Review of AM CVn or Helium Cataclysmic Variable stars
Vayujeet Gokhale
Abstract
We review the observational properties of AM CVn stars and outline
possible theoretical models.
1 Introduction
AM CVn stars are binary helium white dwarfs with extremely short
orbital periods ( ~ few minutes to an hour) in which the less
massive star transfers mass to the more massive one. The systems
show a wide variation in brightness, thus indicating mass
transfer, whilst the spectra is dominated by helium lines with a
total absence of hydrogen, thus betraying its helium composition.
The short periods almost certainly require that both components
are degenerate or at least semi-degenerate (see below). These
systems are thought to be driven by Gravitational Wave Radiation
(GWR), but with increasing periods due to mass transfer and
angular momentum redistribution.
As of today, at least 10 objects are confirmed AM CVn systems.
Recently, 3 objects of extremely short periods have been
observed, which also possibly belong to this class of objects. In
this paper, we first review the formation processes of these
interesting objects and follow-up on some of the observational
characteristics of the sub-classes of Helium cataclysmic stars. In
particular we concentrate on one object in a given sub-class: AM
CVn for the so called "High state" systems, SDSS J1240-01 for the
"low-state", KL Dra for the "Superhump" systems and ES Cet for
the "peculiar" systems. The main characteristics of AM CVn's are:
- Consists of a helium-rich, possibly degenerate, donor
of extremely low mass (< 0.1 M*) and an degenerate helium
accretor of mass ~ 0.7 M* . Thus the mass ratio's are quiet low,
q ~ 0.1.
- Mass transfer is driven by Gravitational Wave
Radiation.
- Periods are of the order of an hour or less and orbits
are expected to evolve to higher periods.
- In most cases, the "state" of the system can
be ascertained from observations, leading to sub classification into "Low
state", "Outbursting" and
"High state" systems.
- Observationally, the spectra are totally devoid of
Hydrogen lines, show a rich Helium spectrum along with processed heavy
element lines.
- A "Superhump" period accompanies the orbital
modulation of the photometric flux.
- The ultra short period systems are comparatively
peculiar: Superhumps are not identifiable, some claims of Hydrogen in the
spectra have been made and the periods of these system seem to decrease with
time; though this claim is under severe scrutiny.
| Name |
V (mag) |
Porb (s) |
Psh (s) |
q |
Spectrum |
Phot. Var. |
dist(pc) |
X-ray |
UV |
| ES Cet |
16.9 |
621 (p/s) |
-- |
-- |
Em |
orb |
-- |
-- |
-- |
| AM CVn |
14.1 |
1029 (s/p) |
1051 |
0.101 |
Abs |
orb |
235 |
-- |
-- |
| HP Lib |
13.7 |
1103 (p) |
1119 |
0.072(6) |
Abs |
orb |
(330) |
-- |
-- |
| CR Boo |
13.0 - 18.0 |
1471 (p) |
1487 |
0.060(5) |
Abs/Em? |
OB/orb |
>250 |
-- |
-- |
| KL Dra |
16.8 - 20.0 |
1500 (p) |
1530 |
-- |
Abs/Em? |
OB/orb |
-- |
-- |
-- |
| V803 Cen |
13.2 - 17.4 |
1612 (p) |
1618 |
-- |
Abs/Em? |
OB/orb |
(250) |
-- |
-- |
| CP Eri |
16.5 - 19.7 |
1701 (p) |
1716 |
0.040(4) |
Abs/Em |
OB/orb |
(800) |
-- |
-- |
| 2003aw |
16.5 - 20.3 |
? |
2042 |
-- |
Abs/Em? |
OB/orb |
-- |
-- |
-- |
| SDSSJ1240-01 |
? |
2242 (s) |
-- |
-- |
Em |
n |
-- |
-- |
-- |
| GP Com |
15.7 - 16.0 |
2794 (s) |
-- |
0.020(6) |
Em |
n |
70 |
-- |
-- |
| CE315 |
17.6 |
3906 (s) |
-- |
-- |
Em |
n |
77 |
-- |
-- |
2 Formations of Helium Cataclysmic stars
AM CVn's are thought to form via 2-3 different "channels".
- A DWD system, itself formed via a series of Common Envelope evolutions, shrinks as a result of angular momentum losses due to GWR. Eventually, the less massive (and thus the bigger degenerate star) star comes into contact with its Roche radius and mass transfer commences. The system then evolves to higher periods due to redestribution of angular momentum.
