorption lines in the ν2 band near 10.5 μm with the Texas Echelon Cross Echelle Spectrograph (TEXES) at the NASA Infrared Telescope Facility (IRTF). We also attempted to search for the rotational transition within the excited vibrational state (v2 = 1) near 2 mm with the IRAM 30m Telescope. Non-LTE radiative transfer modelling, including radiative pumping to the vibrational state, was carried out to derive the radial distribution of NH3 in the CSEs of these targets. Results. We detected NH3 inversion and rotational emission in all four targets. IK Tau and VY CMa show blueshifted absorption in the rovibrational spectra. We did not detect vibrationally excited rotational transition from IK Tau. Spatially resolved VLA images of IK Tau and IRC +10420 show clumpy emission structures; unresolved images of VY CMa and OH 231.8+4.2 indicate that the spatial-kinematic distribution of NH3 is similar to that of assorted molecules, such as SO and SO2, that exhibit localised and clumpy emission. Our modelling shows that the NH3 abundance relative to molecular hydrogen is generally of the order of 10−7, which is a few times lower than previous estimates that were made without considering radiative pumping and is at least ten times higher than that in the carbon-rich CSE of IRC +10216. NH3 in OH 231.8+4.2 and IRC +10420 is found to emit in gas denser than the ambient medium. Incidentally, we also derived a new period of IK Tau from its V-band light curve. Conclusions. NH3 is again detected in very high abundance in evolved stars, especially the oxygen-rich ones. Its emission mainly arises from localised spatial-kinematic structures that are probably denser than the ambient gas. Circumstellar shocks in the accelerated wind may contribute to the production of NH3. Future mid-infrared spectroscopy and radio imaging studies are necessary to constrain the radii and physical conditions of the formation regions of NH3.

New Circumstellar Sources of PO and PN: The Increasing Role of Phosphorus Chemistry in Oxygen-rich Stars

Abstract PO and PN have been newly identified in several oxygen-rich circumstellar envelopes, using the Submillimeter Telescope of the Arizona Radio Observatory. The J = 5 → 4 and J = 6 → 5 transitions of PN near 235 and 282 GHz, and the lambda doublets originating in the J = 5.5 → 4.5 and J = 6.5 → 5.5 lines of PO at 240 and 284 GHz, have been detected toward the shells of asymptotic giant branch (AGB) stars TX Cam and R Cas. A similar set of lines has been observed toward the supergiant NML Cyg, and new transitions of these two molecules were also measured toward the AGB star IK Tau. Along with the previous data from VY Canis Majoris (VY CMa), these spectral lines were analyzed using the non-local thermodynamic equilibrium (non-LTE) circumstellar modeling code, ESCAPADE. For the AGB stars, peak abundances found for PN and PO were f ∼ (1–2) × 10−8 and (0.5–1) × 10−7, respectively, while those for the supergiants were f(PN) ∼ (0.3–0.7) × 10−8 and f(PO) ∼ (5–7) × 10−8. PN was well modeled with a spherical radial distribution, suggesting formation near the stellar photosphere, perhaps enhanced by shocks. PO was best reproduced by a shell model, indicating a photochemical origin, except for VY CMa. Overall, the abundance of PO is a factor of 5–20 greater than that of PN. This study suggests that phosphorus-bearing molecules are common in O-rich envelopes, and that a significant amount of phosphorus (>20%) remains in the gas phase.

The millimeter IRAM-30 m line survey toward IK Tau.

