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Unusual Helium-deficient Dwarf Novae

with a Short Orbital Period

Identified by Gemini/GMOS

Youngdae Lee

Chungnam National University

K-GMT SP UM/ 2024.08.22

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Dwarf novae (DNe)

◎ Semi-detached binary system

◎ 1. Primary star (White Dwarf; WD)

2. Secondary star

(low-mass star; Main sequence or WD)

3. Accretion disk

Primary

Secondary

Accretion Disk

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Dwarf novae (DNe)� - Light Curves

◎ SS Cyg (U Gem)

◎ Light Curve for 5 years

◎ Super/Long outbursts

+ normal/short outbursts

Super/Long

Normal/short

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Thorstensen et al. 2015 AJ 149 128

Patterson et al. 2008 PASP 120 510

Manser & Gansicke 2014 MNRAS 442 23

Dwarf novae (DNe) - Spectrum

◎ Double Hα emission from accretion disk >> radial velocity of primary star

◎ A type or A+M type spectrum

Anderson et al. 2005 AJ 130 2230

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Dwarf novae (DNe)�- Binary Evolution

Ivanova et al. 2013 A&ARv 21 59

ZAMS : zero age main sequence

RLO : Roche-lobe overflow

CE : common envelope

CO WD : carbon-oxygen white dwarf

He : He star

Dwarf Novae

AM CVn, He CVs

◎ Evolution of binary star

◎ Loss of angular momentum by gravitational radiation or magnetic braking

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Dwarf novae (DNe)�- Properties of Secondary Star

Beuermann et al. 1998

Thorstensen et al. 2002

P_min ~ 75 minutes

Gap = 2 ~ 3 hours

◎ Spectral type or temperature of “Secondary star”

◎ Temperature and mass transfer rate decrease with decreasing orbital period

◎ No temperatures of secondary stars have been observed below period minimum (~75 minutes)

Osaki 1996

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Bohm-Vitense 1992

Kennedy et al. 2015 ApJ 815 131

Dwarf novae (DNe)�- Properties of Secondary Star

(Helium Strength; HeI/Hα)

◎ Long period dwarf novae : Strong Hα and weak He emission

◎ Helium dwarf novae : Strong Hα and strong He emission

◎ AM CVn : No Hα emission and strong He emission

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Dwarf novae (DNe)�- Properties of Secondary Star �(Model)

ElBadry et al. 2021 MNRAS 505 2051

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◎ Scientific Objectives

• Infant/Early explosions focused on supernovae

• Peculiar optical transients (e.g., rapidly-evolving extragalactic transients,

AGN related phenomena, novae)

• Variable Stars

• Low-surface brightness objects (dwarf galaxies, GCs) from deep stack images

KMTNet Supernova Program (KSP)

◎ Observations (~17% time of KMTNet):

Accumulated target fields: ~100 galaxies/groups, nearby fields (D<30Mpc)

3 sites(CTIO,SAAO,SSO), high-cadence, multi-color(BVI), 60sec exposure time, wide field of view (2deg x 2deg)

◎ Unique data set with multi-color information and follow-up observations (e.g, Gemini spectra and other data)

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Spectra observed by Gemini GMOS

Observation

- slit width: 1.5 arcsec

- B600/550.0, R400/775.0

- CuAr for wavelength calibration

- Flux Standard stars

- Sequence : target – flat – arc – arc – flat - target

Reduction (Gemini reduction package in IRAF):

- Bias and Flat correction.

- Wavelength calibration

- Background subtraction

- Flux calibration

Target !!

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Observed Dwarf Novae by KSP

◎ If when we investigate 100 observed fields,

~170 DNs including 85 faint DNs are expected to be found.

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KSP-OT-201701a

◎ For 2.6 years, 3 superout bursts and 6 normal outbursts are detected.

Lee et al. 2022 ApJL 925 22

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◎ Orbital period : 51 minutes

KSP-OT-201701a

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KSP-OT-201712a

◎ For 6 months, multiple outbursts and standstills

◎ Showing standstills (cyan shade; about 2 days)

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