R. L. M. Corradi

T Tauri candidates and accretion rates using IPHAS : method and application to IC 1396

The definitive version can be found at : http://onlinelibrary.wiley.com/ Copyright Royal Astronomical Society

Discovery of close binary central stars in the planetary nebulae NGC 6326 and NGC 6778

Original article can be found at : http://www.aanda.org/ Copyright The European Southern Observatory

The Local Group Census: Planetary nebulae in IC 10, Leo A and Sextans A

In the framework of our narrow-band survey of the Local Group galaxies, we present the results of the search for planetary nebulae (PNe) in the dwarf irregular galaxies IC 10, Leo A and Sextans A. Using the standard on-band/off-band technique, sixteen new candidate PNe have been discovered in the closest starburst galaxy, IC 10. The optical size of this galaxy is estimated to be much larger than previously thought, considering the location of the new PNe in an area of 3.6 kpc × 2.7 kpc. We also confirm the results of previous studies for the other two dwarf irregular galaxies, with the detection of one candidate PN in Leo A and another one in Sextans A. We review the number of planetary nebulae discovered in the Local Group to date and their behaviour with metallicity. We suggest a possible fall in the observed number of PNe when (Fe/H) � -1.0, which might indicate that below this point the formation rate of PNe is much lower than for stellar populations of near Solar abundances. We also find non-negligible metallicity effects on the (O ) luminosity of the brightest PN of a galaxy.

The orbital period of V458 Vulpeculae, a post-double common-envelope nova

We present time-resolved optical spectroscopy of V458 Vulpeculae (Nova Vul 2007 No. 1) spread over a period of 15 months starting 301 d after its discovery. Our data reveal radial-velocity variations in the He iiλ5412 and He iiλ4686 emission lines. A period analysis of the radial-velocity curves resulted in a period of 98.096 47 ± 0.000 25 min (0.068 122 55 ± 0.000 000 17 d) which we identify with the orbital period of the binary system. V458 Vul is therefore the planetary nebula central binary star with the shortest period known. We explore the possibility of the system being composed of a relatively massive white dwarf (M1≳ 1.0 M⊙) accreting matter from a post-asymptotic giant branch star which produced the planetary nebula observed. In this scenario, the central binary system therefore underwent two common-envelope episodes. A combination of previous photoionization modelling of the nebular spectra, post-asymptotic giant branch evolutionary tracks and the orbital period favour a mass of M2∼ 0.6 M⊙ for the donor star. Therefore, the total mass of the system may exceed the Chandrasekhar mass, which makes V458 Vul a Type Ia supernova progenitor candidate.

Chemical abundances of Planetary Nebulae in M 33

Using spectroscopic data presented in Magrini et al. (2003), we have analyzed with the photoionization code CLOUDY 94.00 (Ferland et al. 1998) 11 Planetary Nebulae belonging to the spiral galaxy M 33. Central star temperatures and nebular parameters have been determined. In particular the chemical abundances of He/H, O/H, N/H, Ar/H, and S/H have been measured and compared with values obtained via the Ionization Correction Factors (ICF) method, when available. Chemical abundance relationships have been investigated; in particular, a correlation between N/ Ha nd N/O similar to the Galactic one (Henry 1990), and a feeble anti-correlation between O/ Ha nd N/O have been found. A gradient in O/H across the disc of M 33 is consistent with the one found from H  regions in this galaxy (Vilchez et al. 1988). Further studies in the outer parts of M 33 are however needed to ascertain this point. The present result shows that oxygen and helium abundances (with lower accuracy also nitrogen, argon and sulphur) can be actually estimated from the brightest PNe of a galaxy, even if the electron temperature cannot be measured. We also found that the oxygen abundance is quite independent of the absolute magnitude of the PN and consequently the brightest PNe are representative of the whole PN population. This represents an important tool for measuring the metallicity of galaxies at the time of the formation of PNe progenitors.