- A phase in which a non-degenerate, low mass helium donor transfers mass to a white dwarf accretor. The system passes through a minimum in period of ~ 10 mins, when the Helium donor becomes semi-degenrate. The period increases after this minimum and mass transfer keeps falling.
- From Cataclysmic variables with evolved donors. After significant mass loss, the exposed He-core of the donor in a CV evolves similar to Helium star tracks
These channels are depicted in the figure below alongwith observved systems. Note that the shortest periods of a few minutes can only be reached by DWD progenitors.
3 Observational Characteristics
As mentioned, AM CVn systems are characterized by ultrashort
periods, broad spectral lines - mostly helium dominated and by a
total absence of hydrogen. However, not all the known systems have
the exact same characteristics and so we further classify them
into subclasses based on the state of the accretion disc.
3.1 "High state" systems
These are the systems with relatively short periods and thus a
high mass transfer rate due to the high driving rate. Though the
mass transfer rate is high, it is "stable": the depth of contact
is proportional to the rate of driving, which itself is secularly
falling due to the increasing separation. Of the three objects
assigned in this category, AM Cvn - the prototype - and HP Lib
are known to show a Superhump period along with an orbital period.
The third object, ES Cet, is sometimes considered to be of the
`atypical variety' of AM CVn's (see below), but we consider it to
be in a high state because of the high inferred mass transfer rate
and short periods.
Figure 1: Right Panel: Power spectrum of AM CVn of the
four strongest He I lines. The peak at 3.5 cycles/hour corresponds
to a period of 1029 s. Left Panel: Trailed spectrogram of
the He I 4387 and 4471A° line profiles (absorption in white).
After folding on the candidate orbital period of 1028.73 s.
(Bottom panel) After folding on 1051.2 s. The grey-scale is chosen
in order to highlight the weak emission components in the core of
the lines. A clear emission component is visible in both lines
when the data are folded on 1029 s only, indicating that this is
indeed the true orbital period of AM CVn.
We review now the observational characteristics of the prototype,
AM CVn.
AM CVn (HZ 29), the oldest known and probably the most studied
object in this class of CV's, is a faint blue object showing a
rich spectrum of helium lines associated with a variety of
periods. Observations over the past decades seem to confirm two
periods: 1029 s and 1051 s, as the orbital and superhump periods
respectively (Fig. 1 & Fig. 2); this conclusion being reached
after quiet a bit of confusion. Fig. 1 establishes beyond doubt
that the true orbital period of the binary is in fact, 1029 s. In
fact, that AM CVn is a binary CV itself was not well established
till lately. However, compelling evidence in the form of weak
emission lines, variable asymmetric absorption lines and UV
absorption lines from heavy elements along with spectroscopic
evidence clearly demonstrate the nature of AM CVn: its is a helium
binary composed of degenerate helium stars with mass transfer
driven by GWR. A third period associated with this object is 13.5
hrs (Fig. 3). As seen in Fig. 3, the violent and red troughs of
the unblended lines seem to alternate up and down. Patterson et al
define a skewness parameter defined by
where Yi = Fcont -Fi is the difference between the
continuum and observed flux at each pixel. The power contained in
the skewness series is plotted in Fig. 2. The 13.4 hr. period is
essentially the precession period of the accretion disk around the
accretor with precesses due to the tidal pull of the Roche-filling
donor. The superhump period is related to the orbital period and
the precessional period:
With Porb ~ 1028.73 s and Pprec ~ 13.4
hr., we obtain Psh ~ 1051.3 s.
Figure 2: Power spectrum
of AM CVn of the skewness time series for 4 different unblended
lines. The lower panels on the right side are artificial signals
with P = 13.4 hr and 8.6 hr. The 13.4 hr matches the observed
spectra better, indicating the precession period of the disk.
Figure 3: Phase binned
spectra on the 13.4 hr period for AM CVn. The flux calibration for
the bottom spectrum is true, the rest are displaced for
convenience.
From the above discussion it is clear that the dominant
photometric period of 1051.2 s is in fact, the beat between the
orbital period of 1028.73 s and the disc precession period of
13.38-h.
3.2 "Low state" systems
Figure 4: Average spectra of SDSS
J1240.
Low state systems are of the longest periods, believed to be
systems undergoing stable mass transfer at low rates. The
evolution of the system is thus correspondingly slow. Their
optical spectra is dominated by strong Helium emission lines, and
a weak underlying blackbody radiation emanating from the accreting
white dwarf.
