PL
 |
EN
Preferencje help
Polish version English version Język
Widoczny [Schowaj] Abstrakt
Liczba wyników

Znaleziono wyników: 10

Liczba wyników na stronie
first rewind previous Strona / 1 next fast forward last
Wyniki wyszukiwania
Wyszukiwano:
w słowach kluczowych:  Stars: dwarf novae
help Sortuj według:

help Ogranicz wyniki do:
first rewind previous Strona / 1 next fast forward last
1
Content available remote Z Cha and its Superhumps
Smak J.
Acta Astronomica
|
2009
|
Vol. 59, No. 1
109--120
EN
The superhump eclipse light curves are re-determined for five eclipses of Z Cha (E54036, E54037, E67877, E74693, E74694) observed by Warner and O'Donoghue (1988) during its superoutbursts. Qualitatively they are similar to those obtained by O'Donoghue (1990), showing two local minima at φ≈-0.05 and 0.04. Arguments are then presented which imply that the first minimum is not due to an occultation but is produced by absorption effects in the overflowing parts of the stream. The location of the superhump light source (SLS) determined from the analysis of the second minimum coincides with the trajectory of the overflowing parts of the stream. The light curve of the sixth eclipse (E77878) could be simply decomposed into its disk and superhump components. The location of SLS, obtained from the analysis of the SLS eclipse light curve, coincides in this case with the position of the standard hot spot. This implies that superhumps are due to modulated mass transfer rate resulting in periodically enhanced dissipation of the kinetic energy of the stream.
2
Content available remote Variable Stars in the Globular Cluster NGC 6752
Kaluzny J., Thompson I. B.
Acta Astronomica
|
2009
|
Vol. 59, No. 3
273--289
EN
We report time-series photometry for 16 variable stars located in the central part of the globular cluster NGC 6752. The sample includes 13 newly identified objects. The precision of our differential photometry ranges from 1 mmag at V=14.0 mag to 10 mmag at V=18.0 mag. We detected four low amplitude variables located on the extended horizontal branch (EHB) of the cluster. They are candidate binary stars harboring sdB subdwarfs. A candidate degenerate binary was detected about 2 mag below the faint end of the EHB. The star is blue and its light curve is modulated with a period of 0.47 d. We argue that some of the identified variable red/blue stragglers are ellipsoidal binaries harboring degenerate stars. They have low amplitude sine-like light curves and periods from a few hours to a few days. Spectroscopic observations of such objects may lead to the detection of detached inactive binaries harboring stellar mass black holes or neutron stars. No binaries of this kind are known so far in globular clusters although their existence is expected based on the common occurrence of accreting LMXBs and millisecond pulsars. An eclipsing SB1 type binary was identified on the upper main sequence of the cluster. We detected variability of optical counterparts to two X-ray sources located in the core region of NGC 6752. The already known cataclysmic variable B1=CX4 experienced a dwarf nova type outburst. The light curve of an optical counterpart to the X-ray source CX19 exhibited modulation with a period of 0.113 d. The same periodicity was detected in the HST-ACS data. The variable is located on the upper main sequence of the cluster. It is an excellent candidate for a close degenerate binary observed in quiescence.
3
Content available remote On the Origin of Tilted Disks and Negative Superhumps
Smak J.
Acta Astronomica
|
2009
|
Vol. 59, No. 4
419--430
EN
The origin of tilted disks in cataclysmic variables is explained in terms of a model involving the stream-disk interactions. Tilted, precessing disk causes periodically variable asymmetry in the irradiation of the two hemispheres of the secondary component, resulting in variable vertical component of the velocity of the stream. The following stream-disk interactions provide additional vertical acceleration to disk elements needed to produce and maintain disk tilt. Predictions based on this model compare favorably with observations.
4
Content available remote On the Amplitudes of Superhumps
Smak J.
Acta Astronomica
|
2009
|
Vol. 59, No. 1
103--107
EN
Amplitudes of bolometric light curves produced by 2D and 3D SPH simulations are used to determine the corresponding visual amplitudes. They turn out to be ≈10 times lower than typical amplitudes of superhumps. This means a major failure of the tidal model of superhumps.
5
Content available remote New Interpretation of Superhumps
Smak J.
Acta Astronomica
|
2009
|
Vol. 59, No. 1
121--130
EN
Observational evidence is presented for periodically variable irradiation of secondary components. This results in strongly modulated mass outflow. Superhumps are then due to enhanced dissipation of the kinetic energy of the stream. Qualitative interpretation of superhump periods and their variations is also presented.
