arXiv subjects

M. Cervino

Publications and source records attributed to M. Cervino.

50 records · Page 3Linked to original sources

arXiv2002

Gamma-ray line emission from OB associations and young open clusters. II. The Cygnus region

Gamma-ray and microwave observations of the Cygnus region reveal an intense signal of 1.809 MeV line emission, attributed to radioactive decay of 26Al, that is closely correlated with 53 GHz free-free emission, originating from the ionised interstellar medium. We modelled both emissions using a multi-wavelength evolutionary synthesis code for massive star associations that we applied to the known massive star populations in Cygnus. For all OB associations and young open clusters in the field, we determined the population age, distance, and richness as well as the uncertainties in all these quantities from published photometric and spectroscopic data. We propagate the population uncertainties in model uncertainties by means of a Bayesian method. The young globular cluster Cyg OB2 turns out to be the dominant 26Al nucleosynthesis and ionisation source in Cygnus. Our model reproduces the ionising luminosity of the Cygnus region very well, yet it underestimates 26Al production by about a factor of 2. We attribute this underestimation to shortcomings of current nucleosynthesis models, and suggest the inclusion of stellar rotation as possible mechanism to enhance 26Al production. We also modelled 60Fe nucleosynthesis in the Cygnus region, yet the small number of recent supernova events suggests only little 60Fe production. Consequently, a detection of the 1.137 MeV and 1.332 MeV decay lines of 60Fe from Cygnus by the upcoming INTEGRAL observatory is not expected.

astro-ph↗

arXiv2002

Statistics and multiwavelength synthesis models: towards a new generation of synthesis models

In this contribution I present my current work in a new generation of evolutionary synthesis models that compute the multiwavelength energy distribution (from gamma-rays to radio) as well as the associated dispersion for young stellar systems. I will also show some statistical effects that may appear in the analysis of surveys, like bimodal or multi-modal distributions and bias when color indices computed by the codes are compared with observations. Such new generation of synthesis models may be useful for the analysis of the data expected from GTC.

astro-ph↗

arXiv2002

Confidence Limits of Evolutionary Synthesis Models

The probabilistic nature of the IMF in stellar systems implies that clusters of the same mass and age do not present the same unique values of their observed parameters. Instead they follow a distribution. We address the study of such distributions in terms of their confidence limits that can be obtained by evolutionary synthesis models. These confidence limits can be understood as the inherent uncertainties of synthesis models. We will compare such confidence limits arising from the discreteness of the number of stars obtained with Monte Carlo simulations with the dispersion resulting from an analytical formalism. We give some examples of the effects on the kinetic energy, V--K, EW(H$β$) and multiwavelength continuum.

astro-ph↗

arXiv2001

Can stellar winds account for temperature fluctuations in H II regions? The case of NGC 2363

We compare the rate of kinetic energy injected by stellar winds into the extragalactic H II region NGC 2363 to the luminosity needed to feed the observed temperature fluctuations. The kinetic luminosity associated to the winds is estimated by means of two different evolutionary synthesis codes, one of which takes into account the statistical fluctuations expected in the Initial Mass Function. We find that, even in the most favorable conditions considered by our model, such luminosity is much smaller than the luminosity needed to account for the observed temperature fluctuations. The assumptions underlying our study are emphasized as possible sources of uncertainty affecting our results.

astro-ph↗

arXiv2001

Confidence levels of evolutionary synthesis models III: On sampling and Poissonian fluctuations

In terms of statistical fluctuations, stellar population synthesis models are only asymptotically correct in the limit of a large number of stars, where sampling errors become asymptotically small. When dealing with stellar clusters, starbursts, dwarf galaxies or stellar populations within pixels, sampling errors introduce a large dispersion in the predicted integrated properties of these populations. We present here an approximate but generic statistical formalism which allows a very good estimation of the uncertainties and confidence levels in any integrated property, bypassing extensive Monte Carlo simulations, and including the effects of partial correlations between different observables. Tests of the formalism are presented and compared with proper estimates. We derive the minimum mass of stellar populations which is required to reach a given confidence limit for a given integrated property. As an example of this general formalism, which can be included in any synthesis code, we apply it to the case of young (t < 20 Myr) starburst populations. We show that, in general, the UV continuum is more reliable than other continuum bands for the comparison of models with observed data. We also show that clusters where more than 10^5 Mo have been transformed into stars have a relative dispersion of about 10% in Q(He+) for ages smaller than 3 Myr. During the WR phase the dispersion increases to about 25% for such massive clusters. We further find that the most reliable observable for the determination of the WR population is the ratio of the luminosity of the WR bump over the Hbeta luminosity. A fraction of the observed scatter in the integrated properties of clusters and starbursts can be accounted for by sampling fluctuations.

