arXiv subjects

Marcelo Castellanos

Publications and source records attributed to Marcelo Castellanos.

8 recordsLinked to original sources

arXiv2008

Circumnuclear Regions of Star Formation in Early Type Galaxies

Circumnuclear star forming regions, also called hotspots, are often found in the inner regions of some spiral galaxies where intense processes of star formation are taking place. In the UV, massive stars dominate the observed circumnuclear emission even in the presence of an active nucleus, contributing between 30 and 50% to the H$β$ total emission of the nuclear zone. Spectrophotometric data of moderate resolution (3000 < R < 11000) are presented from which the physical properties of the ionized gas: electron density, oxygen abundances, ionization structure etc. have been derived.

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arXiv2007

The metal abundance of circumnuclear star forming regions in early type spirals. Spectrophotometric observations

We have obtained long-slit observations in the optical and near infrared of 12 circumnuclear HII regions (CNSFR) in the early type spiral galaxies NGC 2903, NGC 3351 and NGC 3504 with the aim of deriving their chemical abundances. Only for one of the regions, the [SIII] $λ$ 6312 Åwas detected providing, together with the nebular [SIII] lines at $λλ$ 9069, 9532 Å, a value of the electron temperature of T$_e$([SIII])= 8400$^{+ 4650}_{-1250}$K. A semi-empirical method for the derivation of abundances in the high metallicity regime is presented. We obtain abundances which are comparable to those found in high metallicity disc HII regions from direct measurements of electron temperatures and consistent with solar values within the errors. The region with the highest oxygen abundance is R3+R4 in NGC 3504, 12+log(O/H) = 8.85, about 1.5 solar if the solar oxygen abundance is set at the value derived by Asplund et al. (2005), 12+log(O/H)$_{\odot}$ = 8.66$\pm$0.05. Region R7 in NGC 3351 has the lowest oxygen abundance of the sample, about 0.6 times solar. In all the observed CNSFR the O/H abundance is dominated by the O$^+$/H$^+$ contribution, as is also the case for high metallicity disc HII regions. For our observed regions, however, also the S$^+$/S$^{2+}$ ratio is larger than one, contrary to what is found in high metallicity disc HII regions for which, in general, the sulphur abundances are dominated by S$^{2+}$/H$^+$...

astro-ph↗

arXiv2004

Ionising stellar populations in circumnuclear star forming regions

We present a study of the stellar populations and gas physical conditions in Circumnuclear Star Forming Regions (CNSFRs) based on spectrophotometric data, including the 7000 angstroms to 1 micron spectral range, which is of great interest to the whole diagnostic of the ionized gas. The properties of CNSFRs as a class are inferred from a comparison with those of other star forming emission-line objects: low excitation disc HII regions and HII galaxies.

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arXiv2002

Nebular diagnostics for young stellar populations: Photon escape versus atmosphere blanketing

Emission lines in ionized nebulae can provide strong and useful constraints on the properties of both ionizing and non-ionizing stellar populations in regions with star formation, provided that stellar evolution and stellar atmosphere models can be used in a selfconsistent way. Recently, the application of these techniques has shown important discrepancies between predicted and observed nebular spectra that point to stellar atmosphere models of WR stars which are too energetic and/or to a significant leakageof high energy photons. In this contribution, these two alternatives are analyzed in detail.

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arXiv2002

A comprehensive study of reported high metallicity giant HII regions. II. Ionising stellar populations

The ionising stellar populations of eleven HII regions in the spiral galaxies: NGC628, NGC925, NGC1232 and NGC1637, all of them reported to have solar or oversolar abundances according to empirical calibrations, have been analysed using stellar population synthesis models. Four of the observed regions in the sample, show features which indicate the presence of a population of Wolf-Rayet (WR) stars with ages between 2.3 and 4.1 Myr. This population is sufficient to explain the emission line spectrum of the low metallicity region H13 in NGC628. This is not the case for the rest of the regions for which a second ionising population is required to simultaneously reproduce both the WR features and the emission line spectrum. For two of the regions showing WR features, no consistent solution is found, since the population containing WR stars produces a spectral energy distribution which is too hard to explain the emission of the gas. Several solutions are proposed to solve this problem.

astro-ph↗

arXiv2002

Optical and near IR spectroscopy of low excitation HII regions with GTC

The determination of chemical abundances in ionized gas nebulae is based on the detection of auroral emission lines, from which it is possible to deduce the line temperatures of several ions. These lines are weak in low excitation objects, so its measurement is only possible in great aperture telescopes. The GTC would allow to plan a comprehenssive study of chemical abundances in such objects from mid-high resolution spectra, what would give more accurate observational restrictions to models of chemical evolution.

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arXiv2001

A comprehensive study of reported high metallicity giant HII regions. I. Detailed abundance analysis

We present long-slit observations in the optical and near infrared of fourteen HII regions in the spiral galaxies: NGC 628, NGC 925, NGC 1232 and NGC 1637, all of them reported to have solar or oversolar abundances according to empirical calibrations. For seven of the observed regions, ion-weighted temperatures from optical forbidden auroral to nebular line ratios have been obtained and for six of them, the oxygen abundances derived by standard methods turn out to be significantly lower than solar. The other one, named CDT1 in NGC 1232, shows an oxygen abundance of 12+log(O/H) = 8.95+-0.20 and constitutes, to the best of our knowledge, the first high metallicity HII region for which accurate line temperatures, and hence elemental abundances, have been derived. For the rest of the regions no line temperature measurements could be made and the metallicity has been determined by means of both detailed photoionisation modelling and the sulphur abundance parameter S_23. Only one of these regions shows values of O_23 and S_23 implying a solar or oversolar metallicity. According to our analysis, only two of the observed regions can therefore be considered as of high metallicity. The two of them fit the trends previously found in other high metallicity HII regions, i.e. N/O and S/O abundance ratios seem to be higher and lower than solar respectively.

astro-ph↗

arXiv2000

Chemical Abundances and Ionizing Clusters of HII Regions in the LINER Galaxy NGC 4258

We present long-slit observations in the optical and near infrared of eight HII regions in the spiral galaxy NGC 4258. Six of the observed regions are located in the SE inner spiral arms and the other two are isolated in the northern outer arms. A detailed analysis of the physical conditions of the gas has been performed. For two of the regions, an electron temperature has been derived from the [SIII] 6312 A line. For the rest, an empirical calibration based on the red and near infrared sulphur lines has been used. The oxygen abundances derived by both methods are found to be significantly lower (by a factor of two) than previously derived by using empirical calibrations based on the optical oxygen lines. In the brightest region, 74C, the observation of a prominent feature due to Wolf-Rayet stars provides an excellent constraint over some properties of the ionizing clusters. In the light of the current evolutionary synthesis models, no consistent solution is found to explain at the same time both the WR feature characteristics and the emission line spectrum of this region. In principle, the presence of WR stars could lead to large temperature fluctuations and also to a hardening of the ionizing radiation. None of these effects are found in region 74C for which the electron temperatures found from the [SIII] 6312A and the Paschen discontinuity at 8200A are equal within the errors and the effective temperature of the ionizing radiation is estimated at around 35,300K. Both more observations of confirmed high metallicity regions and a finer metallicity grid for the evolutionary synthesis models are needed in order to understand the ionizing populations of HII regions.

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