We aim to investigate the physical and chemical properties of the molecular envelope of the oxygen-rich AGB star IK Tau. We carried out a millimeter wavelength line survey between ~79 and 356 GHz with the IRAM-30 m telescope. We analysed the molecular lines detected in IK Tau using the population diagram technique to derive rotational temperatures and column densities. We conducted a radiative transfer analysis of the SO2 lines, which also helped us to verify the validity of the approximated method of the population diagram for the rest of the molecules. For the first time in this source we detected rotational lines in the ground vibrational state of HCO+, NS, NO, and H2CO, as well as several isotopologues of molecules previously identified, namely, C18O, Si17O, Si18O, 29SiS, 30SiS, Si34S, H13CN, 13CS, C34S, H234S, 34SO, and 34SO2. We also detected several rotational lines in vibrationally excited states of SiS and SiO isotopologues, as well as rotational lines of H2O in the vibrationally excited state ν2=2. We have also increased the number of rotational lines detected of molecules that were previously identified toward IK Tau, including vibrationally excited states, enabling a detailed study of the molecular abundances and excitation temperatures. In particular, we highlight the detection of NS and H2CO with fractional abundances of f(NS)~10-8 and f(H2CO)~[10-7-10-8 ]. Most of the molecules display rotational temperatures between 15 and 40 K. NaCl and SiS isotopologues display rotational temperatures higher than the average (~65 K). In the case of SO2 a warm component with Trot~290 K is also detected. With a total of ~350 lines detected of 34 different molecular species (including different isotopologues), IK Tau displays a rich chemistry for an oxygen-rich circumstellar envelope. The detection of carbon bearing molecules like H2CO, as well as the discrepancies found between our derived abundances and the predictions from chemical models for some molecules, highlight the need for a revision of standard chemical models. We were able to identify at least two different emission components in terms of rotational temperatures. The warm component, which is mainly traced out by SO2, is probably arising from the inner regions of the envelope (at ≲8R∗) where SO2 has a fractional abundance of f(SO2)~10-6. This result should be considered for future investigation of the main formation channels of this, and other, parent species in the inner winds of O-rich AGB stars, which at present are not well reproduced by current chemistry models.

Oxygen-rich dust production in IC 10

We report the detection of oxygen-rich circumstellar envelopes in stars of the nearby (700 kpc) starburst galaxy IC 10. The star formation history and the chemical environment of this galaxy makes it an ideal target to observe dust production by high-mass stars in a low-metallicity environment. The goal of this study is to identify oxygen-rich stars in IC 10 and to constrain their nature between asymptotic giant branch stars (AGBs), red supergiants (RSGs), and other infrared bright sources. We examine the mass-loss rate of the stars and compare to results obtained for the Magellanic Clouds. Our objectives are to (1) assess whether RSGs can be significant dust producers in IC 10, and (2), solve the discrepancy between the star formation history of IC 10 and the relatively low number of RSGs detected in the optical. We search for silicate dust in emission by using the spectral map observed with the Infrared Spectrograph on board the Spitzer Space Telescope. The optical (UBVRI) and infrared (JHK, Spitzer/IRAC and Spitzer/MIPS) photometry is used to assert the membership of the stars to IC 10 and disentangle between AGBs and RSGs. Radiative models are used to infer mass-loss rates and stellar luminosities. The luminosity and colors of at least 9 silicate emission sources are consistent with stars within IC 10. Furthermore, the photometry of 2 of these sources is consistent with RSGs. We derive dust mass-loss rates similar to the values found in the Magellanic Clouds. Accounting for the sample completeness, RSGs are not important contributors to the dust mass budget in IC 10.

