Z Cam stars: A particular response to a general phenomenon

Buat-Ménard, V.; Hameury, J. M.; Lasota, Jean–Pierre

doi:10.1051/0004-6361:20010176

Cited by 38 publications

(45 citation statements)

References 21 publications

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“…A very similar behaviour occurs in Z Cam-type dwarf novae in the intervals when theiṙ m approachesṁ crit (Buat-Ménard et al 2001). Figure 11 is an example: the amplitude of the outbursts largely diminishes and also the brightness at minimum increases considerably because the mass accretion onto the white dwarf still proceeds.…”

Section: Discussionsupporting

confidence: 52%

The complex long-term X-ray activity of the compact binary 4U 1915–05

Šimon¹

2005

A&A

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Astronomy & AstrophysicsThe complex long-term X-ray activity of the compact binary 4U 1915-05 Abstract. An analysis of the long-term X-ray activity of the compact binary 4U 1915-05 from 1996-2003, using the ASM/RXTE data, shows that the behaviour of the long-term X-ray light curve inside this interval was considerably different from that reported previously. We find that the X-ray light curve is dominated by complicated variations in the form of outbursts with rather sharp peaks, each lasting for about 20-30 days. The period search by weighted wavelet transform (WWZ) reveals that the cycle-length of the activity is highly variable and the periods can persist only through several epochs. We find that a cycle of about 230-250 days (similar to the ∼199 day cycle reported by Priedhorsky & Terrell 1984, ApJ, 280, 661) occurred only in a limited time interval -this cycle manifests in the variations of the recurrence time and amplitude of the outbursts. Generally, the system spends more time in the low state than in the high state (outbursts), and the standard deviation and skewness display variations on the time scale of the order of hundreds of days. We show that the decays of the individual outbursts in 4U 1915-05 display a relatively broad range of slopes. The slow average decay rate τ D ≈ 0.2 ct/s/day can be related to the viscous time of the disk. We discuss the models and offer an interpretation in terms of the thermal instability of the disk, modified by irradiation by the neutron star, following the model by King & Ritter (1998, MNRAS, 293, L42). We also compare the activity of 4U 1915-05 with that of a similar system, 4U 1820-30, and interpret the largely divergent kinds of activity in terms of the different mean mass transfer rates. This rate is lower in 4U 1915-05 and can allow thermal instability of the disk, unlike the case of 4U 1820-30. We argue that the disk in 4U 1915-05 can oscillate between thermally stable and unstable state.

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Section: Discussionsupporting

confidence: 52%

The complex long-term X-ray activity of the compact binary 4U 1915–05

Šimon¹

2005

A&A

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“…However, Equation (33) is only an approximation to numerical results that themselves depend on various assumptions. Perhaps most importantly, it does not include the heating of the outer disk that may be expected due to tidal dissipation and irradiation (e.g., Stehle et al 2001;Buat-Ménard et al 2001). Depending on the system parameters, this can lower theṀ crit,hi (P orb ) by a factor of several (Buat-Ménard et al 2001), which might be enough to push the predictedṀ 2 (P orb ) into the stable regime above the period gap.…”

