Koji Mukai on X-rays and Dwarf Novae

This is the second time Koji Mukai has granted me
an interview. The first time we discussed magnetic CVs,
intermediate polars specifically. That interview can be
read here.

Now Koji is back to discuss RU Peg and the X-ray
behavior of dwarf novae with massive white dwarfs.




CVnet: Hi, Koji. Thank you for granting us another interview. Let's start with
where are you working now and what are your primary responsibilities?
Also, what are you current areas of research?

Mukai: I work at NASA's Goddard Space Flight Center, although my employer
is University of Maryland, Baltimore County. I work at the
US Guest Observer Facility for the joint Japan-US Suzaku mission,
and also work on the education and public outreach group of the
astrophysics science division here. My research has always focused
on accreting white dwarfs - it still does, but over the last few
years it has expanded from just CVs to CVs and symbiotic stars.
I'm interested both in accretion and mass ejection during nova
outbursts.

CVnet: Are you still maintaining the Intermediate Polars pages?

Mukai: Yes, although I haven't had the time to make a substantial update
for the last year or so. There are quite a few new confirmed
and candidate IPs to add to the site!

CVnet: AAVSO Alert Notice 459 states you are requesting monitoring of the dwarf nova,
RU Peg, in anticipation of the next outburst. Let's discuss why RU Peg is so interesting,
and what you hope to learn by observing it with Swift.

Mukai: RU Peg is a bright dwarf nova that has been neglected, relatively
speaking, for X-ray observations. For dwarf novae, it is very
important to conduct X-ray monitoring campaigns through an outburst.
Now that RXTE has been decomissioned, Swift is the only observatory
for this type of campaign.

CVnet: Since your observations will be in the X-ray, where do X-rays in dwarf novae originate?

Mukai: In a dwarf nova, half the available gravitational potential energy is
radiated away in the accretion disk - that's a source of infrared,
visible, and ultraviolet light. The other half of the potential
energy has been converted into the kinetic energy of the disk material,
moving at several thousand kilometers per second. Since the white
dwarf is rotating much more slowly than this, that motion must suddenly
cease in a very small region - what we call the boundary layer. That's
where the X-rays originate in dwarf novae.


CVnet: How does the amount of X-rays emitted change between the quiescent and outburst
phases of the dwarf novae?

Mukai: That actually depends on what you mean by "X-rays." But if you mean
X-rays in the traditional band (photon energies of 2-10 keV, or
wavelengths of about 1-5 Angstroms), dwarf novae become fainter during
outburst than in quiescence.

Below are the AAVSO and RXTE light curves of WW Cet from
a recent paper I was involved in. This shows what I now think of

as "typical" behavior. X-ray bright in quiescence, X-ray faint in outburst, 

with sudden a transition and no intermediate states.

From 2011PASP..123.1054F  Fertig, D.; Mukai, K.; Nelson, T.; Cannizzo, J. K. 

The Fall and the Rise of X-Rays from Dwarf Novae in Outburst: RXTE Observations of VW Hydri and WW Ceti

CVnet: What do we think is happening as the outburst begins in the accretion disc
to cause this X-ray suppression?

Mukai: In quiescence, the boundary layer is optically thin - that is, X-ray
photons, once emitted, escape the boundary layer without interacting
with matter. In outburst, much more matter is flowing through the
boundary layer, so the density is much higher. In this case, the
boundary layer becomes optically thick - X-ray photons emitted by
the ions interact with surrounding matter several times before
they are able to escape. In this situation, the temperature of
the boundary layer drops, and only lower energy X-rays ("soft"
X-rays, as in X-rays that cannot penetrate matter that much) are
emitted - with energies below 0.5 keV. The optically thin case
is like the corona of the sun, the optically thick case is like
the photosphere of the sun. In fact, during outburst, the boundary
layer has both the photosphere-like region and the corona-like region.

If the line of sight to the dwarf nova is relatively free of
interstellar matter, then we can observe dwarf novae brighten
dramatically during outburst in soft X-rays and extreme ultraviolet.

CVnet: Isn't this the opposite of what has been observed in prior campaigns on SS Cygni?

Mukai: No, not really. During the peak of the outburst (as determined by
visible light observers), SS Cyg is fainter in hard X-rays and brighter
in soft X-rays. It's in the time of transitions that SS Cyg has
shown a behavior pattern that has not been seen in other dwarf novae.
Other systems have shown "quiescent" (hard X-ray bright) and
"outburst" (hard X-ray dim) states, and nothing else. SS Cyg,
on the other hand, initially brightens in hard X-rays (near the
time of the peak visible light) before switching to hard X-ray
faint/soft X-ray bright state. There is another hard X-ray brightening
near the end of the outburst. So, in hard X-rays, it goes from
bright-brighter-faint-brighter-bright through an outburst.

