CAPÍTULO 2: DISEÑO Y EVALUACIÓN DEL AULA VIRTUAL DE LA ASIGNATURA
2.4 Implementación del curso
Until now I considered the evolution of CVs whereq < 1. For systems in which the mass of the donor is larger than that of the white dwarf accretor,q> 1, the evolution differs from the standard scenario of CVs.
Thermal time scale mass transfer
The stability for mass transfer from the companion depends on the radius-mass exponents (see Section 1.2.3) of the Roche lobe,ζRL = dln(RRL)/dln(MMS), and the MS companion,
ζMS = dln(RMS)/dln(MMS). In general, the Roche lobe reduces (ζRL>0) when material is transferred from the donor to a less massive accretor. If the donor star has a radiative envelope, it will shrink (ζMS>0) in response to mass loss, but if it has a deep convective en-
Figure 1.16: Critical mass ratio for thermal (solid) and dynamical (dashed) time scale mass transfer as a function of the mass of the main sequence donor. The mass ratio limit has been calculated assuming conservative (grey) and non-conservative (black) mass transfer. The red cross indicates the position of TYC 6760-497-1, which is a system that will undergo a phase of thermal time-scale mass transfer. Figure taken from Parsons et al. (2015).
velope, it will expand rapidly (ζMS<0). These parameters regulate whether the mass transfer proceeds on a dynamical or thermal time scale. IfζMS<ζRLthe Roche lobe radius shrinks more rapidly than the adiabatic radius, and the mass transfer proceeds on the dynamical time scale, which leads very quickly to a common envelope (Section 1.2.2). If, in contrast,ζMS
>ζRL, the mass transfer occurs on the thermal time scale. Thermal time scale mass transfer can drive very high mass transfer rates.
The orbit shrinks faster to a point that the mass-losing star is no longer able to adjust its thermal equilibrium inside the rapidly reducing Roche lobe, resulting in a drastic increase of the rate of mass transfer. The response of the Roche lobe to the mass transfer depends on the mass ratio, whereas the response of the stellar radius of the companion to the mass trans- fer is a function of the mass transfer rate. The critical mass ratio for marginal stability against thermal time-scale mass transfer can be calculated by equating the mass-radius exponents of the Roche lobe and the star (Figure 1.16). Therefore, a pre-CV with a mass ratio ofq=2, is likely to go through a phase of thermal time scale mass transfer. Since the average mass of the white dwarfs in pre-CVs is 0.67±0.21M (Zorotovic et al., 2011), the mass of their companions need to be as large as∼1.3Mto ensure a thermal time scale mass transfer phase.
Figure 1.17: Accretion rate versus the mass of the white dwarf. The chemical composition of accreting material isX=0.7,Y=0.28, andZ=0.02. The red lines indicate the narrow region where white dwarfs can support steady hydrogen nuclear burning, which corresponds to the location of the supersoft X-ray sources. Below this band the material builds up on the white dwarf and eventually most of it is ejected in explosive flashes (nova eruptions). The recurrence period of these flashes is shorter for larger masses and higher accretion rates, and vice versa. Above the steady burning band, the matter piles up on the white dwarf forming a red-giant-like structure. Figure taken from Wolf et al. (2013).
Population synthesis models show that a sufficiently high accretion rate can allow efficient mass retention on the white dwarf through quasi-steady or episodic nuclear shell burning. Figure 1.17 shows that, depending on the accretion rate, there are three distinctive groups of nuclear burning white dwarfs (Nomoto et al., 2007):
• Classical and recurrent novae, i.e. systems accreting below the stable nuclear H burning limit (MÛ < MÛstable), which will undergo sporadic and explosive nuclear burning. The recurrence times for nova eruptions are indicated by the dashed lines in Figure 1.17.
• Steady shell burning on white dwarfs. If the accretion rate falls within a narrow band (MÛstable 6 MÛ 6 MÛcr) the material forms a thick, non-degenerate shell on the white
dwarf surface and therefore H burns into He at the rate it is supplied by the donor. The peak of emission of these systems is in the soft X-rays, therefore they are referred to as Supersoft X-rays sources (SSSs).
• In systems where the accretion rate is above the steady nuclear burning limit (MÛ > MÛcr, dotted line), the large amount of accreted material piles up forming a red-giant-like structure. The photosphere turns optically thick producing winds that remove some of the accreted material.
In this thesis, the emphasis is on the evolution of systems that undergo the steady nuclear burning phase.
Supersoft X-rays Sources as progenitors of Supernova type Ia
The first SSSs were discovered in the Large Magellanic Cloud by Long et al. (1981) with theEINSTEINsatellite and the absence of photons detected above∼0.5 KeV withROSAT
confirms them as a new class of supersoft source emitters (Trümper et al., 1991; Greiner et al., 1991). However, their binary nature was only revealed with optical observations that established the orbital periods of 1.04 d and 0.44 d for CAL 83 and CAL 87, respectively (Smale et al., 1988; Cowley et al., 1990). van den Heuvel et al. (1992) proposed the so-called close-binary supersoft source (CBSS) model that states that SSSs are the result of steady nuclear burning of H accreted onto massive white dwarfs, supplied by a more massive MS star or subgiant2. Indeed, the high effective temperatures (105–106K) and luminosities
(1036–1038erg s−1) derived from the X-ray data suggest that these sources have effective radii comparable to those of white dwarfs.