IPHAS and the symbiotic stars - III. New discoveries and their IR spectral energy distributions

Context. The IPHAS Hα survey provides a rich database to search for emission-line sources in the northern Galactic plane. Aims. We are systematically searching for symbiotic stars in the Milky Way using IPHAS. Our final goal, a complete census of this class of objects in the Galaxy, is a fundamental figure for discussing their overall properties and relevance to other classes of stars. Methods. Candidate symbiotic stars were selected using a refined combination of IPHAS and 2MASS photometric colours. Optical spectroscopy, together with the analysis of their spectral energy distribution in the IR, were obtained to confirm their nature and determine their main properties. Results. Five new symbiotic stars have been confirmed from spectroscopy at the 10.4 m GTC telescope. In one case, confirming the presence of a red giant star required near infrared spectroscopy. In another case, its symbiotic nature was adopted based on the strong similarity of its optical spectrum and spectral energy distribution to those of other genuinely symbiotic stars. The spectral energy distribution of the two S-types found is well fitted by red-giant model atmospheres up to 22 μm without evidence of IR excesses due to dust. In contrast, the three D-types mostly show emission from hot dust with a temperature around 1000 K. We also present the spectroscopic and photometric monitoring of the symbiotic star IPHASJ190832.31+051226.6 that was originally discovered in outburst, and it has now returned to a lower luminosity status. The spectra of thirteen other sources, all classified as young stellar objects except for a new compact planetary nebula, are also presented. Conclusions. The refinement of our discovery method, the completion of the IPHAS survey and photometric calibration, and the start of the twin survey in the south, VPHAS+, provide excellent perspectives for completing a reliable census of symbiotic stars in the Galaxy in the next few years.

Automatic spectral classification of stellar spectra with low signal-to-noise ratio using artificial neural networks

Context. As part of a project aimed at deriving extinction-distances for thirty-five planetary nebulae, spectra of a few thousand stars were analyzed to determine their spectral type and luminosity class. Aims. We present here the automatic spectral classification process used to classify stellar spectra. This system can be used to classify any other stellar spectra with similar or higher signal-to-noise ratios. Methods. Spectral classification was performed using a system of artificial neural networks that were trained with a set of line-strength indices selected among the spectral lines most sensitive to temperature and the best luminosity tracers. The training and validation processes of the neural networks are discussed and the results of additional validation probes, designed to ensure the accuracy of the spectral classification, are presented. Results. Our system permits the classification of stellar spectra of signal-to-noise ratio (S /N) significantly lower than it is generally considered to be needed. For S /N ≥ 20, a precision generally better than two spectral subtypes is obtained. At S /N < 20, classification is still possible but has a lower precision. Its potential to identify peculiar sources, such as emission-line stars, is also recognized.

K 4−47: a planetary nebula excited by photons and shocks

K 4-47 is an unusual planetary nebula composed of a compact high-ionization core and a pair of low-ionization knots. Long-slit medium-resolution spectra of the knots and core are analyzed in this paper. Assuming photoionization from the central star, we have derived physical parameters for all the nebular components, and the (icf) chemical abundances of the core, which appear similar to Type-I PNe for He and N/O but significantly deficient in oxygen. The nebula has been further modelled using both photoionization (CLOUDY) and shock (MAPPINGS) codes. From the photoionization modelling of the core, we find that both the strong auroral [O iii] 4363u and [N ii] 5755u emission lines observed and the optical size of the core cannot be accounted for if a homogeneus density is adopted. We suggest that a strong density stratification, matching the high-density core detected at radio wavelengths and the much lower density of the optical core, might solve the problem. From the bow-shock modelling of the knots, on the other hand, we find that knots’ chemistry is also represented by Type-I PN abundances, and that they would move with velocities of 250 - 300kms 1 .

The Local Group Census: Planetary nebulae in Sextans B ?

Five planetary nebulae (PNe) have been discovered in the nearby dwarf irregular galaxy. Emission line images were obtained using the Wide Field Camera of the 2.5 m Isaac Newton Telescope (INT) at La Palma (Spain). The candidate PNe were identied by their point-like appearance and relatively strong (O iii) emission-line fluxes. They are located within a galactocentric distance of 2.8 arcmin, corresponding to 1.1 kpc at the distance of Sextans B. Luminosities are in the range 1800{5600 L. Sextans B is one of the smallest dwarf irregular galaxies with a PN population. The number of PNe detected suggest an enhanced star formation rate between 1 and 5 Gyr ago.

New candidate planetary nebulae in M 81

A3 4 0 34 0 eld centred on the spiral galaxy M 81 has been searched for emission-line objects using the prime focus wide eld camera (WFC) of the 2.54 m Isaac Newton Telescope (La Palma, Spain). A total of 171 candidate planetary nebulae (PNe) are found, 54 of which are in common with the ones detected by Jacoby et al. (1989). The behaviour of PNe excitation as a function of galactocentric distance is examined, and no signicant variations are found. The PNe luminosity function is built for the disk and bulge of M 81, separately. A distance modulus of 27:92 0:23 mag is found for disk PNe, in good agreement with previous distance measurements for M 81 (Jacoby et al. 1989; Huterer et al. 1995).