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Title: Yonsei Evolutionary Population Synthesis (YEPS). II. Spectro-photometric Evolution of Helium-enhanced Stellar Populations

Abstract

The discovery of multiple stellar populations in Milky Way globular clusters (GCs) has stimulated various follow-up studies on helium-enhanced stellar populations. Here we present the evolutionary population synthesis models for the spectro-photometric evolution of simple stellar populations (SSPs) with varying initial helium abundance ( Y {sub ini}). We show that Y {sub ini} brings about dramatic changes in spectro-photometric properties of SSPs. Like the normal-helium SSPs, the integrated spectro-photometric evolution of helium-enhanced SSPs is also dependent on metallicity and age for a given Y {sub ini}. We discuss the implications and prospects for the helium-enhanced populations in relation to the second-generation populations found in the Milky Way GCs. All of the models are available at http://web.yonsei.ac.kr/cosmic/data/YEPS.htm.

Authors:
; ;  [1]
  1. Center for Galaxy Evolution Research, Yonsei University, Seoul 03722 (Korea, Republic of)
Publication Date:
OSTI Identifier:
22663493
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 842; Journal Issue: 2; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ABUNDANCE; COMPUTERIZED SIMULATION; HELIUM; METALLICITY; MILKY WAY; SPECTROPHOTOMETRY; STAR CLUSTERS; STAR EVOLUTION; STARS; SYNTHESIS

