[Paper Review] Observation of ER UMa Stars

Taichi Kato, Daisaku Nogami+4 more|arXiv (Cornell University)|Jan 15, 2013

Astrophysical Phenomena and Observations3 citations

TL;DR

This paper identifies ER UMa stars as a rare subgroup of SU UMa-type dwarf novae characterized by extremely short supercycle lengths (19–48 days) and high mass-transfer rates, challenging the canonical thermal-tidal disk instability model. The authors demonstrate that these systems exhibit unique superhump evolution, including large-amplitude early superhumps, and propose they represent a transitional link between classical SU UMa stars and permanent superhumpers, with implications for mass-transfer mechanisms in cataclysmic variables below the period gap.

ABSTRACT

ER UMa stars are a recently recognized small subgroup of SU UMa-type dwarf novae, which are characterized by the extremely high outburst frequency and short (19--48 d) supercycles. From the current thermal-tidal disk instability scheme, they are considered to be high mass-transfer SU UMa-type dwarf novae, and comprise a link to "permanent superhumpers" below the period gap. They do not only provide an opportunity to test the applicability of thermal-tidal instability model but also pose problems on the origin of high mass-transfer in short orbital-period cataclysmic variables. A historical review of this subgroup and recent topics of ER UMa stars, the unique pattern of superhump evolution and the "helium ER UMa analog" (CR Boo), are also discussed.

Motivation & Objective

• To characterize ER UMa stars as a distinct subgroup of SU UMa-type dwarf novae with anomalously short supercycle lengths.
• To investigate the origin of high mass-transfer rates in systems below the period gap, challenging the standard gravitational wave-driven angular momentum loss model.
• To explore the evolutionary significance of ER UMa stars as a potential bridge between classical SU UMa-type dwarf novae and permanent superhumpers.
• To examine the unusual superhump behavior, including large-amplitude early-stage superhumps and their rapid decay.
• To evaluate the existence and properties of helium-rich analogs, such as CR Boo, in the context of helium-rich cataclysmic variables.

Proposed method

• Analysis of long-term light curves from amateur and professional observers, including visual and CCD time-series data, to detect periodic variability and superhump signals.
• Compilation and cross-verification of superhump periods and supercycle lengths across multiple systems (ER UMa, V1159 Ori, RZ LMi, DI UMa) using pre-VSNET collaborations.
• Modeling of disk instability using the thermal-tidal disk instability framework (Osaki 1989), with adjustments to mass-transfer rates to reproduce observed supercycle lengths.
• Phase-resolved analysis of superhump evolution, particularly the amplitude and period behavior during early superoutbursts, to identify distinct superhump states.
• Comparison of observed supercycle lengths and superhump periods with theoretical expectations to assess deviations from standard SU UMa behavior.
• Investigation of CR Boo’s light curve folded over a 46.3-day supercycle to identify analogies with ER UMa stars, suggesting a helium-rich counterpart.

Experimental results

Research questions

• RQ1 Can the extremely short supercycle lengths (19–48 days) of ER UMa stars be explained by the standard thermal-tidal disk instability model?
• RQ2 What physical mechanism enables such high mass-transfer rates in systems below the period gap, given the dominance of gravitational wave radiation in standard models?
• RQ3 Why are ER UMa stars concentrated near the minimum orbital period, and what does this imply about their evolutionary origin?
• RQ4 How do the early-stage superhumps in ER UMa stars differ from those in other systems like WZ Sge stars, and what do they reveal about disk dynamics?
• RQ5 Do helium-rich cataclysmic variables, such as CR Boo, exhibit similar supercycle and superhump behavior, suggesting a broader analog population?

Key findings

• ER UMa stars exhibit supercycle lengths as short as 19 days (RZ LMi), significantly shorter than the canonical 130-day minimum (YZ Cnc), challenging the standard mass-transfer model.
• The superhump period in ER UMa stars remains nearly constant throughout the superoutburst, contrasting with the period evolution seen in typical SU UMa stars.
• Large-amplitude superhumps appear during the early phase of superoutbursts and decay rapidly within a few days, followed by the emergence of standard superhumps with reversed phase.
• Modeling by Osaki (1995a) shows that increasing the mass-transfer rate by a factor of ~10 can reproduce the observed supercycle lengths, suggesting ER UMa stars are high mass-transfer systems.
• The discovery of CR Boo with a 46.3-day supercycle and similar outburst alternation to ER UMa suggests the existence of a helium-rich analog population, termed 'helium ER UMa stars'.
• The observed number density of ER UMa stars (4 out of ~40 SU UMa-type dwarf novae) implies a 10-fold lower evolutionary timescale than typical SU UMa stars, raising questions about their origin and the true mass-transfer rate distribution.