[Paper Review] Spitzer/IRAC Characterization of Galactic AGB Stars

M. Marengo, Joseph L. Hora+5 more|arXiv (Cornell University)|Nov 10, 2006

Astronomy and Astrophysical Research3 citations

TL;DR

This paper presents the first empirical characterization of Spitzer/IRAC colors for Galactic AGB stars using Guaranteed Time Observations, developing a technique to measure saturated magnitudes via PSF fitting to derive accurate fluxes. The key contribution is a calibrated dataset of IRAC magnitudes and colors for AGB stars, enabling improved identification in infrared surveys despite complex molecular absorption features and variability.

ABSTRACT

The Spitzer Space Telescope and in particular its InfraRed Array Camera (IRAC) is an ideal facility to study the distribution of AGB stars in our own and other galaxies because of its efficiency in surveying vast areas of the sky and its ability to detect sources with infrared excess. The IRAC colors of AGB stars, however, are not well known because cool stars have numerous molecular absorption features in the spectral region covered by the IRAC photometric system. The presence and strength of these features depends on the chemistry of the stellar atmosphere and the mass loss rate and can change with time due to the star's variability. To characterize the IRAC colors of AGB stars, we are carrying out a Spitzer Guaranteed Time Observation program to observe a sample of AGB stars with IRAC. The results will be made available to the community in the form of template magnitudes and colors for each target with the goal of aiding the identification of AGB stars in already available and future IRAC surveys. We present here the first results of this project.

Motivation & Objective

• To address the lack of reliable IRAC colors for AGB stars due to complex molecular absorption features and variability.
• To validate and cross-calibrate ISO SWS spectra for AGB stars using direct Spitzer/IRAC observations.
• To develop a high-accuracy photometry technique for saturated AGB stars using PSF fitting to overcome instrumental saturation.
• To produce a community-accessible catalog of template magnitudes and colors for AGB stars in IRAC bands.
• To distinguish AGB stars from other red objects in IRAC surveys by characterizing their unique color-color distributions.

Proposed method

• Conducted Guaranteed Time Observations (GTO) with Spitzer/IRAC on 52 AGB stars (O-rich, carbon, S-type, and M-type supergiants) with known distances and mass loss rates.
• Performed two-epoch observations spaced two months apart to assess variability effects on IRAC colors.
• Developed a PSF subtraction technique using a model PSF derived from bright stars to measure fluxes of saturated AGB stars with 1–3% accuracy.
• Calibrated the PSF model using Vega’s known magnitude, enabling absolute flux calibration independent of standard IRAC calibration.
• Convolved ISO SWS spectra of 155 sources with IRAC bandpasses to generate synthetic IRAC colors for comparison.
• Used DUSTY and Groenewegen’s radiative transfer models to interpret observed colors and assess the impact of molecular features.

Experimental results

Research questions

• RQ1 How do IRAC colors of AGB stars differ from simple reddened photospheric models due to molecular absorption features?
• RQ2 To what extent do IRAC colors vary with pulsation phase in variable AGB stars?
• RQ3 Can PSF fitting of saturated IRAC images yield flux measurements accurate to 1–3% for the brightest AGB stars?
• RQ4 How well do theoretical models (DUSTY and Groenewegen) reproduce observed IRAC colors, especially for carbon stars with strong C3 absorption?
• RQ5 What are the distinguishing IRAC color-color signatures of O-rich AGB stars, carbon stars, S-stars, and M-type supergiants?

Key findings

• IRAC colors of AGB stars are significantly affected by molecular absorption features, such as C2H2 and HCN in the 3.6 μm band for carbon stars, leading to bluer [3.6]–[4.5] colors than expected.
• Carbon stars exhibit anomalous [4.5]–[5.8] colors as low as −0.5 mag due to transient C3 absorption at 5.8 μm, which varies with pulsation phase.
• The PSF fitting technique achieved 1–3% accuracy in flux measurement for saturated AGB stars, independent of standard IRAC calibration.
• Synthetic colors from ISO SWS spectra closely match the first-epoch IRAC observations, validating the use of SWS data for color prediction.
• Groenewegen’s models using realistic stellar atmospheres better reproduce observed colors than DUSTY models with black-body central sources.
• O-rich AGB stars with high mass loss show strong infrared excess, particularly in the [3.6]–[8.0] color, while S-stars and M-type supergiants have colors similar to M-type AGB stars.