Citation Formats

Chung, Chul, Yoon, Suk-Jin, and Lee, Young-Wook, E-mail: chulchung@yonsei.ac.kr, E-mail: sjyoon0691@yonsei.ac.kr. Yonsei Evolutionary Population Synthesis (YEPS). II. Spectro-photometric Evolution of Helium-enhanced Stellar Populations. United States: N. p., 2017. Web. doi:10.3847/1538-4357/AA6F19.
Chung, Chul, Yoon, Suk-Jin, & Lee, Young-Wook, E-mail: chulchung@yonsei.ac.kr, E-mail: sjyoon0691@yonsei.ac.kr. Yonsei Evolutionary Population Synthesis (YEPS). II. Spectro-photometric Evolution of Helium-enhanced Stellar Populations. United States. doi:10.3847/1538-4357/AA6F19.
Chung, Chul, Yoon, Suk-Jin, and Lee, Young-Wook, E-mail: chulchung@yonsei.ac.kr, E-mail: sjyoon0691@yonsei.ac.kr. Tue . "Yonsei Evolutionary Population Synthesis (YEPS). II. Spectro-photometric Evolution of Helium-enhanced Stellar Populations". United States. doi:10.3847/1538-4357/AA6F19.
@article{osti_22663493,
title = {Yonsei Evolutionary Population Synthesis (YEPS). II. Spectro-photometric Evolution of Helium-enhanced Stellar Populations},
author = {Chung, Chul and Yoon, Suk-Jin and Lee, Young-Wook, E-mail: chulchung@yonsei.ac.kr, E-mail: sjyoon0691@yonsei.ac.kr},
abstractNote = {The discovery of multiple stellar populations in Milky Way globular clusters (GCs) has stimulated various follow-up studies on helium-enhanced stellar populations. Here we present the evolutionary population synthesis models for the spectro-photometric evolution of simple stellar populations (SSPs) with varying initial helium abundance ( Y {sub ini}). We show that Y {sub ini} brings about dramatic changes in spectro-photometric properties of SSPs. Like the normal-helium SSPs, the integrated spectro-photometric evolution of helium-enhanced SSPs is also dependent on metallicity and age for a given Y {sub ini}. We discuss the implications and prospects for the helium-enhanced populations in relation to the second-generation populations found in the Milky Way GCs. All of the models are available at http://web.yonsei.ac.kr/cosmic/data/YEPS.htm.},
doi = {10.3847/1538-4357/AA6F19},
journal = {Astrophysical Journal},
number = 2,
volume = 842,
place = {United States},
year = {Tue Jun 20 00:00:00 EDT 2017},
month = {Tue Jun 20 00:00:00 EDT 2017}
}
  • We present a series of papers on the 2012 version of the Yonsei Evolutionary Population Synthesis (YEPS) model, which was constructed based on over 20 years of research. This first paper delineates the spectroscopic aspect of integrated light from stellar populations older than 1 Gyr. The standard YEPS is based on the most up-to-date Yonsei-Yale stellar evolutionary tracks and BaSel 3.1 flux libraries, and provides absorption line indices of the Lick/IDS system and high-order Balmer lines for simple stellar populations as functions of stellar parameters, such as metallicity, age, and {alpha}-element mixture. Special care has been taken to incorporate amore » systematic contribution from horizontal-branch (HB) stars, which alters the temperature-sensitive Balmer lines significantly, resulting in up to a 5 Gyr difference in the age estimation of old, metal-poor stellar populations. We also find that HBs exert an appreciable effect not only on the Balmer lines but also on the metallicity-sensitive lines, including the magnesium index. This is critical in explaining the intriguing bimodality found in index distributions of globular clusters in massive galaxies and to accurately derive spectroscopic metallicities from various indices. A full set of the spectroscopic and photometric YEPS model data of the entire parameter space is currently downloadable at http://web.yonsei.ac.kr/cosmic/data/YEPS.htm.« less
  • Models for the formation and evolution of galaxies readily predict physical properties such as star formation rates, metal-enrichment histories, and, increasingly, gas and dust content of synthetic galaxies. Such predictions are frequently compared to the spectral energy distributions of observed galaxies via the stellar population synthesis (SPS) technique. Substantial uncertainties in SPS exist, and yet their relevance to the task of comparing galaxy evolution models to observations has received little attention. In the present work, we begin to address this issue by investigating the importance of uncertainties in stellar evolution, the initial stellar mass function (IMF), and dust and interstellarmore » medium (ISM) properties on the translation from models to observations. We demonstrate that these uncertainties translate into substantial uncertainties in the ultraviolet, optical, and near-infrared colors of synthetic galaxies. Aspects that carry significant uncertainties include the logarithmic slope of the IMF above 1 M{sub sun}, dust attenuation law, molecular cloud disruption timescale, clumpiness of the ISM, fraction of unobscured starlight, and treatment of advanced stages of stellar evolution including blue stragglers, the horizontal branch, and the thermally pulsating asymptotic giant branch. The interpretation of the resulting uncertainties in the derived colors is highly non-trivial because many of the uncertainties are likely systematic, and possibly correlated with the physical properties of galaxies. We therefore urge caution when comparing models to observations.« less
  • Broad-band photometric data and infrared line indices have been combined with new results on giant-branch luminosity functions to yield population synthesis for giant elliptical galaxies. If the main-sequence mass function is a power law of slope x, a value x<1 is indicated. This yields rather rapid luminosity evolution and a large correction to the deceleration parameter q$sub 0$ as derived from the Hubble diagram for first-ranked cluster ellipticals. The uncertainties are discussed. (AIP)
  • The evolutionary synthesis technique is used to construct population models for giant elliptical galaxies, using detailed spectrophotometric data for the galaxies and for stars obtained with the Oke multichannel spectrometer on the Hale telescope. We find that ellipticals are well represented by an old metal-rich population with a turnoff at B-Vapprox.0.80 and a turnoff mass function slope x< or approx. =1, plus a quite significant contribution from stars above the turnoff. The nature of these objects is discussed, and it is concluded that the present data and astrophysical constraints cannot distinguish between a small young population and a blue stragglermore » population augmented by a few O stars.« less
  • A new grid of 450 theoretical models for old simple stellar populations is presented. Evolutionary synthesis explores the influence of relevant parameters such as age, chemical composition, initial mass function, and stellar mass loss, on the integrated spectral energy distribution. Ages range between 4 and 18 Gyr, and the metallicity Z ranges from 0.0001 to 0.03, with helium content Y = 0.23 and 0.25, respectively. Three values are considered for the initial mass function, assumed as a power law, N(M) proportional to M exp-s; s = 1.35, 2.35, and 3.35. The computational code takes into account in a quantitative waymore » the contributions from all the relevant stellar evolutionary phases according to the theory of the stellar evolution. Thus, late stage in the life of low- mass stars, such as the horizontal branch and the asymptotic and postasymptotic giant branches are accounted for. Furthermore, a simplified treatment for the evolution of the horizontal branch is developed, and the influence of different morphologies are investigated. 109 refs.« less