[Paper Review] The Optical Gravitational Lensing Experiment. Small Amplitude Variable Red Giants in the Magellanic Clouds

I. Soszyński, A. Udalski+6 more|arXiv (Cornell University)|Jul 2, 2004

Astronomical Observations and Instrumentation21 citations

TL;DR

This paper analyzes small amplitude variable red giants (OSARGs) in the Large and Small Magellanic Clouds using OGLE-II and OGLE-III photometry, identifying two distinct pulsation groups. It finds that stars below the tip of the red giant branch (TRGB) are a mixture of first-ascent RGB and second-ascent AGB stars, with pulsation properties in the Magellanic Clouds and Galactic bulge being remarkably similar, indicating universal pulsation mechanisms across different metallicities.

ABSTRACT

We present analysis of the large sample of variable red giants from the Large and Small Magellanic Clouds detected during the second phase of the Optical Gravitational Lensing Experiment (OGLE-II) and supplemented with OGLE-III photometry. Comparing pulsation properties of detected objects we find that they constitute two groups with clearly distinct features. In this paper we analyze in detail small amplitude variable red giants (about 15400 and 3000 objects in the LMC and SMC, respectively). The vast majority of these objects are multi-periodic. At least 30% of them exhibit two modes closely spaced in the power spectrum, what likely indicates non-radial oscillations. About 50% exhibit additional so called Long Secondary Period. To distinguish between AGB and RGB red giants we compare PL diagrams of multi-periodic red giants located above and below the tip of the Red Giant Branch (TRGB). The giants above the TRGB form four parallel ridges in the PL diagram. Among much more numerous sample of giants below the TRGB we find objects located on the low luminosity extensions of these ridges, but most of the stars are located on the ridges slightly shifted in logP. We interpret the former as the second ascent AGB red giants and the latter as the first ascent RGB objects. Thus, we empirically show that the pulsating red giants fainter than the TRGB are a mixture of RGB and AGB giants. Finally, we compare the Petersen diagrams of the LMC, SMC and Galactic bulge variable red giants and find that they are basically identical indicating that the variable red giants in all these different stellar environments share similar pulsation properties.

Motivation & Objective

• To characterize the pulsation properties of small amplitude variable red giants (OSARGs) in the Large and Small Magellanic Clouds.
• To distinguish between red giants on the first ascent of the red giant branch (RGB) and the second ascent of the asymptotic giant branch (AGB) using period-luminosity (PL) diagrams.
• To compare pulsation behavior of OSARGs in the Magellanic Clouds with those in the Galactic bulge to assess environmental dependence.
• To determine whether the observed period-amplitude relations and Petersen diagrams indicate non-radial oscillations or convection-driven pulsations.

Proposed method

• Photometric data from OGLE-II and OGLE-III surveys were used, covering the Large and Small Magellanic Clouds with multi-epoch observations.
• Periodogram analysis identified pulsation periods and amplitudes, with particular focus on multi-periodic stars and closely spaced frequency pairs.
• Period-luminosity (PL) diagrams were constructed for stars above and below the tip of the red giant branch (TRGB), revealing distinct ridges.
• Petersen diagrams were used to compare period ratios across different stellar populations and environments.
• The sample was divided into groups based on period-amplitude relations and PL diagram morphology to infer evolutionary status.
• Comparison with Galactic bulge OSARGs from Wray et al. (2004) was performed using Petersen diagrams and period-amplitude diagrams to assess consistency across environments.

Experimental results

Research questions

• RQ1 Are small amplitude variable red giants in the Magellanic Clouds predominantly first-ascent RGB or second-ascent AGB stars?
• RQ2 What fraction of OSARGs exhibit non-radial pulsations, as indicated by closely spaced frequency pairs in the power spectrum?
• RQ3 How do the period-luminosity relations of OSARGs above and below the TRGB differ, and what do they reveal about stellar evolution?
• RQ4 Are the pulsation properties of OSARGs in the Magellanic Clouds and Galactic bulge fundamentally similar despite differences in metallicity and distance?
• RQ5 Can the two distinct period-amplitude sequences observed in Galactic bulge OSARGs be explained by a mixture of RGB and AGB stars?

Key findings

• Approximately 15,400 and 3,000 small amplitude variable red giants were identified in the Large and Small Magellanic Clouds, respectively.
• At least 30% of the OSARGs exhibit two closely spaced pulsation modes, suggesting non-radial oscillations.
• Giants above the TRGB form four parallel ridges in the PL diagram, interpreted as second-ascent AGB stars.
• Giants below the TRGB are a mixture of first-ascent RGB stars (slightly shifted ridges) and second-ascent AGB stars (low-luminosity extensions of the ridges).
• Petersen diagrams of OSARGs in the LMC, SMC, and Galactic bulge are nearly identical, indicating similar pulsation properties across different stellar environments.
• The two distinct period-amplitude sequences in the Galactic bulge OSARG sample correspond to stars in the b3 and b2 sequences, with the b4 sequence matching AGB stars fainter than the TRGB.