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ASTR 5610, Majewski [SPRING 2016]. Lecture Notes

ASTR 5610 (Majewski) Lecture Notes


One of the goals of stellar population studies is to unravel the (typically complicated) histories of galaxies through observation of the stars and gas in these systems visible today.

Age as the "Independent Variable" in Evolutionary Models

In a perfect situation, age (time) would serve as the preferred primary, independent variable in constructing a galactic history, and we would tie all other observed properties (e.g., chemical, dynamical, spatial) to this independent variable to derive the evolution of galactic properties and reconstruct "snapshots of the past".

An estimation of the Dynamical Population Box for the Milky Way, using a variety of available data. See Majewski (1999, in Globular C lusters, eds. C. Martinez Roger et al., Cambridge Univ. Press).

Nevertheless, undaunted, we must go on!

In this lecture we ask, what can we broadly ascertain about the distribution of stellar ages in a system by simple observation of what kinds of stars are present?

CMDs and Isochrones

In the preceding lectures, we talked about the evolution of individual stars.

Obviously, with a stellar population things are more complicated:

For simple (or composite) stellar systems whose populations we can resolve into individual stars down past the main sequence turn-off (MSTO), we are at a great advantage since many details of the stellar population(s) are discernible and can be used to constrain a fit to the CMD distribution of stars:

(Left) Isochrone fit to the open cluster NGC 2420 by Salaris et al. (2004, A&A, 414, 163). (Right) Isochrone fit to the globular cluster M68 by Salaris et al. (1997, ApJ, 479, 665).
However, very often we are in a situation where such detail is not possible:
The cluster BH176 is a "low latitude" (i.e., small |b |) cluster with a CMD that reveals substantial foreground contamination (visible as the arcing MSTO sequence of field stars above the cluster MSTO), obscuring the MSTO region of the cluster. In addition, the CMD shows evidence of substantial foreground reddening (how??). However, note the presence of the red clump, which still is obvious amidst the noise.
In these types of cases, even knowledge of he presence of a few "interesting" (telltale) stars can help identify the presence of a particular stellar population age.

Simple Age-Dating: Detecting Hallmark Stellar Species of Specific Population Ages

As we have seen, it is possible, under certain evolutionary phases, to make a good guess about the age if individual stars when placed in the context of their populations's CMD.

To make a summary of some of these "telltale" stars, we make use of the knowledge we have now acquired (reviewed) about stellar evolution of stars by mass.

We can build something like a "population age dating logic chart" which helps us to ascertain the presence of stars that are identified with specific ages.

From Grebel (1997, Reviews in Modern Astronomy, 10, 29).

This chart shows key stellar types for specific ages.

Some notes to the figure (working from left to right):

Note that:

A Simple Example: Decoding a CMD for a System that is a Composite of Simple Stellar Populations

Color-magnitude diagram of the Carina dwarf spheroidal galaxy by Smecker-Hane et al. (1994, AJ, 108, 507) showing evidence of the superposition of multiple simple stellar populations.

Your eye should now be trained to pick out immediately the hallmarks of specific aged populations.

In fact, it turns out that many galaxies in the Local Group of the dwarf spheroidal variety seem to have star formation histories similar to that of Carina:

But many galaxies -- especially larger ones, but including some dSphs -- have more complex star formation histories.

Unraveling these SFHs requires modeling the flow of stars into and out of various evolutionary phases.

The latter is called the Initial Mass Function.

Here is another example of a dwarf galaxy with an interesting CMD that can be shown to be made of discrete populations:

From Siegel et al. (2007, ApJL, 667:L57-L60), a CMD of the Sagittarius dwarf galaxy, which has the star cluster M54 at its center. Photometry of this M54 field is from the Advanced Camera for Surveys on the Hubble Space Telescope. (a) CMD of 60,000 stars selected to be PSF-like and to have less than a 10% contribution of neighbor stars to their integrated light. (b) Hess diagram of the field with an overlaid schematic describing the various clues to populations in the M54+Sgr field obtained from "population synthesis". The dotted line is the Sgr Metal Poor Population as defined in Layden & Sarajedini (2000). The dashed lines demarcate regions where foreground stars from the Milky Way bulge appear. (c) A computer-simulated CMD of the system using models of simple stellar populations. (d) Hess diagram overlaid with theoretical isochrones describing the inferred stellar populations.
The inferred Hodge Population box shown as an age-metallicity relation with contours giving the strength of the star formation bursts, as derived from population synthesis (that shown in the previous figure).

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All material copyright © 2003,2006,2008,2010,2012,2014,2016 Steven R. Majewski. All rights reserved. These notes are intended for the private, noncommercial use of students enrolled in Astronomy 551 and Astronomy 5610 t the University of Virginia.