The fact that these accreting white dwarf can grow in mass makes the close binary SSSs potential candidates of SNIa progenitors (Whelan & Iben, 1973). Simulations show that on a CO white dwarf the accreted H fuses into He, the He layer grows in mass and will eventually fuse steadily to C and O (Starrfield et al., 2004). Therefore, the increasing mass of the CO white dwarf can eventually reach the Chandrasekhar mass limit and lead to the SNIa explosion. However, the accretion rate decreases drastically once that the ratio of the masses of the two stars come close to unity, and therefore the H burning on the surface stops. If the white dwarf has not exploded by then, the binary system will continue its evolution as a CV towards shorter orbital periods.
2The existence of stable shell burning white dwarfs, and their appearance as luminous soft X-rays emitters
was predicted by Shara et al. (1977) and (Iben, 1982), but those works were not connected to the sources discovered by Long et al. (1981)
Failing the Supernova type Ia explosion
As mentioned above, systems undergoing a thermal time scale mass transfer phase contain a companion star typically more massive than the Sun. Hence, these donor stars have undergone a considerable nuclear evolution before the onset of the mass transfer. Binary population syntheses show that CVs that start mass transfer with a slightly evolved sec- ondary (with the mass fraction of the central H content less than 0.3) will not experience the period gap, or if they do, it will be at periods shorter than∼2 h (Pylyser & Savonije, 1989). Moreover, these CVs with nuclear evolved secondaries can evolve towards ultrashort orbital periods (∼7 min) contributing to the population of AM CVn stars (Podsiadlowski et al., 2003).
The high initial masses of the companions in these failed SNIa imply that they were powered by the CNO burning during their lifetime as pre-CVs.
Products from CNO nuclear process
As introduced in Section 1.1.1 the CNO cycle is one of the two main nuclear fusion reactions through which the star converts H into He. When the temperature in the core reaches approximately 15×106K, the CNO burning becomes the dominant energy source, which corresponds to stars with masses larger than'1.3M. CNO burning is a catalytic process which fuses four protons (p), using C, Nitrogen (N), and O isotopes as catalysts, to produce oneαparticle (He nucleus), two positrons (e+), and two electron neutrinos (νe)
12C+p→13 N+γ+1.95 MeV (1.17) 13N→13 C+e++ν e+1.20 MeV (1.18) 13C+p→14 N+γ+7.54 MeV (1.19) 14N+p→15 O+γ+7.35 MeV (1.20) 15O→15 N+e++ν e+1.73 MeV (1.21) 15N+p→12 C+4He+4.96 MeV (1.22)
Reactions involving νe are weaker interactions and the slowest step is the proton
capture on14N. During the evolution of the star,14N and4He become enriched, while12C
and15O are depleted.
The Roche lobe overflow of the donor results that its outer layers are removed, and if the CV undergoes the thermal time scale mass transfer, the donor is rapidly stripped off a
large fraction of its mass. In addition, due to the decreasing mass of the donor during the evolution it will start to experience convective mixing episodes with deeper extents, which will move material from the layers where CNO burning has operated, altering the surface composition which is what can be observed of the star.
In CVs, material from the outer layers of the secondary is accreted onto the white dwarf, and therefore enhancing the N and He abundances in the white dwarf atmosphere. A example is AE Aquarii which is considered to be a descendent of super-soft X-rays sources, since its ultraviolet spectrum shows strong N emission lines, and weak to absent C emission lines (Schenker et al., 2002).
Thesis structure
The structure of this thesis is organised as follows. Most of the data analysed corresponds to ultraviolet spectroscopy taken withHST, and therefore both HSTspectrographs will be
described in Chapter 2. The codes used to generate synthetic white dwarf spectra together with the fitting technique that models the ultraviolet spectroscopy is explained in Chapter 3. At the end of both of these chapters I briefly explain the analysis ofHSTdata, which led
to a number of side projects in which I was involved. In Chapter 4, I describe the work on G29-38, a pulsating white dwarf which shows metal abundances in its atmosphere due to the accretion of a disrupted planetary. In Chapter 5, I explain my work on GW Librae, a CV containing a pulsating white dwarf which the pulsations were damped out due to a dwarf nova outburst. In Chapter 6, I explain my work on two descendants of super-soft X-rays sources and the evolutionary models for N and C abundances that can be seen in CVs that are post super-soft X-rays sources. Finally, in Chapter 7 I summarise the most important conclusion of the three topics presented in this thesis.