CIRCUMSTELLAR ION-MOLECULE CHEMISTRY: OBSERVATIONS OF HCO+IN THE ENVELOPES OF O-RICH STARS AND IRC+10216

Millimeter-wave observations of HCO+ have been conducted toward oxygen-rich circumstellar envelopes, as well as IRC+10216, using the facilities of the Arizona Radio Observatory (ARO). The J = 1 → 0 and 2 → 1 transitions of this molecule were measured with the ARO 12 m antenna, while the J = 3 → 2 and 4 → 3 lines were observed using the ARO Sub-Millimeter Telescope. HCO+ was detected toward the supergiant NML Cyg and the asymptotic giant branch (AGB) stars IK Tau, TX Cam, and W Hya in at least two transitions. The J = 2 → 1 and 3 → 2 lines of this ion were also detected toward IRC+10216, confirming the identification of HCO+ in this object. The line profiles measured for HCO+ toward NML Cyg consist of red- and blueshifted components, suggesting a non-spherical shell. Based on a radiative transfer analysis, the abundances in the O-rich envelopes were f(HCO+/H2) ∼ 0.15–1.3 × 10−7, with the AGB stars typically exhibiting the higher values. In IRC+10216, f(HCO+/H2) ∼ 4.1 × 10−9, lower than the O-rich counterparts. The abundances of HCO+ were also found to peak at considerable distances from the star, indicative of an outer envelope molecule. Comparison with H2O and CO, the main precursor species, suggests that HCO+ is more prevalent in envelopes that have substantial water, but CO also plays a role in its formation. The abundance of HCO+ appears to increase inversely with mass-loss rate, provided the rate is >10−6 M☉ yr−1. The common appearance of HCO+ in circumstellar gas indicates that, at some level, ion–molecule reactions influence the chemistry of evolved stellar envelopes.

A PILOT IMAGING LINE SURVEY OF RW LMi AND IK Tau USING THE EXPANDED VERY LARGE ARRAY

We report on a pilot imaging line survey (36.0 - 37.0 GHz, with ~1 km/s spectral channels) with the Expanded Very Large Array for two asymptotic giant branch stars, RW LMi (= CIT6, which has a carbon-rich circumstellar envelope) and IK Tau (= NML Tau, with an oxygen-rich circumstellar envelope). Radio continuum emission consistent with photospheric emission was detected from both stars. From RW LMi we imaged the HC3N (J = 4 -> 3) emission. The images show several partial rings of emission; these multiple shells trace the evolution of the CSE from 400 to 1200 years. SiS (J = 2 -> 1) emission was detected from both RW LMi and IK Tau. For both stars the SiS emission is centrally condensed with the peak line emission coincident with the stellar radio continuum emission. In addition, we have detected weak HC7N (J = 32 -> 31) emission from RW LMi.

IRAS 19227+1700: A carbon-rich or oxygen-rich start?

In the new version of the catalogue of carbon stars, IRAS 19227+1700 is listed as a carbon-rich sta. However, in the IRAS infrared two-color diagram, it is located in the typical region of stars with rather thick oxygen-rich circumstellar envelopes, and not in the region commonly occupied by silicate carbon stars. Moreover, there are other pieces of evidence that show that this star not only has the main and satellite line OH maser emissions, but also has water emission close to the central star. Its IRAS low- resolution spectrum (LRS) displays dust emission features in the range of 9–12 μm, and near infrared observation has confirmed that it really possesses a hot circumstellar envelope. Therefore, the circumstellar envelope of IRAS 19227+1700 should be completely oxygen-rich in nature, and quite possibly its central star is also oxygen-rich.

A Circumbinary Reservoir around BM Geminorum?

We report the detection of both (J = 1-0) and (J = 2-1) CO emission from the oxygen-rich circumstellar envelope around a carbon star with an oxygen-rich circumstellar envelope, BM Gem. The line has two distinct components. Unique among mass-losing carbon stars, there is a feature that is much narrower (FWHM ~ 1 km s-1) than the normal CO emission detected from winds. This narrow feature sits on top of a broader (FWHM = 15 km s-1) line, which is probably the emission from the normal wind. The most plausible interpretation of the narrow feature is that we are observing emission from a circumbinary reservoir of material, probably in the form of a distorted or puffed-up disk. We propose that this circumbinary reservoir is composed of oxygen-rich material left over from a previous phase of mass loss that was trapped in the system by the action of an as yet undetected companion.

