Observational studies of highly evolved cataclysmic variables

Cataclysmic Variables (CV) are binary systems where a main-sequence star transfers mass onto a white dwarf (WD). According to standard evolutionary theory, angular momentum loss drives CVs to initially evolve from longer to shorter orbital periods until a minimum period is reached (? 80 minutes). At roughly this stage, the donors becomes degenerate, expand in size, and the systems move towards longer orbital periods. Theory predicts that 70% of all CVs should have passed their minimum period and have sub-stellar donors, but until recently, no such systems were known. I present one CV showing evidence of harbouring a sub-dwarf donor, SDSS J1507+52. Due to the system’s unusually short orbital period of ~ 65 minutes, and very high space velocity, two origins for SDSS J1507+52 have been proposed; either the system was formed from a young WD/brown-dwarf binary, or the system is a halo CV. In order to distinguish between these two theories, I present UV spectroscopy and find a metallicity consistent with halo origin. Systems close to the minimum period are expected to be faint and have low accretion rates. Some of these CVs show absorption in their spectra, implying that the underlying WD is exposed. This yields a rare opportunity to study the WD in a CV. I introduce two new systems showing WD signatures in their light curves and spectra, SDSS J1457+51 and BW Sculptoris. Despite the fact that CVs close to the minimum period should be faint, we find systems that aremuch too bright for their orbital periods. Such a system is T Pyxidis – a recurrent nova with an unusually high accretion rate and a photometrically determined period < 2 hours. The system is ~ 2 times brighter than any other CV at its period. However, to confirm the status of this unusual star, a more reliable period determination is needed. Here, I present a spectroscopic study of T Pyxidis confirming its evolutionary status as a short-period CV. In this thesis, I discuss what implications these systems may have on the current understanding of CV evolution, and the importance of studying individual systems in general

The orbital period and system parameters of the recurrent nova T Pyx

T Pyx is a luminous recurrent nova that accretes at a much higher rate than is expected for its photometrically determined orbital period of about 1.8 h. We here provide the first spectroscopic confirmation of the orbital period, P = 1.8295 h (f = 13.118368 +/- 1.1 x 10(-5) c d(-1)), based on time-resolved optical spectroscopy obtained at the Very Large Telescope and the Magellan telescope. We also derive an upper limit of the velocity semi-amplitude of the white dwarf, K 1 = 17.9 +/- 1.6 kms(-1), and estimate amass ratio of q = 0.20 +/- 0.03. If the mass of the donor star is estimated using the period-density relation and theoretical main-sequence mass-radius relation for a slightly inflated donor star, we find M-2 = 0.14 +/- 0.03 M-circle dot. This implies a mass of the primary white dwarf of M-1 = 0.7 +/- 0.2 M-circle dot. If the white-dwarf mass is > 1 M-circle dot, as classical nova models imply, the donor mass must be even higher. We therefore rule out the possibility that T Pyx has evolved beyond the period minimum for cataclysmic variables. We find that the system inclination is constrained to be i approximate to 10 degrees, confirming the expectation that T Pyx is a low-inclination system. We also discuss some of the evolutionary implications of the emerging physical picture of T Pyx. In particular, we show that epochs of enhanced mass transfer (like the present) may accelerate or even dominate the overall evolution of the system, even if they are relatively short-lived. We also point out that such phases may be relevant to the evolution of cataclysmic variables more generally

The cataclysmic variable SDSS J1507+52: An eclipsing period bouncer in the Galactic halo

SDSS J1507+52 is an eclipsing cataclysmic variable consisting of a cool, non-radially pulsating white dwarf and an unusually small sub-stellar secondary. The system has a high space velocity and a very short orbital period of about 67 minutes, well below the usual minimum period for CVs. To explain the existence of this peculiar system, two theories have been proposed. One suggests that SDSS J1507+52 was formed from a detached white-dwarf/brown-dwarf binary. The other theory proposes that the system is a member of the Galactic halo-population. Here, we present ultraviolet spectroscopy of SDSS J1507+52 obtained with the Hubble Space Telescope with the aim of distinguishing between these two theories. The UV flux of the system is dominated by emission from the accreting white dwarf. Fits to model stellar atmospheres yield physical parameter estimates of T(eff) = 14200 \pm 500 K, log(g)=8.2 \pm 0.3, vsin(i)=180 \pm 20 kms-1 and [Fe/H]=-1.2 \pm 0.2. These fits suggest a distance towards SDSS J1507+52 of d = 250 \pm 50 pc. The quoted uncertainties include systematic errors associated with the adopted fitting windows and interstellar reddening. Assuming that there is no contribution to the UV flux from a hot, optically thick boundary layer, we find a T(eff) much higher than previously estimated from eclipse analysis. The strongly sub-solar metallicity we infer for SDSS J1507+52 is consistent with that of halo stars at the same space velocity. We therefore conclude that SDSS J1507+52 is a member of the Galactic halo

