arXiv subjects

Jonas Johansson

Publications and source records attributed to Jonas Johansson.

21 records · Page 2Linked to original sources

arXiv2010

Chemical abundance ratios of galactic globular clusters from modelling integrated light spectroscopy

We use our new, flux-calibrated stellar population model of absorption-line indices to derive ages, metallicities, and various element abundance ratios from integrated light spectroscopy of galactic globular clusters. The ages agree well with the literature and are all consistent with the age of the universe. There is a considerable scatter, though, and we obtain systematically larger ages than CMD determinations mostly for metal-rich globular clusters. The metallicities agree well with literature values on the Zinn & West scale, if we adopt iron abundance [Fe/H] for those clusters whose ages agree with the CMD ages. It turns out that the derivation of individual element abundance ratios is not reliable at [Fe/H]<-1 dex, while the [alpha/Fe] ratio is robust at all metallicities. We find general enhancement of light and alpha elements, as expected, with significant variations for some elements. The elements O and Mg follow the same general enhancement with almost identical distributions of [O/Fe] and [Mg/Fe]. We obtain slightly lower [C/Fe] and very high [N/Fe] ratios, instead. This chemical anomaly, commonly attributed to self-enrichment, is well known in globular clusters from individual stellar spectroscopy. It is the first time that this pattern is obtained also from the integrated light. The alpha elements follow a pattern such that the heavier elements Ca and Ti are less enhanced. More specifically, the [Ca/Fe] and [Ti/Fe] ratios are lower than [O/Fe] and [Mg/Fe] by about 0.2 dex. This trend is also seen in recent determinations of element abundances in globular cluster and field stars of the Milky Way. This suggests that Type Ia supernovae contribute significantly to the enrichment of the heavier alpha elements as predicted by nucleosynthesis calculations and galactic chemical evolution models.

astro-ph.CO↗

arXiv2010

Flux-calibrated stellar population models of Lick absorption-line indices with variable element abundance ratios

We present new stellar population models of Lick absorption-line indices with variable element abundance ratios. The models are based on our new calibrations of absorption-line indices with stellar parameters derived from the MILES stellar library. The key novelty compared to our previous models is that they are now available at the higher spectral resolution of MILES (~2.7A FWHM) and flux-calibrated, hence not tied anymore to the Lick/IDS system. This is essential for the interpretation of galaxy spectra where calibration stars are not available, such as large galaxy redshift surveys or other high-redshift observations. We note that the MILES resolution appears to be comparable to SDSS resolution, so that our models can be applied to SDSS data without any corrections for instrumental spectral resolution. For the first time we provide random errors for the model predictions based on the uncertainties in the calibration functions and the underlying stellar parameter estimates. We show that random errors are small except at the edges of the parameter space (high/low metallicities and young ages <1 Gyr) where the stellar library is under-sampled. We calibrate the base model for the parameters age, metallicity and alpha/Fe ratio with galactic globular cluster and galaxy gradient data. We discuss two model flavours with different input stellar evolutionary tracks from the Frascati and Padova groups. The new model release now includes abundance variations of the elements C, N, Mg, Na, Si, Ca, Ti, Cr, and Fe. The individual elements that are best accessible with these models and the standard set of Lick absorption features are C, N, Mg, Ca, Ti, and Fe. The model data is available at www.icg.port.ac.uk/~thomasd.

astro-ph.CO↗

arXiv2010

Empirical calibrations of optical absorption line indices based on the stellar library MILES

Stellar population models of absorption line indices are an important tool for the analysis of stellar population spectra. They are most accurately modelled through empirical calibrations of absorption line indices with the stellar parameters effective temperature, metallicity, and surface gravity, the so-called fitting functions. Here we present new empirical fitting functions for the 25 optical Lick absorption line indices based on the new stellar library MILES. The major improvements with respect to the Lick/IDS library are the better sampling of stellar parameter space, a generally higher signal- to-noise, and a careful flux calibration. In fact we find that errors on individual index measurements in MILES are considerably smaller than in Lick/IDS. Instead we find the rms of the residuals between the final fitting functions and the data to be dominated by errors in the stellar parameters. We provide fitting functions for both Lick/IDS and MILES spectral resolutions, and compare our results with other fitting functions in the literature. A Fortran 90 code is available online in order to simplify the implementation in stellar population models. We further calculate the offsets in index measurements between the Lick/IDS system to a flux calibrated system. For this purpose we use the three libraries MILES, ELODIE, and STELIB. We find that offsets are negligible in some cases, most notably for the widely used indices Hbeta, Mgb, Fe5270, and Fe5335. In a number of cases, however, the difference between flux calibrated library and Lick/IDS is significant with the offsets depending on index strengths. Interestingly, there is no general agreement between the three libraries for a large number of indices, which hampers the derivation of a universal offset between the Lick/IDS and flux calibrated systems.

astro-ph.CO↗