astro-ph↗

arXiv2001

On some Uncertainties in Evolutionary Synthesis Models

Ranging from track interpolation techniques through model atmospheres to the stochastic nature of the IMF, there are many uncertainties which need to be taken into account when modelling HR diagrams or performing population synthesis, particularly if comparison with actual data is sought. In this paper, we highlight (1) the problem of discontinuities along evolutionary tracks of massive stars (M> 8 Mo), showing that inconsistencies appear in the computation of the corresponding isochrones, and (2) the sampling fluctuations produced by the stochastic nature of the IMF, presenting a statistical formalism to estimate the dispersion in any given observable of a stellar population due to sampling effects which bypasses the need of performing Monte Carlo simulations.

astro-ph↗

arXiv2001

Diffuse gamma-ray line emission from multiple OB Associations in Cygnus

The COMPTEL observations of the diffuse galactic 1.809 MeV emission attributed to the radioactive decay of 26Al have confirmed the diffuse nature of this interstellar emission line. One of the most significant features of the reconstructed intensity pattern is a flux enhancement in the direction of the Cygnus region. This region is fairly young and contains a wealth of massive stars, most of them grouped in the Cygnus OB associations. Multi-frequency model fitting strongly supports the hypothesis of massive stars and their descendent supernovae being the dominant sources of interstellar 26Al as observed by COMPTEL. Massive stars and supernovae are known to impart a large amount of kinetic energy into the surrounding ISM which lead to shockregions and large cavities. In addition, a large fraction of the electro-magnetic radiation of massive stars lies in the extreme ultra-violet causing photoionisation of the surrounding interstellar medium. We applied a population synthesis model in combination with an analytic model of the expansion of superbubbles to the Cygnus OB associations. The model predicts the expected 1.809 MeV flux and the gamma-ray line intensity due to interstellar 60Fe. We compute the sizes and expansion parameters of the expected HI-structures and the free-free emission intensities due to the photoionizing radiation from massive stars within this region of the sky. We discuss our present understanding of the Cygnus region with respect to the massive star census and history. Our model assigns about 70% of the 1.809 MeV intensity to six known OB associations, about 20% to known isolated sources and roughly 10% to an unkown diffuse component.

astro-ph↗

arXiv2001

Confidence limits of evolutionary synthesis models III. On time-integrated quantities

Evolutionary synthesis models are a fundamental tool to interpret the properties of observed stellar systems. In order to achieve a meaningful comparison between models and real data, it is necessary to calibrate the models themselves, i.e. to evaluate the dispersion due to the discreteness of star formation as well as the possible model errors. In this paper we show that linear interpolations in the log M - log t_k plane, that are customary in the evaluation of isochrones in evolutionary synthesis codes, produce unphysical results. We also show that some of the methods used in the calculation of time-integrated quantities (kinetic energy, and total ejected masses of different elements) may produce unrealistic results. We propose alternative solutions to solve both problems. Moreover, we have quantified the expected dispersion of these quantities due to stochastic effects in stellar populations. As a particular result, we show that the dispersion in the 14N/12C ratio increases with time.

astro-ph↗

arXiv2001

The Cygnus X region: a nucleosynthesis laboratory for INTEGRAL

The detection of 1.809 MeV gamma-ray line emission from the Cygnus X complex by the COMPTEL telescope is one of the most convincing proves of massive star nucleosynthesis in our Galaxy. The Cygnus X complex is an extremely active nearby region, containing several star forming regions, OB associations and young open star clusters. It houses some of the most massive stars known in our Galaxy and concentrates a large number of Wolf-Rayet stars. Thermal radio continuum emission and intense [C II] line emission reveals widespread ionisation, and at least 60 individual H II regions have been identified. In order to understand the 1.809 MeV gamma-ray line emission from the Cygnus X complex, and to compare the observations to theoretical nucleosynthesis calculations, we modelled the multi-wavelength spectrum of the region by means of an evolutionary synthesis model. Our investigation leads us to the following conclusions: 1. Stellar wind ejection is the dominant mechanism for the observed 26Al enrichment in the Cygnus X region. 2. Cyg OB2 is by far the dominating massive star association in Cygnus X and 1.809 MeV line emission from 26Al produced in this association should be detectable by the spectrometer SPI on INTEGRAL. 3. There is only low supernova activity in the Cygnus X complex and the 60Fe lines should be below INTEGRAL's detection sensitivity. The detectability of an individual massive star cluster (Cyg OB2) by INTEGRAL provides a fantastic opportunity for future nucleosynthesis studies using gamma-ray line spectroscopy. We will explore the scientific potential of such observations, and provide estimates for gamma-ray line intensity distributions based on the massive star census of the Cygnus X region.

