[Paper Review] A search for Symbiotic Stars in the Local Group

L. Magrini, R. L. M. Corradi+1 more|arXiv (Cornell University)|Aug 5, 2002

Astronomy and Astrophysical Research2 references3 citations

TL;DR

This paper presents a methodology to detect symbiotic stars in external Local Group galaxies using photometric data from the Local Group Census survey. By leveraging narrow- and broad-band filters (Hα, [O iii], Strömgren y, i′), the authors define diagnostic diagrams to distinguish symbiotic stars from planetary nebulae and develop distance sensitivity estimates, showing detectability up to ~1.6 Mpc in Hα for bright systems.

ABSTRACT

The Local Group Census is a narrow- and broad-band survey of all the galaxies of the Local Group above dec = -30 deg, in progress at the 2.5m Isaac Newton telescope on La Palma. We discuss here the ability of the survey to detect symbiotic star candidates in the Local Group, by deriving detection limits in each of the narrow- and broad-band frames used in the survey, and by estimating the total number of objects expected in each galaxy. We present two diagnostic diagrams, based on the adopted photometric filters, to discriminate between symbiotic stars and other emission-line objects such as planetary nebulae.

Motivation & Objective

• To identify symbiotic star candidates in external galaxies using photometric data from the Local Group Census survey.
• To address the challenge of distinguishing symbiotic stars from planetary nebulae based on photometric color-color diagrams.
• To estimate the detectability distance of symbiotic stars in the survey's narrow- and broad-band filters.
• To predict the expected number of symbiotic stars per galaxy based on red giant population estimates and a 0.5% symbiotic fraction.
• To assess the reliability of these predictions across different galaxy types, especially low-mass and irregular systems.

Proposed method

• Utilizes narrow-band filters (Hα, [O iii], He ii, [S ii]) and broad-band filters (g′, r′, i′, Strömgren y) from the Isaac Newton Telescope's Wide Field Camera.
• Employs synthetic photometry with filter profiles from Moro & Munari (2000) and distance estimates via infrared spectrophotometric parallax for prototype symbiotic stars.
• Constructs two diagnostic color-color diagrams: (Hα - Strömgren y) vs. (i′ - Strömgren y) and ([O iii] - Strömgren y) vs. (i′ - Strömgren y) to separate symbiotic stars from PNe.
• Computes signal-to-noise ratios (S/N ≥ 5) for detection limits using exposure times (1200–3600 s) from the LGC survey.
• Estimates the number of red giants per galaxy using K-band and B-band magnitudes from LEDA and Fioc & Rocca-Volmerange (1999), assuming 100 L⊙ per giant.
• Applies a 0.5% symbiotic star fraction relative to red giant counts to predict total expected symbiotic systems per galaxy.

Experimental results

Research questions

• RQ1 Can symbiotic stars in external Local Group galaxies be reliably identified using only broad- and narrow-band photometry?
• RQ2 What are the photometric diagnostic diagrams that effectively separate symbiotic stars from planetary nebulae in the LGC filter system?
• RQ3 How far can a typical symbiotic star be detected in the LGC survey, given its depth and filter sensitivity?
• RQ4 What is the expected number of symbiotic stars per galaxy in the Local Group based on red giant population estimates?
• RQ5 How reliable are these predictions for low-mass and irregular galaxies with high B/K light ratios?

Key findings

• The Hα filter enables detection of typical symbiotic stars up to ~0.5 Mpc and the brightest up to ~1.6 Mpc with S/N ≥ 5.
• The [O iii] filter allows detection of typical symbiotic stars up to ~0.2 Mpc and the brightest up to ~1.4 Mpc.
• The Strömgren y filter detects typical systems up to ~0.1 Mpc and the brightest up to ~0.8 Mpc.
• The i′ filter detects typical symbiotic stars up to ~0.6 Mpc and the brightest up to ~1.6 Mpc.
• The diagnostic diagrams (Hα - Strömgren y vs. i′ - Strömgren y and [O iii] - Strömgren y vs. i′ - Strömgren y) successfully separate symbiotic stars from planetary nebulae and field stars.
• Predicted symbiotic star counts range from 0 to 660,000 per galaxy, with M31 hosting the highest estimated number (660,000), while low-mass and irregular galaxies show unreliable estimates due to high B/K light ratios.