Methane in oxygen-rich circumstellar envelopes: a search for CH3OH and CCH

Methane in oxygen-rich circumstellar envelopes: a search for CH3OH and CCH

Aluminum Oxide and the Opacity of Oxygen‐rich Circumstellar Dust in the 12–17 Micron Range

Amorphous alumina (Al2O3) was produced by a sol-gel technique in order to make available its optical constants for possible astrophysical applications. Gradual annealing showed that the X-ray amorphousness of alumina ended somewhere between 723 and 873 K. Above this transition point, the structure changes into disordered γ-Al2O3. At T > 1273 K, crystalline α-Al2O3 (corundum) is formed. Mie calculations show that amorphous alumina exhibits a wide Al-O vibrational band, peaking at 11.5-11.8 μm and having a steep blue and an extended red wing. It may be an important contributor to the continuous opacity between the silicate bands in oxygen-rich circumstellar envelopes, whereas it is ruled out for the explanation of the 13 μm band. An average 13 μm band profile was derived from 51 IRAS low-resolution spectra of bright Mira stars and semiregular variables. Its shape, which is satisfactorily represented by a Lorentz profile, can be reproduced by Mie calculations with the data of α-Al2O3, but not with those of γ-Al2O3. The calculations show that the 13 μm band profile of α-Al2O3 is sensitive to grain shape. If α-Al2O3 is the absorber, a second band should be present at 21 μm. A close correlation was found between the strengths of the 13 μm band and the 10 μm silicate band. It suggests that the 13 μm band carrier could also be somehow connected with silicate dust. Experimental arguments supporting this attribution are presented.

Disks Around Post-Main Sequence Binaries

We propose that at least two stars on or near the AGB have long-lived orbiting disks: HD 44179, the central star in the Red Rectangle, and BM Gem, a carbon-rich star with an oxygen-rich circumstellar envelope. The CO emission from both of these disks has a spike with a width near ∼2 km s−1, indicating disk radii of ∼1016 cm. The dust in such disks is therefore quite cold (near T ∼ 50 K for the Red Rectangle) and may emit primarily at submillimeter wavelengths. The disks around stars where there is also substantial mass loss may not be easily observable; there could be many as yet undiscovered disks around AGB stars

Carbon-Bearing Molecules in Oxygen-Rich Circumstellar Envelopes

We are currently conducting a search of oxygen-rich circumstellar envelopes for carbon-bearing molecules with the ISO SWS instrument. CH4 is the molecule in which we are most interested, because it has been shown to be capable of providing the reactive carbon which is needed produce the carbon-bearing molecules in these envelopes (Willacy & Millar 1997). However, CO2 and C2H2 are also possible candidates. The spectral ranges for our observations have been chosen specifically to include ro-vibrational bands in these molecules, but many other molecules, such as HCN, NH3, OH and HO2 have bands in these regions and hopefully there will be a few interesting surprises.

The Galactic distribution of O-rich AGB stars

A sample of AGB stars with oxygen-rich circumstellar envelopes is selected from the IRAS PSC in the regions of IIIa, IIIb and IV defined by van der Veen (1988). The total flux for each object is derived from the flux at 12μm and the IRAS colour C 12 using the bolometric correction formula given by van der Veen. The distance for each object is obtained assuming that the luminosities of O-rich AGB stars are identical. The spatial distribution of the sample shows a similar spiral structure as that of HII regions derived from optical and radio observations. This confirms that the O-rich AGB stars are a disk population and that they are predominantly located in the arms. Fitting the spiral structure based on the distribution of HII regions we derive that the average luminosity of the objects in the sample is 8300 L⊙( R o = 8.0 kpc). It is a little large than normal AGB stars, we consider that the objects in the arms which are used to derive the average luminosity are given more weight than others.

Chemical modelling of molecular sources - VI. Carbon-bearing molecules in oxygen-rich circumstellar envelopes

On etudie la possibilite de production de molecules porteuses de carbone dans les enveloppes exterieures d'etoiles riches en oxygene a travers les reactions entre les especes formees par photodissociation des molecules parents stables