astro-ph↗

arXiv2001

Confidence limits of SN, Kinetic Energy and chemical yields in evolutionary synthesis models

When evolutionary synthesis models take into account the stochastic nature of the IMF together with the discrete number of stars in real stellar clusters, typical output turns to dispersion band (where real data can be placed) instead of narrow lines. We present here a qualitative analysis of such dispersion in the SN rate, the kinetic energy and the 14N/12C ratio for different amounts of mass transformed into stars.

astro-ph↗

arXiv2000

Gamma-ray line emission from OB associations and young open clusters: I. Evolutionary synthesis models

We have developed a new diagnostic tool for the study of gamma-ray emission lines from radioactive isotopes (such as 26Al and 60Fe in conjunction with other multi-wavelength observables of Galactic clusters, associations, and alike objects. Our evolutionary synthesis models are based on the code of Cervino & Mas-Hesse (1994), which has been updated to include recent stellar evolution tracks, new stellar atmospheres for OB and WR stars, and nucleosynthetic yields from massive stars during hydrostatic burning phases and explosive SN II and SN Ib events. The temporal evolution of 26Al and 60Fe production, the equivalent yield of 26Al per ionising O7 V star (yal), and other observables are predicted for a coeval population. We also employed our model to estimate the steady state nucleosynthesis of a population of solar metallicity. To allow for a fully quantitative analysis of existing and future multi-wavelength observations, we propose a Bayesian approach that allows the inclusion of IMF richness effects and observational uncertainties in the analysis. In particular, a Monte Carlo technique is adopted to estimate probability distributions for all observables of interest. We outline the procedure of exploiting these distributions by applying our model to a fictive massive star association. Applications to existing observations of the Cygnus and Vela regions will be discussed in companion papers.

astro-ph↗

arXiv2000

Confidence levels of evolutionary synthesis models

The stochastic nature of the IMF in young stellar clusters implies that clusters of the same mass and age do not present the same unique values of their observed parameters. Instead they follow a distribution. We address the study of such distributions, parameterised in terms of their confidence limits, in evolutionary synthesis models. These confidence limits can be understood as the inherent uncertainties of the synthesis models. Here we concentrate on some parameters such as EW(Hb) in emission. For instance, we show that for a cluster where 10^5 Mo have been transformed into stars, the dispersion of EW(Hb) is about 18% within the 90% confidence levels at ages between 3.5 and 5 Myrs.

astro-ph↗

arXiv1999

Evolutionary population synthesis: the effect of binary systems

We present in this contribution our set of multiwavelength synthesis models including the evolution of single and binary stars. The main results we have obtained can be summarized as follows: (a) massive close-binary systems will start to experience mass transfer episodes after the first 4 Myr of the starburst evolution; (b) as a result of these mass transfer processes, stars of relatively low initial mass can loose completely their envelope and become a WR. In this way, the formation of WR stars is extended over longer than 15 Myr, and does not stop at 6 Myr as predicted by models including only single stars; (c) WR stars can thus be coeval with red supergiants, which peak at around 10 Myr for solar metallicities; (d) the accretion of mass will originate relatively massive stars at ages for which they should have already disappeared; these stars, together with the WR stars formed in rather evolved clusters, increase the production of ionizing photons, so that the H$β$ equivalent width will not drop as rapidly as predicted by models considering only individual stars; and (e) the mass transfer to compact companions will produce an additional source of high-energy radiation in the form of high-mass X-ray binaries, not predicted either by standard synthesis models.

astro-ph↗

arXiv1999

Hard X-ray-to-radio energy distributions in starburst galaxies

We present in this contribution the predictions on the multiwavelength spectral energy distribution of our evolutionary population synthesis models including single and binary stellar systems. The high energy computations include the emission associated to X-ray binaries and supernovae remnants, as well as the mechanical energy released into the interstellar medium, which can be partially reprocessed into thermal X-rays. With these components we compute the spectral energy distribution of starburst galaxies from X-ray to radio ranges, and analyze finally the effects of the high energy emission on the H and He ionizing continuum.

astro-ph↗