You can see this in the light curves here.

CVnet: Does this mean SS Cygni is actually the exception to the rule, and not the
prototype as most people have always assumed?

Mukai: You can still consider SS Cyg to be the prototype of the hard X-ray
bright (quiescence) - dim (outburst) behavior. It appears to be
an exception in showing the bright-brighter-faint-brighter-bright
behavior.

CVnet: How does the mass of the white dwarf come into play in the whole process?

Mukai: The accretion rate at which the boundary layer switches from the
optically thin regime to the optically thick regime is believed to
be a strong function of the white dwarf mass, according to theoretical
studies. The higher the white dwarf mass, the higher the accretion
rate at which the transition occurs. The state change of the disk,
between quiescence and outburst, is governed by the conditions in
the disk, and is far less sensitive to the white dwarf mass. When
the disk goes into outburst, the accretion rate through the boundary
layer rises, making it optically thick for an average mass white
dwarf, while making it brighter but still optically thin for a
high mass white dwarf - at least that''s a physically motivated
explanation of why SS Cyg might behave differently from the average
dwarf novae.

CVnet: Is this the main reason for selecting RU Pegasi as your target for the Swift campaign?

Mukai: Yes, we believe that the white dwarf in the RU Peg system is among the
most massive for a dwarf nova. Also, it is one of the X-ray brightest
dwarf novae for which an X-ray monitoring campaign has never been
done.

CVnet: How do you know the mass of the white dwarf in RU Peg?

Mukai: In the optical spectra of RU Peg, you can see both the mass donor and
the accretion disk, so the radial velocity motion of both stars can
be measured, with the usual caveats.

CVnet: So what if we don't see the same X-ray behavior as SS Cyg when RU Peg goes into outburst?
Will the campaign still prove useful scientifically?

Mukai: That would be a very important result, because it would have disproved
our current hypothesis. We will have to go back to square one in terms
of trying to understand why SS Cyg is different, but that's how science
is supposed to work.

CVnet: Thanks, Koji. Any final comments or advice for our observers?

Mukai: Thank you, and thanks to all the AAVSO observers out there who make
this kind of research possible!

The Furor Over FUOrs

FU Orionis and its associated nebula. Image credit: ESO

In 1937, an ordinary 16th magnitude star in the constellation Orion began to brighten steadily. Thinking it was a nova, astronomers were astounded when the star just kept getting brighter and brighter over the course of a year. Most novae burst forth suddenly and then begin to fade within weeks. But this star, now glowing at 9th magnitude, refused to fade. Adding to the puzzle, astronomers could see there was a gaseous nebula nearby shining from the reflected light of this mysterious star, now named FU Orionis. What was this new kind of star?

FU Ori has remained in this high state, around 10th magnitude ever since. This was a from of stellar variability never seen before. Since there were no other examples of this kind of variable star astronomers were forced to learn what they could from the only known example, or wait for another event to provide more clues.

Finally, more than 30 years later, FU Ori-like behavior appeared again in 1970 when the star now known as V1057 Cyg increased in brightness by 5.5 magnitudes over 390 days. Then in 1974, a 3rd example was discovered when V1515 Cyg rose from 17th magnitude to 12th magnitude over an interval lasting years. Astronomers began piecing the puzzle together from these clues.

FU Orionis stars are pre-main sequence stars in the early stages of stellar development. They have only just formed from clouds of dust and gas in interstellar space, which occur in active star- forming regions. They are all associated with reflection nebulae, which become visible as the star brightens.

This artist's concept shows a young stellar object 

and the whirling accretion disk surrounding it.  

NASA/JPL-Caltech

 

Astronomers are interested in these systems because FUOrs may provide us with clues to the early history of stars and the formation of planetary systems. At this early stage of evolution, a YSO is surrounded by an accretion disk, and matter is falling onto the outer regions of the disk from the surrounding interstellar cloud. Thermal instabilities, most likely in the inner portions of the accretion disk, initiate an outburst and the young star increases its luminosity. Our Sun probably went through similar events as it was developing.

One of the major challenges in studying FU Orionis stars is the relatively small number of known examples. Although approximately 20 FU Orionis candidates have been identified, only a handful of these stars have been observed to rise from their pre-outburst state to their eruptive state.

