連星系・変光星研究会2020 (2021.1.30)

IW And型矮新星の光度変動の研究について、最近の進展
発表者：木邑 真理子（理化学研究所）
共同研究者：尾崎 洋二（東京大学）、加藤 太一、嶺重 慎（京都大学）

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‣ Dwarf-nova outbursts
Outbursts = sudden brightening of the disk
(WD)

The thermal-viscous instability in the disk
triggered by partial ionization of hydrogen
Thermal equilibrium curve
mass accretion(outburst)

Hot state(HII)

BrightUnstable(HI*)
mass accumulation(quiescence)

Cool state(HI)

DimTime [days](e.g., Warner 1995; Osaki 1996; Osaki 2005)
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‣ IW And-type phenomenon
Relative magnitudeThe 2018 behavior in IM Eri (Kato et al. 2019)
Brightening
Anomalous
Z Cam stars
BJD-2400000
standstill with
damping oscillations

Mass transfer burst ?

What is the origin of this light variability ?
Discovery of negative superhumps
Tilted disks(Gies et al. 2013; Armstrong et al. 2013)(Hameury & Lasota 2014)3

‣ Tilted disks & negative superhumps
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The disk enters nodal & retrograde precession
The bright spot moves with a period
slightly shorter than the orbital period
Negative superhumps
3456(Wood et al. 2000; Wood & Burke 2007)4‣ What happens in tilted disks ?
The gas stream from the secondary
often flows into the inner disk.

Do tilted disks achieve the hot
inner disk & cool outer disk ?
hot inner disk?

?

cool outer disksuggested in
Kato (2019)

We examined this idea by numerical simulations of the disk instability.

5‣ Numerical simulations
One-dimensional time-dependent viscous disk (Ichikawa & Osaki 1992)
Correct treatment of the conservation of angular momentum~

mass input from the secondary(tilted disks)WD

Variable width ofthe outermost ring(mass conservation)

mass input rate from secondary
input angular momentum(angular-momentum conservation)(energy conservation)

tidal torque by secondary
thermal diffusionenergy of mass input

advection heating6

‣ Results of moderate tilt case
Disk luminosity

V magnitude

Disk radius

Disk mass

Disk angularmomentum7‣ Results of moderate tilt case
How many times each region
goes to the hot state.

The entire disk is hot.

Brightening

Only the inner disk is hot.
Mid-brightness interval
The entire disk is cool.

Dips

log10 (Temperature)outermiddle

inner(Kimura, Osaki, Kato and Mineshige, 2020,PASJ, 72, 22)

log10 (Disk angular momentum)
8hot‣ Observations vs. Simulations
Repetition of oscillations andbrightening

Reminiscent of IW And-type
phenomenon

Thermal instability in tilted disks could explain IW And-type phenomenon.

• However, there is a gap between the observations and our results.
✓ Luminosity dips soon after brightening✓ Small amplitudes✓ Z Cam-type standstills

9‣ Observational study of an IW And star
• KIC 9406652 observed by the Kepler satellite for ~1500 days (Gies et al. 2013)

Negative superhumps
Let’s investigate the nature of this tilted disk by NSH !
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Frequency [1/d]

Amplitude[flux]

Amplitude[mag]

Relativemagnitude‣ Time evolution of negative superhumps

11‣ Time evolution of negative superhumps
Profiles change with time

Almost constant flux

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2

9

3

10

4

11

5

12

6

13

7

14

Rapid decrease during brightening&
Slow increase during quasi-standstills
12‣ Gas-stream overflow in a slightly-tilted disk ?
• The flux amplitude is almost constant.
Rule out the mass-transfer burst model by Hameury & Lasota (2014)• The light curve profile varies with time. (flat-top, triangular, sinusoidal waveforms)• The tilt angle of the disk is small.
• The gas stream from the secondary star overflows the disk.
Highly-tilted disk

Slightly-tilted disk

(Armitage & Livio 1998; Kunze et al. 2001)

Kimura et al. (2020) did not consider the gas-stream overflow.

13‣ What does the frequency variation mean ?
• The frequency regularly varies per cycle of the IW And-type phenomenon.
1. Rapid decrease during brightening
2. Slow increase during quasi-standstills

Tilt angle

Depends on the disk radius &radial mass distribution ( Σ(r) )Constant(Larwood 1998)14

‣ What does the frequency variation mean ?
• The frequency regularly varies per cycle of the IW And-type phenomenon.
Depends on the disk radius &radial mass distribution ( Σ(r) )(Larwood 1998)

1. Rapid decrease during brightening

Quasistandstills

Rapid change in Σ(r)Brightening(Standard disk)

Consistent with the prediction in
Kimura et al. (2020)

Light curves

Disk radiusFrequency(Kimura et al. 2020)15‣ What does the frequency variation mean ?
• The frequency regularly varies per cycle of the IW And-type phenomenon.
Depends on the disk radius &radial mass distribution ( Σ(r) )(Larwood 1998)

2. Slow increase during quasi-standstills
The model in Kimura et al. (2020) cannot explain this variation ..
Light curves

The disk slowly expands ?
Disk radiusFrequency(Kimura et al. 2020)16

‣ Slow expansion of the disk radius ?
• It is hard to reproduce the gradual expansion of the disk radius by simulations of
the thermal instability ..

1. If the outer disk is in the cool state,
the disk slowly shrinks.
2. If the outer disk is in the hot state,
the disk radius expands rapidly.

• The outer disk may be not in the hot state in quasi-standstills ?
Quasi-standstills

Orbital hump by the brightspot at the disk outer rim• Irradiation + orbital hump

Orbital phase(Kimura, Osaki and Kato, 2020, PASJ, 72, 94)
17‣ Summary• We proposed one possible model for IW And-type phenomenon by numericalsimulations. The thermal-viscous instability in the tilted disk• We obtained supportive and unsupportive results for that model by optical
data analyses of KIC 9406652.
Slowly expanding disk radius in quasi-standstills cannot be explained.
Gas-stream overflow should be included.
Future works• Improve the simulation work by dealing with the gas-stream overflow• Find more IW And stars by long-term optical monitoring• Investigate the time evolution of the disk radius and the temperature distribution(Shibata-kun’s presentation)

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