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Papers by Roberto Valois Mendez

Monthly Notices of the Royal Astronomical Society

Rev Chil Obstet Ginecol, 1994

Base de dados : LILACS. Pesquisa : 143921 [Identificador único]. Referências encontradas : 1 [ref... more Base de dados : LILACS. Pesquisa : 143921 [Identificador único]. Referências encontradas : 1 [refinar]. Mostrando: 1 .. 1 no formato [Detalhado]. página 1 de 1, 1 / 1, LILACS, seleciona. para imprimir. Fotocópia. experimental, Documentos relacionados. Id: 143921. ...

Astronomy & Astrophysics, 2015

Context. At least 492 central stars of Galactic planetary nebulae (CSPNs) have been assigned spec... more Context. At least 492 central stars of Galactic planetary nebulae (CSPNs) have been assigned spectral types. Since many CSPNs are faint, these classification efforts are frequently made at low spectral resolution. However, the stellar Balmer absorption lines are contaminated with nebular emission; therefore in many cases a low-resolution spectrum does not enable the determination of the H abundance in the CSPN photosphere. Whether or not the photosphere is H deficient is arguably the most important fact we should expect to extract from the CSPN spectrum, and should be the basis for an adequate spectral classification system. Aims. Our purpose is to provide accurate spectral classifications and contribute to the knowledge of central stars of planetary nebulae and stellar evolution. Methods. We have obtained and studied higher quality spectra of CSPNs described in the literature as weak emission-line star (WELS). We provide descriptions of 19 CSPN spectra. These stars had been previously classified at low spectral resolution. We used medium-resolution spectra taken with the Gemini Multi-Object Spectrograph (GMOS). We provide spectral types in the Morgan-Keenan (MK) system whenever possible. Results. Twelve stars in our sample appear to have normal H rich photospheric abundances, and five stars remain unclassified. The rest (two) are most probably H deficient. Of all central stars described by other authors as WELS, we find that at least 26% of them are, in fact, H rich O stars, and at least 3% are H deficient. This supports the suggestion that the denomination WELS should not be taken as a spectral type, because, as a WELS based on low-resolution spectra, it cannot provide enough information about the photospheric H abundance.

Planetary Nebulae, 1997

Küstner (1921) catalogued K648 in his photographic survey of M15, but it was not recognized as a ... more Küstner (1921) catalogued K648 in his photographic survey of M15, but it was not recognized as a PN central star until Pease (1928) discovered the nebula, denoted Psl. As one of very few PN known in globular clusters-it was the only known until Gillet et al. (1989) reported the discovery of a second in M22 and Jacoby et al. (this meeting) announced two new but very faint objects-K648 offers one of the better opportunities to study the post-AGB evolution of extreme Pop. II stars. Previous investigations of the nebula and star (Adams et al. 1984; Pena, Torres-Peimbert, & Ruiz 1992; Heber, Dreizier, & Werner 1993) all concluded that the stellar temperature is slightly less than 40000 K. Heber et al also concluded the photospheric He and C abundances were 3 χ and 5 χ higher than solar. We have conducted a NLTE model atmosphere analysis of K648 based on optical spectra obtained with the 10 m Keck HIRES échelle spectrograph. The high resolution (R = λ/Δλ = 35,000 [4 pixels]), high S/N (140 at H7) Keck spectra of K648 revealed HI Balmer, Hei, Hell, CIII, and CIV absorption lines, among others. Also evident in the HIRES spectra were weak narrow lines indicating contamination by a metal poor red giant spectrum; by comparing the equivalent widths of the contaminating features to a reference red giant spectrum (metallicity similar to M15), we successfully removed the red giant light (contributing 15% to 25% of the observed continuum) from the K648 spectrum. Lines of Hel (e.g., A4471Â) present in the HIRES spectra were particularly significant, enabling use of the Hel/Hell ionization equilibrium to determine T e ff more reliably than possible previously. We derive T e fr = 43000 K, log g = 3.9, and helium abundance y = 0.08 (nearly solar) for K648; nor does our analysis of the CIII and CIV lines in the HIRES spectra support the high C abundance derived by Heber et al. Comparison to evolutionary tracks in the log(#)-log(T e ff) plane yields distance-independent estimates of the stellar mass (0.60 Μ Θ) and post-AGB evolutionary age (2000 y). Finally, we derive a spectroscopic distance to K648, d = 11.8 ± 1.5 kpc, which compares favorably to the 10.4 ± 0.8 kpc accepted distance to M15 (Durrell & Harris 1993) to within the quoted errors. This result strongly supports the reliability of central star spectroscopic distances to PN generally.

Proceedings of the International Astronomical Union, 2011

Using the Subaru telescope and its FOCAS spectrograph in multi-object mode, we have obtained spec... more Using the Subaru telescope and its FOCAS spectrograph in multi-object mode, we have obtained spectra of a selection of bright PNs in two galaxies: the Virgo giant elliptical M 60 (NGC 4649), and the starburst spiral M 82. We report on individual extinctions, as determined from the Balmer decrement, and also on the intensities of [O III] 4959 and 5007 relative to Hβ.

Astronomy and Astrophysics Supplement Series, 2000

Radiation-driven winds of hot luminous stars XVIII. The unreliability of stellar and wind paramet... more Radiation-driven winds of hot luminous stars XVIII. The unreliability of stellar and wind parameter determinations from optical vs. UV spectral analysis of selected central stars of planetary nebulae and the possibility of some CSPNs as single-star supernova Ia progenitors

Astronomy & Astrophysics, 2010

(Abridged) We trace the interaction processes of galaxies at intermediate redshift by measuring t... more (Abridged) We trace the interaction processes of galaxies at intermediate redshift by measuring the irregularity of their ionized gas kinematics, and investigate these irregularities as a function of the environment (cluster versus field) and of morphological type (spiral versus irregular). Our sample consists of 92 distant galaxies. 16 cluster (z~0.3 and z~0.5) and 29 field galaxies (mean z=0.44) of these have velocity fields with sufficient signal to be analyzed. We find that the fraction of galaxies that have irregular gas kinematics is remarkably similar in galaxy clusters and in the field at intermediate redshifts. The distribution of the field and cluster galaxies in (ir)regularity parameters space is also similar. On the other hand galaxies with small central concentration of light, that we see in the field sample, are absent in the cluster sample. We find that field galaxies at intermediate redshifts have more irregular velocity fields as well as more clumpy and less centrally concentrated light distributions than their local counterparts. Comparison with a SINS sample of 11 z ~ 2 galaxies shows that these distant galaxies have more irregular gas kinematics than our intermediate redshift cluster and field sample. We do not find a dependence of the irregularities in gas kinematics on morphological type. We find that two different indicators of star formation correlate with irregularity in the gas kinematics. More irregular gas kinematics, also more clumpy and less centrally concentrated light distributions of spiral field galaxies at intermediate redshifts in comparison to their local counterparts indicate that these galaxies are probably still in the process of building their disks via mechanisms such as accretion and mergers. On the other hand, they have less irregular gas kinematics compared to galaxies at z ~ 2.

AIP Conference Proceedings, 2006

Spectra and Neutron Dosimetry Inside a PET Cyclotron Vault Room. [AIP Conference Proceedings 854,... more Spectra and Neutron Dosimetry Inside a PET Cyclotron Vault Room. [AIP Conference Proceedings 854, 203 (2006)]. Héctor René Vega‐Carrillo, Roberto Méndez, María Pilar Iñiguez, Joseph Martí‐Climent, Iván Peñuelas, Raquel Barquero. ...