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Context. Born-again stars provide a unique possibility to study the evolution of the circumstellar envelope of evolved stars in human timescales. Up until now, most of the observations of the circumstellar material in these stars have been limited to studying the relatively hot gas and dust. In other evolved stars, the emission from rotational transitions of molecules, such as CO, is commonly used to study the cool component of their circumstellar envelopes. Thus, the detection and study of molecular gas in born-again stars is of great importance when attempting to understand their composition and chemical evolution. In addition, the molecular emission is an invaluable tool for exploring the physical conditions, kinematics, and formation of asymmetric structures in the circumstellar envelopes of these evolved stars. However, up until now, all attempts to detect molecular emission from the cool material around born-again stars have failed. Aims. We searched for emission from rotational transitions of molecules in the hydrogen-deficient circumstellar envelopes of bornagain stars to explore the chemical composition, kinematics, and physical parameters of the relatively cool gas. Methods. We carried out observations using the APEX and IRAM 30 m telescopes to search for molecular emission toward four well-studied born-again stars, V4334 Sgr, V605 Aql, A30, and A78, that are thought to represent an evolutionary sequence. Results. For the first time, we detected emission from HCN and H13CN molecules toward V4334 Sgr, and CO emission in V605 Aql. No molecular emission was detected above the noise level toward A30 and A78. The detected lines exhibit broad linewidths ≳150 km s-1, which indicates that the emission comes from gas ejected during the born-again event, rather than from the old planetary nebula. A first estimate of the H12CN/H13CN abundance ratio in the circumstellar environment of V4334 Sgr is 3, which is similar to the value of the 12C/13C ratio measured from other observations. We derived a rotational temperature of Trot = 13 ± 1 K, and a total column density of NHCN = 1.6 ± 0.1 × 1016 cm-2 for V4334 Sgr. This result sets a lower limit on the amount of hydrogen that was ejected into the wind during the born-again event of this source. For V605 Aql, we obtained a lower limit for the integrated line intensities I12CO/I13CO> 4. [ABSTRACT FROM AUTHOR]

We present a morphological study of nebulae around Wolf-Rayet (WR) stars using archival narrow-band optical and Wide-field Infrared Survey Explorer (WISE) infrared images. The comparison among WISE images in different bands and optical images proves to be a very efficient procedure to identify the nebular emission from WR nebulae, and to disentangle it from that of the ISM material along the line of sight. In particular, WR nebulae are clearly detected in the WISE W4 band at 22 µm. Analysis of available mid-IR Spitzer spectra shows that the emission in this band is dominated by thermal emission from dust spatially coincident with the thin nebular shell or most likely with the leading edge of the nebula. TheWR nebulae in our sample present different morphologies that we classified into well defined WR bubbles (bubble B-type nebulae), clumpy and/or disrupted shells (clumpy/disrupted C-type nebulae), and material mixed with the diffuse medium (mixedM-type nebulae). The variety of morphologies presented by WR nebulae shows a loose correlation with the central star spectral type, implying that the nebular and stellar evolutions are not simple and may proceed according to different sequences and time-lapses. We report the discovery of an obscured shell around WR35 only detected in the infrared. [ABSTRACT FROM AUTHOR]

We present an analysis of XMM-Newton Reflection Grating Spectrometer (RGS) observations of the planetary nebula (PN) NGC6543. These observations render it the second PN with high-resolution X-ray spectroscopic observations after BD+303639. The observations consist of 26 pointings, 14 of which included RGS observations for a total integration time of 435 ks. Many of these observations, however, were severely affected by high-background levels, and the net useful exposure time is drastically reduced to 25 ks. Only the O vii triplet at 22 Å is unambiguously detected in the RGS spectrum of NGC6543. We find this spectrum consistent with an optically thin plasma at 0.147 keV (1.7 MK) and nebular abundances. Unlike BD+303639, the X-ray emission from NGC6543 does not reveal overabundances of C and Ne. The results suggest that the N/O ratio of the hot plasma is consistent with that of the stellar wind, that is, it is lower than the nebular N/O ratio, but this result is not conclusive. [ABSTRACT FROM AUTHOR]

We present the analysis of XMM-Newton archival observations towards the Wolf-Rayet (WR) bubble around WR16. Despite the closed bubble morphology of this WR nebula, the XMM-Newton observations show no evidence of diffuse emission in its interior as in the similar WR bubbles NGC6888 and S 308. We use the present observations to estimate a 3-σ upper limit to the X-ray luminosity in the 0.3-1.5 keV energy band equal to 7.4× 1032 erg s-1 for the diffuse emission from the WR nebula, assuming a distance of 2.37 kpc. TheWR nebula around WR16 is the fourth observed by the current generation of X-ray satellites and the second not detected. We also examine FUSE spectra to search for nebular O vi absorption lines in the stellar continuum of WR16. The present far-UV data and the lack of measurements of the dynamics of the optical WR bubble do not allow us to confirm the existence of a conductive layer of gas at T ∼ 3× 105 K between the cold nebular gas and the hot gas in its interior. The present observations result in an upper limit of ne< 0.6 cm-3 on the electron density of the X-ray emitting material within the nebula. [ABSTRACT FROM AUTHOR]