[Paper Review] IPHAS and the symbiotic stars. I. Selection method and first discoveries

R. L. M. Corradi, E. R. Rodríguez–Flores+8 more|ArXiv.org|Dec 14, 2007

Stellar, planetary, and galactic studies22 references58 citations

TL;DR

This paper presents a novel photometric selection method combining IPHAS and 2MASS near-infrared colors to identify candidate symbiotic stars in the northern Galactic plane, successfully isolating 1,183 candidates from 4,338 Hα-emitting stars; spectroscopic confirmation of three new systems validates the method's effectiveness for systematic discovery of these rare binaries.

ABSTRACT

The study of symbiotic stars is essential to understand important aspects of stellar evolution in interacting binaries. Their observed population in the Galaxy is however poorly known, and is one to three orders of magnitudes smaller than the predicted population size. IPHAS, the INT Photometric Halpha survey of the Northern Galactic plane, gives us the opportunity to make a systematic, complete search for symbiotic stars in a magnitude-limited volume, and discover a significant number of new systems. A method of selecting candidate symbiotic stars by combining IPHAS and near-IR (2MASS) colours is presented. It allows us to distinguish symbiotic binaries from normal stars and most of the other types of Halpha emission line stars in the Galaxy. The only exception are T Tauri stars, which can however be recognized because of their concentration in star forming regions. Using these selection criteria, we discuss the classification of a list of 4338 IPHAS stars with Halpha in emission. 1500 to 2000 of them are likely to be Be stars. Among the remaining objects, 1183 fulfill our photometric constraints to be considered candidate symbiotic stars. The spectroscopic confirmation of three of these objects, which are the first new symbiotic stars discovered by IPHAS, proves the potential of the survey and selection method.

Motivation & Objective

• Address the significant discrepancy between the predicted and observed numbers of symbiotic stars in the Milky Way, which remains poorly constrained due to lack of systematic surveys.
• Overcome the challenge of distinguishing symbiotic stars from other Hα-emitting objects—especially T Tauri stars—using photometric color criteria.
• Leverage the IPHAS survey’s deep, wide-field Hα photometry combined with 2MASS near-infrared data to enable a magnitude-limited, systematic search for symbiotic stars.
• Develop a robust selection criterion that separates S-type (stellar) and D-type (dusty) symbiotic stars from normal stars and other emission-line populations.
• Initiate a spectroscopic follow-up campaign to confirm candidate systems and improve the census of symbiotic stars in the Galaxy.

Proposed method

• Apply a color-color diagram analysis using IPHAS g, r, i and Hα magnitudes alongside 2MASS J, H, Ks magnitudes to distinguish symbiotic stars from other Hα emitters.
• Define photometric selection criteria based on the distinct location of symbiotic stars in the (r-i) vs. (H-K) and (g-r) vs. (J-H) color-color diagrams, separating them from normal stars and Be stars.
• Use the clustering parameter—defined as the mean distance to the four nearest Hα emitters—to distinguish isolated symbiotic stars from spatially clustered T Tauri stars.
• Apply the method to a list of 4,338 Hα-emitting stars from Witham et al. (2008), filtering out Be stars (1,500–2,000 candidates) and retaining 1,183 as candidate symbiotic stars.
• Prioritize candidates with high spatial isolation and consistent near-infrared excess to minimize contamination from T Tauri stars.
• Conduct spectroscopic follow-up on a subset of candidates to confirm symbiotic nature through detection of both cool stellar absorption features and high-excitation emission lines.

Experimental results

Research questions

• RQ1 Can a combination of optical and near-infrared photometry from IPHAS and 2MASS effectively isolate candidate symbiotic stars from other Hα-emitting stars in the Galactic plane?
• RQ2 To what extent do T Tauri stars contaminate the candidate symbiotic star sample, and can spatial clustering be used to disentangle them?
• RQ3 How effective is the color-color selection method in distinguishing S-type (stellar) from D-type (dusty) symbiotic stars?
• RQ4 What is the yield of new symbiotic star discoveries using this method, and can spectroscopic confirmation validate its reliability?
• RQ5 Can this method enable a statistically complete census of symbiotic stars in a magnitude-limited volume of the Galaxy?

Key findings

• The method successfully isolates 1,183 candidate symbiotic stars from 4,338 Hα-emitting stars, with 817 classified as S-type and 366 as D-type based on photometric criteria.
• Spectroscopic follow-up confirms three new symbiotic stars—designated IPHAS Sy 1, Sy 2, and Sy 3—validating the photometric selection method.
• The confirmed systems exhibit the characteristic spectral signature of symbiotic binaries: absorption lines from a cool giant and emission lines from highly ionized species.
• T Tauri stars are the primary contaminant, overlapping with S-type symbiotics in color-color diagrams, but can be distinguished via spatial clustering and spectral morphology.
• The estimated radius of IPHAS Sy 3 (~30 R☉) at a distance of ~1 kpc is inconsistent with a pre-main sequence star but consistent with a red giant, supporting its classification as a symbiotic star.
• The clustering parameter effectively separates isolated symbiotic stars from groupings typical of T Tauri stars, improving the purity of the candidate sample.