3.2.1 SDSS J1240-01
SDSS J1240-01 was discovered in a search of the Sloan Digital Sky
Survey database. Its characteristic broad helium emission lines -
some with double humps - clearly indicating an accretion disk,
and hence mass transfer. It has however, been observed in past
surveys usually having a V-magnitude of ~ 19.7. The continuum
helium absorption lines are consistent with a single white dwarf
temperature of 17000 K, usually attributed to the accretor. The
orbital period of SDSS J1240 is determined by generating a power
spectrum of the red-wing/blue-wing ratios. These are
shown in Fig. 6 and a peak at 37.35 mins is clearly seen.
Figure 5: Trailed spectra and Doppler
tomograms of the R-band He I and Si II features. A pattern of two
bright spots, at approximately the same radial velocity but at a
120 degree angle, appears in the trails and tomograms. The central
spikes are clearly visible in the trailed spectra of He I 5875, He
I 6678 and He II 7065. The possible central spike feature in the
He I 7281 line is strongly affected by night sky
lines.
Figure 6: Power spectrum for SDSS
J1240. A clear peak is seen at 38.57 cycles/day corresponding to a
period of 37.35 minutes. The main peaks are shown in detail on the
bottom panel.
An interesting feature seen in this particular object is a
double bright spot in the Doppler tomogram (Fig. 5,
bottom panel). Also, a central spike is observed at zero
velocities, between the doppler shifted double hump feature (Fig.
5, top panel). The central spike is believed to be located either
on or very near to the primary, but its origin is unknown. This
feature is also seen in other Low state AM CVn's: GP Com and CE
315. The origin of the second bright spot on the Doppler tomogram
is also unknown, though it is speculated that it is a result of
overshooting of the accretion stream: it misses the edge of the
accretion disk, and instead impact the rim of the disk on the
other side of the disk. This plausible, since the radial
velocities of both the bright spots are identical, thus implying a
similar origin. It is also possible that the second bright spot is
caused by standing waves on the disk, or due to tidal effects on
the donor on the disk.
3.3 "Outbursting" systems
These are systems of intermediate periods and their optical
brightness varies between that of high and low state systems.
These are considered to be a result of the unstable accretion
disks. Thus in high state, these systems exhibit He absorption
lines whilst in the low state they show He emission lines.
At first considered a Supernova candidate, KL Dra was confirmed
later to be an AM CVn in the outburst state. Presence of He I
absorption lines and total absence of hydrogen and the observation
that the objects brightness varied from ~ 20th magnitude to
about 16.8 suggested that the object was in fact a helium dwarf
nova. In Fig. 7, the peak corresponds to about 25.03 min., which
is thought to be the orbital period in the low state, while a
period of 25.5142 min. is the superhump period in the high state.
Figure 7: Power spectra of KL
Dra.
Figure 8: Sine curve fitted to single
night data for KL Dra, yielding a period of 25.421
mins.
V803 Cen typifies the "Cyclic state objects" in the AM CVn class,
and proving their dwarf nova credentials vis-a-vis the regular
Cataclysmic variables - it has a thermally unstable disk
resulting in variable mass transfer rates, leading the system to
oscillate between a high state (V ~ 13, top panel in Fig. 9)
and a low state (V ~ 17, middle panel). It also has a truly
"cyclical" state, lasting for about 50% of the time, where the
system oscillates between a V ~ 13.5-14.5 (lower panel).
Again, this resembles the recurrent nova phase in usual CV's.
Interestingly, the recurrence phase has two periods, a 0.94 day
and a ~ 5 day period. This is probably a reflection of the
fact that the recurrence time is inversely proportional to the
mass transfer rate.
V803 Cen has an orbital period of 1611 s, and a superhump period
of 1618 s. Strangely, the superhump period persists in both the
high and low states. Thus, this particular object has the strange
property to spend most of its time in the cyclic state, and then
periodically jump to a more or less steady brightness state -
which can be either high or low. This behavior is unique to V803
Cen and CR Boo as compared to regular dwarf novae in CV's.
Figure 9: Power spectra for V803
Cen - top: in the high state, middle: in the
low state, bottom: in the cyclic state.
3.4 "Candidate/Ultra-short period" systems
4 Models
4.1 Helium disk model
4.2 Direct Impact model
4.3 Electric Star model
5 Discussion
References
- [1]
- Lai, Rasio, Shapiro, 1993, ApJ, 406, L63.
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