BK Lyncis: The Oldest Old Nova?... And a Bellwether for Cataclysmic-Variable Evolution

We summarize the results of a 20-year campaign to study the light curves of BK Lyncis, a nova-like star strangely located below the 2-3 hour orbital period gap in the family of cataclysmic variables. Two apparent "superhumps" dominate the nightly light curves - with periods 4.6% longer, and 3.0% shorter, than P_orb. The first appears to be associated with the star's brighter states (V~14), while the second appears to be present throughout and becomes very dominant in the low state (V~15.7). Starting in the year 2005, the star's light curve became indistinguishable from that of a dwarf nova - in particular, that of the ER UMa subclass. Reviewing all the star's oddities, we speculate: (a) BK Lyn is the remnant of the probable nova on 30 December 101, and (b) it has been fading ever since, but has taken ~2000 years for the accretion rate to drop sufficiently to permit dwarf-nova eruptions. If such behavior is common, it can explain other puzzles of CV evolution. One: why the ER UMa class even exists (because all members can be remnants of recent novae). Two: why ER UMa stars and short-period novalikes are rare (because their lifetimes, which are essentially cooling times, are short). Three: why short-period novae all decline to luminosity states far above their true quiescence (because they're just getting started in their postnova cooling). Four: why the orbital periods, accretion rates, and white-dwarf temperatures of short-period CVs are somewhat too large to arise purely from the effects of gravitational radiation (because the unexpectedly long interval of enhanced postnova brightness boosts the mean mass-transfer rate). These are substantial rewards in return for one investment of hypothesis: that the second parameter in CV evolution, besides P_orb, is time since the last classical-nova eruption.Comment: PDF, 46 pages, 4 tables, 10 figures; in preparation; more info at http://cbastro.org

Gender differences in natural science and technology educational paths in Sweden

I denna uppsats presenterar jag statistik över könsskillnader på utbildningar inom naturvetenskap och teknik i Sverige mellan åren 2000-2018, från gymnasienivå till doktorandnivå. På doktorandnivå rapporterar jag statistik mellan åren 1973-2017. Mellan 2011- 2018 rapporterar jag även behörighet mellan könen för gymnasiets statistik och sätter detta i relation till andelen flickor och pojkar som antagits. Denna statistik jämför jag sedan med aktuella vetenskapliga studier som publicerats under samma tidsintervall. Jag finner stora könsskillnader mellan gymnasiets naturvetenskapliga program och tekniska program. På det tekniska programmet var antalet flickor år 2000 endast 10%. På det naturvetenskapliga programmet var  år 2000 45% flickor medan 55% var flickor år 2018. Alltså hade andelen flickor och pojkar bytt plats under tidsintervallet för studien. Jag finner att könsskillnaderna på eftergymnasiala program inom fysik och teknik har varit relativt konstant, på 20% kvinnor under samma tidsintervall. Dock har könsskillnaderna på doktorandnivå förändrats kraftigt till att år 2018 kvinnor utgör 40%. Jag jämför detta med könsskillnaderna på samtliga doktorandutbildningar i Sverige och finner att könsskillnaderna minskar stadigt från 70-talet fram till år 2006 då en normalisering sker. Efter 2006 förändras inte den totala andelen män och kvinnor som doktorerar nämnvärt och de utgör i princip 50%. Att färre kvinnor väljer att gå eftergymnasiala utbildningar i fysik/teknik är inte unikt för Sverige, dock visar studier att skillnaderna är som störst i länder som kan anses mer jämlika. Detta är den så kallade jämställdhetsparadoxen. Att flickor väljer bort eftergymnasiala utbildningar  i fysik/teknik beror inte på sämre betyg i dessa ämnen, men kan enligt studier spegla flickors självförtroende samt att de rapporterar att ha andra intressen, där kvinnor av STEM ämnena är mer intresserade av biologi (oavsett om dessa skillnader i intresse är socialt inlärda eller inte).