Cataclysmic Variables

Thesis Research: "Exploring Stellar Activity on the Secondary star of Cataclysmic Variables"
Synopsis:The light curves of Cataclysmic Variables (CVs) exhibit a plethora of behaviors, from ephemeral changes (flickering) to large nova explosions; most are attributed to the accretion process. During the last couple of decades, it has been realized that activity (starspots) and activity cycles on the secondary star can be responsible for (quasi-periodic) modulations observed in long-term CV light curves, and may moderate, even halt accretion from the secondary star. The latter can lead to observed large drops in the system luminosity (VY Scl low states); their presence in both magnetic and disk systems direct attention to the secondary star for their cause. Activity on the K-M dwarf mass-losing secondary is commonly invoked to explain sustained mass transfer caused by system angular momentum loss via a magnetized stellar wind, quasi-periodic changes in the mass transfer rate, the elusive CV period gap, and widely differing mass transfer rates among CVs at the same orbital period. On the other hand, there is a substantial lack of observational support for these scenaria, since conventional activity indicators (X-ray and H-alpha emission; flares) are usually mimicked by accretion.
My thesis aims to explore alternative methods to infer stellar activity on CV secondary stars. For such a complicated problem, both photometric and spectroscopic indicators of activity are sought, focusing on systems showing large (2-5 mag) drops in their optical light curves (VY Scl systems), presumably due to disruptions of the mass transfer from starspots near the L1 point. One and two-slope fits to the shapes of transitions from and to the high state have been successfully related to the presence of starspots, if the slopes are interpreted as being due to migration of the umbra and penumbra portion of the starspot in front of L1. The speeds and shapes of the transitions of both disk and magnetic systems above the CV period gap are consistent with the aforementioned scenario. For systems below the gap, the characteristics of the transitions point to X-ray irradiation on the L1 point for the cause of the low states. Our results are in accord with the nature of the secondary star, which is expected to be fully convective for systems below the gap.
Furthermore, an extended monitoring campaign of the 2003/2004 low states of the polar prototype AM Herculis was conducted, at a time where accretion is negligible. Resolved photometric events during the 2004 low state are attributed to activity (flares) on the secondary star. Their phase concentration during the 2004 low state, indicate the presence of two active regions on the secondary star. Spectroscopic monitoring in both 2003 and 2004 reveal that the H$\alpha$ line is often triple-peaked, with all three components following the motion of the secondary. The central peak likely originates from the irradiated inner hemisphere of the donor star. The blue/red satellite components, reaching velocities of $\sim$300 km/sec with respect to the motion of the secondary, coincide in phase with the photometric flaring events. The variablity, phasing and velocity of the satellites, indicate that they possibly arise from loop-like coronal structures connecting active regions on the secondary. If confirmed, this would be good evidence for the presence of activity on a CV secondary star.
RoboScope

RoboScope is an automated 41-cm telescope in Indiana, equipped for unattended CCD stellar photometry, including scheduling of the observations, decisions for opening/closing of the dome, liquid nitrogen filling, telescope focusing, detector calibration and field identification.

As part of the program, incomplete ensemble photometry is performed, automatically updating the light curves of the stars observed.

More than 14 years of V band photometry (the telescope is operating since June 1990) allows the examination of the system's long term behavior. We generally have one data point each clear night at some random orbital phase, but the orbital light curve and its changes can nevertheless be studied when the data are grouped into intervals of several months, ensuring that the observed features are not just transient phenomena.

VY Scl low states

About 13 years of photometric monitoring of nova-like and magnetic CVs allowe analysis of occasional VY Scl-type low states, defined as a radical drop in the magnitude of the system (more than 1.5 mag) in less than 150 days.

Out of the 65 Nova-likes included in the program, 8 systems exhibiting these low states provide us with 29 transitions to and from the low state. A thorough study of the transitions reveal that they can be characterized by either single or dual slopes; the dual-sloped transitions are always faster when fainter.

This behavior is consistent with the low states being due to a starspot on the secondary star drifting under the inner Lagrangian point, if the two slopes are interpreted as being due to the passage of the umbral and penumbral portions of the starspot.

image credit: http://antwrp.gsfc.nasa.gov/apod/ap001128.html

Studying CV outflows in the Optical

Cataclysmic variables (CVs) often have evidence for bi-polar winds arising from the accretion process, which carry away mass and angular momentum, likely affecting the evolution of the system. For the most part, it has only been possible to study such winds by means of the P Cygni profiles seen in space-UV resonance lines. Recently, P Cygni profiles were discovered in the He I triplets (5876 & 7065AA) and in Ha, providing a new method of detecting outflows in CVs in the optical.

So far, 6 CVs (BZ Cam, Q Cyg, HR Del, DI Lac, BT Mon and AT Cnc) have been discovered to display P-Cygni profiles in their optical lines, in which wind signatures indicate terminal velocities up to ~4600 km/sec. More to come...

Publications:
  • "Spectroscopic Study of Q Cyg: Surprises from an Old Nova", Kafka, S., Tappert, C., Honeycutt, R.K. & Bianchini, A. 2003, AJ, 126, 1472
  • "The Puzzling Optical Light Curve of the Polar QQ Vulpeculae", Kafka, S. & Honeycutt, R.K. 2003, AJ, 125, 2188
  • "VY Scl transitions in nova-like cataclysmic variables", Honeycutt, R.K. & Kafka, S. 2003, ASP Conf.Ser.292: Interplay of Periodic, Cyclic and Stochastic Variability in Selected Areas of the H-R Diagram, 279
  • "QQ Vul: variable features in the orbital light curve", Kafka, S.& Honeycutt, R.K. 2003, ASP Conf.Ser.292: Interplay of Periodic, Cyclic and Stochastic Variability in Selected Areas of the H-R Diagram, 319
  • "Long-term Changes in the Orbital Light Curve of the Polar QQ Vulpeculae" Kafka, S.& Honeycutt, R.K. 2000, American Astronomical Society Meeting, 197
  • "Outburst Characteristics in the Dwarf Nova SU Ursae Majoris" Rosenzweig, P., Mattei, J.A., Kafka, S., Turner, G.W., & Honeycutt, R.K. 2000, pasp, 112, 632

    The animations of CVs displayed here were taken from Dr. A.Beardmore's site, at: http://www.astro.keele.ac.uk/~apb/OGL_CV/oglcv.html

    Updated: Sept. 2004