Variations in Star Formation History and the Red Giant Branch Tip

Stellar Evolution theory is based upon well understood physics and provides clear predictions as to how a Colour-Magnitude Diagram (CMD) will change due to effects of age and metallicity. The theory has been tested by looking at nearby coeval star clusters. The power of applying CMD analysis to galaxies has been demonstrated in studies of the Carina dSph (Smecker-Hane et al. 1996). In Carina the observation of separate, distinct Main Sequence (MS) Turnoffs has forced us to believe that this small, nearby companion of our Galaxy has had a very complex star formation history. No similar direct evidence for “bursting” behaviour on a global scale has been seen indisputably in larger systems. Partly this is due to the greater distances of larger systems, but also to the complications in distinguishing old star formation events (> 1 Gyr old) in systems which are currently forming stars. Where we lack MS turnoffs we have to resort to statistical modeling of the CMD. This has been applied using a a number of different approaches, but all generally based on Monte-Carlo techniques (e.g. Tosi et al. 1992; Bertelli et al. 1992; Tolstoy &; Saha 1996). However, even using these more sophisticated analysis techniques, it is difficult to find unique solutions. This is mostly due to the age-metallicity degeneracy on the Red Giant Branch (RGB). The RGB is usually the most populated, easiest to observe phase of stellar evolution. The Carina CMD reveals the dangers of blindly interpreting the RGB, because from the RGB alone it is impossible to extract the information revealed by the MS Turnoffs.

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Breaking the age–metallicity degeneracy: The metallicity distribution and star formation history of the Large Magellanic Cloud

Proceedings of the International Astronomical Union ◽

10.1017/s174392130802855x ◽

2008 ◽

Vol 4 (S256) ◽

pp. 263-268 ◽

Cited By ~ 4

Author(s):

Andrew A. Cole ◽

Aaron J. Grocholski ◽

Doug Geisler ◽

Ata Sarajedini ◽

Verne V. Smith ◽

...

Keyword(s):

Star Formation ◽

Near Infrared ◽

Large Magellanic Cloud ◽

Star Formation History ◽

Red Giants ◽

Formation History ◽

History Of ◽

Red Giant ◽

Metallicity Distribution ◽

Modelling Process

AbstractWe have obtained metallicities from near-infrared calcium triplet spectroscopy for nearly a thousand red giants in 28 fields spanning a range of radial distances from the center of the bar to near the tidal radius. We have used these data to investigate the radius-metallicity and age-metallicity relations. A powerful application of these data is in conjunction with the analysis of deep HST color–magnitude diagrams (CMDs). Most of the power in determining a robust star-formation history from a CMD comes from the main-sequence turnoff and subgiant branches. The age-metallicity degeneracy that results is largely broken by the red giant branch color, but theoretical model RGB colors remain uncertain. By incorporating the observed metallicity distribution function into the modelling process, a star-formation history with massively increased precision and accuracy can be derived. We incorporate the observed metallicity distribution of the LMC bar into a maximum-likelihood analysis of the bar CMD, and present a new star formation history and age–metallicity relation for the bar. The bar is certainly younger than the disk as a whole, and the most reliable estimates of its age are in the 5–6 Gyr range, when the mean gas abundance of the LMC had already increased to [Fe/H] ≳ −0.6. There is no obvious metallicity gradient among the old stars in the LMC disk out to a distance of 8–10 kpc, but the bar is more metal-rich than the disk by ≈0.1–0.2 dex. This is likely to be the result of the bar's younger average age. In both disk and bar, 95% of the red giants are more metal-rich than [Fe/H] = −1.2.

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Hubble Space Telescope Imaging of Antlia B: Star Formation History and a New Tip of the Red Giant Branch Distance

The Astrophysical Journal ◽

10.3847/1538-4357/ab58d2 ◽

2020 ◽

Vol 888 (1) ◽

pp. 31 ◽

Cited By ~ 2

Author(s):

J. R. Hargis ◽

S. Albers ◽

D. Crnojević ◽

D. J. Sand ◽

D. R. Weisz ◽

...

Keyword(s):

Star Formation ◽

Hubble Space Telescope ◽

Star Formation History ◽

Space Telescope ◽

Formation History ◽

Red Giant

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The HST/ACS star formation history of the Tucana dwarf spheroidal galaxy: clues from the horizontal branch

Astronomy and Astrophysics ◽

10.1051/0004-6361/201936077 ◽

2019 ◽

Vol 630 ◽

pp. A116 ◽

Cited By ~ 2

Author(s):

A. Savino ◽

E. Tolstoy ◽

M. Salaris ◽

M. Monelli ◽

T. J. L. de Boer

Keyword(s):

Star Formation ◽

Main Sequence ◽

Star Formation History ◽

Horizontal Branch ◽

Low Mass Stars ◽

Formation History ◽

History Of ◽

Low Mass ◽

Ancient Times ◽

Red Giant

We report a new star formation history for the Tucana dwarf spheroidal galaxy, obtained from a new look at a deep HST/ACS colour-magnitude diagram. We combined information from the main sequence turn-off and the horizontal branch to resolve the ancient star formation rates on a finer temporal scale than previously possible. We show that Tucana experienced three major phases of star formation, two very close together at ancient times and the last one ending between 6 and 8 Gyr ago. We show that the three discrete clumps of stars on the horizontal branch are linked to the distinct episodes of star formation in Tucana. The spatial distribution of the clumps reveals that each generation of stars presents a higher concentration than the previous one. The simultaneous modelling of the horizontal branch and the main sequence turn-off also allows us to measure the amount of mass lost by red giant branch stars in Tucana with unprecedented precision, confirming dwarf spheroidals to be excellent laboratories to study the advanced evolution of low-mass stars.

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Strömgren Photometry of Field Red Giants in the LMC

Symposium - International Astronomical Union ◽

10.1017/s0074180900118273 ◽

1999 ◽

Vol 190 ◽

pp. 368-369

Author(s):

Andrew A. Cole ◽

John S. Gallagher ◽

Tammy A. Smecker-Hane

Keyword(s):

Star Formation ◽

Star Formation History ◽

Red Giants ◽

Strong Constraint ◽

Preliminary Results ◽

Field Star ◽

Abundance Estimates ◽

Formation History ◽

Red Clump ◽

Red Giant

We present here preliminary results of our program to obtain rough abundance estimates for > 104 red giant branch and red clump stars in four fields of the LMC. The derived abundance distributions will be a strong constraint on models of the LMC's star-formation history based on field star color-magnitude diagrams.

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