6
Content available remote Hα Imaging of X-ray Sources in Selected Globular Clusters with the SOAR Telescope
Pietrukowicz P.
Acta Astronomica
|
2009
|
Vol. 59, No. 3
291--305
EN
We present results of a search for objects with Hα excess, such as cataclysmic variables (CVs) and chromospherically active binaries (ABs), as counterparts to X-ray sources detected with Chandra satellite observatory in six Galactic globular clusters (GCs): M4, M28, M30, M71, M80, NGC 6752. Binary systems play a critical role in the evolution of GCs, serving as an internal energy source countering the tendency of GC cores to collapse. Theoretical studies predict dozens of CVs in the cores of some GCs (eg., 130 for M28, 40 for M30). A number of such binaries is also expected outside the core radius. However, few CVs are known so far in GCs. Using subtraction technique applied to images taken with the 4.1-m SOAR telescope we have found 27 objects with Hα excess in the field of the observed clusters, of which nine are likely associated with the clusters. Four are candidate CVs, four are candidate ABs, one could be either a CV or an AB. One Hα object seems to be a background galaxy, while other 17 detected objects are probably foreground or background stars.
7
Content available remote Are Disks in Dwarf Novae during their Superoutbursts Really Eccentric?
Smak J.
Acta Astronomica
|
2009
|
Vol. 59, No. 1
89--101
EN
The evidence presented earlier by several authors for the substantial disk eccentricity in dwarf novae during their superoutbursts is shown to result either from errors, or from arbitrary, incorrect assumptions. (1) The evidence for Z Cha and WZ Sge based on radial velocities measured from absorption components, was an artifact, resulting from miscalculated beat phases. (2) The evidence for OY Car and IY UMa based on the observed dependence of eclipse parameters on the beat phase, involved an implicit assumption that the observed eclipses are pure disk eclipses, which is not true. In particular, the observed variations of eclipse parameters are likely due to the contributions from the hot spot and from the superhump source, which depend strongly on the beat phase. (3) The evidence for OY Car and WZ Sge resulting from the analysis of hot spot eclipses, was based on the assumption that the spot distances are identical with the radius of the disk, which is not always correct. In particular, in the case of eclipses of "peculiar" spots (involving the stream overflow), observed at beat phases away from φb≈0.5, the resulting spot distances are smaller than the radius of the disk. New determination of disk eccentricity in Z Cha, using Vogt's radial velocities measured from emission components, gives e=0.05±0.05.
8
Content available remote The Hot Spot in OY Car during its Superoutburst
Smak J.
Acta Astronomica
|
2008
|
Vol. 58, No. 1
65--67
EN
Detection of a "standard" hot spot in OY Car during one of its superoutbursts is reported. Its presence implies that the mass transfer rate was strongly enhanced.
9
Content available remote Superoutbursts of Z Cha and their Interpretation
Smak J.
Acta Astronomica
|
2008
|
Vol. 58, No. 1
55--64
EN
Disk eclipses of Z Cha during its superoutbursts are analyzed. The resulting flux/temperature distributions in the outer parts of the disk show peculiarities which are interpreted as being due to absorption effects in the overflowing parts of the stream. The temperatures of the inner parts of the disk are used to determine the accretion rates as a function of time since superoutburst maximum. They are practically identical with mass transfer rates determined from the hot spot. Results are summarized in the form of the following, purely observational scenario: Superoutbursts are due and begin with a major enhancement in the mass transfer rate. During the "flat-top" part of the superoutburst the mass transfer rate decreases slowly, causing the observed luminosity to decline. The superoutburst ends when the mass transfer rate decreases below its critical value, resulting in a transition to the quiescent state of the dwarf nova cycle.
10
Content available remote The 1985 Superoutburst of U Geminorum. Modulation of the Superhump Amplitude
Smak J. I.
Acta Astronomica
|
2006
|
Vol. 56, No. 4
365--371
EN
The superhump amplitude was modulated with phase of the beat period. The highest amplitudes (2Amax≈0.4 mag) occurred when the superhump maximum coincided with the orbital phases φorb≈0.15 and 0.65, while the lowest (2Amin≈0.2 mag) - with φorb≈0.40 and 0.90. The directions defined by those two pairs of orbital phases appear to be parallel and perpendicular to the direction of the stream.
first rewind previous Strona / 1 next fast forward last
JavaScript jest wyłączony w Twojej przeglądarce internetowej. Włącz go, a następnie odśwież stronę, aby móc w pełni z niej korzystać.