Now, in the last year, several new FUOrs have been discovered. In November 2009, two newly discovered objects were announced in Central Bureau Electronic Telegrams (CBET) #2033. Patrick Wils, John Greaves and the Catalina Real-time Transient Survey (CRTS) collaboration had discovered them in CRTS images.

The first of these objects appears to coincide with the infrared source IRAS 06068-0641.  Discovered by the CRTS on Nov. 10, it had been continuously brightening from at least early 2005 (when it was mag 14.8 on unfiltered CCD images) to its present mag 12.6. A faint cometary reflection nebula was visible to the east.  A spectrum taken with the SMARTS 1.5-m telescope at Cerro Tololo, on Nov. 17, confirmed it to be a young stellar object.  The object lies inside a dark nebula to the south of the Monocerotis R2 association, and is likely related to it.  

Also inside this dark nebula, a second object, coincident with IRAS 06068-0643, had been varying between mag 15 and 20 over the past few years, reminiscent of UX-Ori-type objects with very deep fades.  This second object is also associated with a variable cometary reflection nebula, extending to the north.  The spectrum of this object also shows H_alpha and the strong Ca II infrared triplet in emission. 

Light curves, spectra and images can be found here.

In August 2010, two new eruptive, pre-main sequence stars were discovered in Cygnus.  The first object was an outburst of the star HBC 722. The object was reported to have risen by 3.3 magnitudes from May 13 to August 16, 2010. Spectroscopy reported by U. Munari et al in ATel #2808, Aug 23, 2010 support this object's classification as an FU Ori star. Munari and his team reported the object at 14.04V on Aug 21, 2010.

The second object, coincident with the infrared source IRAS 20496+4354, was discovered by K. Itagaki (Yamagata, Japan) on August 23, 2010 and reported in CBET 2426.  The object appears very faint (magnitude 20) in a DSS image taken in 1990.  Subsequent spectroscopy and photometry of this object by U. Munari showed that this object also has the characteristics of an FU Ori star. Munari reported the object at 14.91V on August 26, 2010. 
 
Both these objects are now the subjects of an AAVSO observing campaign announced October 1, 2010 in AAVSO Alert Notice 425. Dr. Colin Aspin (U. Hawai'i) has requested the help of AAVSO observers in performing long-term photometric monitoring of these two new YSOs in Cygnus. AAVSO observations will be used to help calibrate optical and near-infrared spectroscopy to be obtained during the next year. 

Since these stars are newly discovered, very little is known about their behavior. Their classification as FU Ori variables is based on spectroscopy by U. Munari et al. Establishing a good light curve and maintaining it, over the next several years, will be crucial to understanding these stars. This kind of long-term monitoring is one of the things at which amateur astronomers excel.

November 10, 2010, results presenting rare pre and post outburst observations from the Palomar Transient Factory (PTF) show that HBS722 is a bona fide FU Ori type star that was a classical T-Tauri star before eruption, providing strong evidence that FU Orionis eruptions represent periods of enhanced disk accretion and outflow, likely triggered by instabilities in the accretion disk. 

Another paper, released the next day, also based on observations from the PTF, shows IRAS 20496+4354 brightened by more than 5 magnitudes, reaching 13.5R in September 2010. Near-infrared spectra appear quite similar to a spectrum of McNeil's Nebula/V1647 Ori, a FUOr which has undergone several brightenings in recent decades.


So after a very slow start, discoveries of new YSOs and our understanding of the dusty disk environments around them are starting to heat up. With new tools and new examples to study we are peering into the the early stages of stellar and planetary formation and finding some of our models have been pretty close to the truth. We expect to find more and similar objects as new all-sky surveys begin to cover the sky, but these objects will still be relatively rare and therefore interesting, because this period in a star's evolution is short-lived and only takes place in the active star forming regions of galaxies.


Images of HBC722 and  IRAS 20496+4354 from  
Discovery of possible FU-Ori and UX-Ori type objects
Wils, P., Greaves, J. and the CRTS collaboration, Nov 18th 2009.
http://crts.caltech.edu/CSS091110.html

Accretion

Accretion- The term accretion describes the growth of a massive object by gravitationally attracting more matter. This is commonly done through the formation of an accretion disk of gaseous matter. We find accretion disks around smaller stars or stellar remnants in a close binary, or around black holes in the centers of galaxies.

Accretion disks form in non-magnetic cataclysmic variable binaries

Accretion also refers to the collision and sticking of microscopic dust and ice particles in protoplanetary discs and protoplanet systems, leading to planetesimals which gravitationally accrete more small particles and other planetesimals.

Images courtesy Mark A. Garlick

http://www.space-art.co.uk/index.php

Pleas do not use without permission from the artist