Section: Reconciling Theory and Observationmentioning

confidence: 99%

The Evolution of Cataclysmic Variables as Revealed by Their Donor Stars

Knigge

Baraffe

Patterson

2011

ApJS

568

879

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We present an attempt to reconstruct the complete evolutionary path followed by cataclysmic variables (CVs), based on the observed mass-radius relationship of their donor stars. Along the way, we update the semi-empirical CV donor sequence presented previously by one of us, present a comprehensive review of the connection between CV evolution and the secondary stars in these systems, and reexamine most of the commonly used magnetic braking (MB) recipes, finding that even conceptually similar ones can differ greatly in both magnitude and functional form. The great advantage of using donor radii to infer mass-transfer and angular-momentum-loss (AML) rates is that they sample the longest accessible timescales and are most likely to represent the true secular (evolutionary average) rates. We show explicitly that if CVs exhibit long-term mass-transfer-rate fluctuations, as is often assumed, the expected variability timescales are so long that other tracers of the mass-transfer rate-including white dwarf (WD) temperatures-become unreliable. We carefully explore how much of the radius difference between CV donors and models of isolated main-sequence stars may be due to mechanisms other than mass loss. The tidal and rotational deformation of Roche-lobe-filling stars produces 4.5% radius inflation below the period gap and 7.9% above. A comparison of stellar models to mass-radius data for non-interacting stars suggests a real offset of 1.5% for fully convective stars (i.e., donors below the gap) and 4.9% for partially radiative ones (donors above the gap). We also show that donor bloating due to irradiation is probably smaller than, and at most comparable to, these effects. After calibrating our models to account for these issues, we fit self-consistent evolution sequences to our compilation of donor masses and radii. In the standard model of CV evolution, AMLs below the period gap are assumed to be driven solely by gravitational radiation (GR), while AMLs above the gap are usually described by an MB law first suggested by Rappaport et al. We adopt simple scaled versions of these AML recipes and find that these are able to match the data quite well. The optimal scaling factors turn out to be f GR = 2.47 ± 0.22 below the gap and f MB = 0.66 ± 0.05 above (the errors here are purely statistical, and the standard model corresponds to f GR = f MB = 1). This revised model describes the mass-radius data significantly better than the standard model. Some of the most important implications and applications of our results are as follows. (1) The revised evolution sequence yields correct locations for the minimum period and the upper edge of the period gap; the standard sequence does not. (2) The observed spectral types of CV donors are compatible with both standard and revised models. (3) A direct comparison of predicted and observed WD temperatures suggests an even higher value for f GR , but this comparison is sensitive to the assumed mean WD mass and the possible existence of mass-transfer-rate fluctuations. (4) The pred...

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“…The main difference is that Z Cam stars have no superoutbursts. We will therefore adapt the Z Cam model (Buat-Ménard et al 2001b) by combining its mass-transfer modulations with the EMT.…”

Section: Dips "Cycling" and Standstillsmentioning

confidence: 99%

“…Adapting the Z Cam models of Buat-Ménard et al (2001b) to AM CVn stars, we assumed the mass-transfer to be modulated as ΔṀ/ Ṁ = 40% around an average rate close to the critical (Ṁ 0,tr = 1.1 × 10 17 g s −1 ) with the modulations occurring at t = 10 + k · 70 days. To take into account the presence of irradiation-induced super-outbursts, we combined this modulation with the mass-transfer modulations given by Eq.…”

Section: "Z Cam -Type" Modulations and (γ) Superoutburstsmentioning

confidence: 99%

“…The prescription for mass-transfer modulations used in the previous section has been chosen not for its realism but because it has been used in modelling the Z Cam star outburst cycle by Buat-Ménard et al (2001b). Since in AM CVn light curves other peculiar features are present in addition to standstills, we tried other simple forms of mass-transfer rate modulations.…”

Section: Sinusoidal Plus γ Modulationmentioning

confidence: 99%

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Models of AM Canum Venaticorum star outbursts

Kotko

Lasota

Dubus

et al. 2012

A&A

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Context. Outbursting AM CVn stars exhibit outbursts similar to those observed in different types of dwarf novae. Their light-curves combine the characteristic features of SU UMa, ER UMa, Z Cam, and WZ Sge-type systems but also show a variety of properties never observed in dwarf novae. The compactness of AM CVn orbits and their unusual chemical composition make these systems valuable testbeds for outburst models. Aims. We aim for a better understanding of the role of helium in the accretion disc instability mechanism, testing the model for dwarf novae outbursts in the case of AM CVn stars, and aim to explain the outburst light-curves of these ultra-compact binaries. Methods. We calculated the properties of the hydrogen-free AM CVn stars using our previously developed numerical code adapted to the different chemical composition of these systems and supplemented with formulae accounting for mass transfer rate variations, additional sources of the disc heating, and the primary's magnetic field. Results. We discovered how helium-dominated discs react to the thermal-viscous instability and were able to reproduce various features of the outburst cycles in the light-curves of AM CVn stars. Conclusions. The AM CVn outbursts can be explained by the suitably adapted dwarf-nova disc instability model but, as in the case of its application to hydrogen-dominated cataclysmic variables, one has to resort to additional mechanisms to account for the observed superoutbursts, dips, cycling states, and standstills. We show that the enhanced mass-transfer rate, due presumably to variable irradiation of the secondary, must not only be taken into account but is a determining factor that shapes AM CVn star outbursts. The cause of the variable secondary's irradiation has yet to be understood; the best candidate is the precession of a tilted/warped disc.

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Z Cam stars: A particular response to a general phenomenon

Cited by 38 publications

References 21 publications

The complex long-term X-ray activity of the compact binary 4U 1915–05

The complex long-term X-ray activity of the compact binary 4U 1915–05

The Evolution of Cataclysmic Variables as Revealed by Their Donor Stars

Models of AM Canum Venaticorum star outbursts

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