ABSTRACT
'Cataclysmic variables' are binary star systems in which one star of the pair is a white dwarf, and which often generate bright and energetic stellar outbursts. Classical novae are one type of outburst: when the white dwarf accretes enough matter from its companion, the resulting hydrogen-rich atmospheric envelope can host a runaway thermonuclear reaction that generates a rapid brightening. Achieving peak luminosities of up to one million times that of the Sun, all classical novae are recurrent, on timescales of months to millennia. During the century before and after an eruption, the 'novalike' binary systems that give rise to classical novae exhibit high rates of mass transfer to their white dwarfs. Another type of outburst is the dwarf nova: these occur in binaries that have stellar masses and periods indistinguishable from those of novalikes but much lower mass-transfer rates, when accretion-disk instabilities drop matter onto the white dwarfs. The co-existence at the same orbital period of novalike binaries and dwarf novae-which are identical but for their widely varying accretion rates-has been a longstanding puzzle. Here we report the recovery of the binary star underlying the classical nova eruption of 11 March AD 1437 (refs 12, 13), and independently confirm its age by proper-motion dating. We show that, almost 500 years after a classical-nova event, the system exhibited dwarf-nova eruptions. The three other oldest recovered classical novae display nova shells, but lack firm post-eruption ages, and are also dwarf novae at present. We conclude that many old novae become dwarf novae for part of the millennia between successive nova eruptions.
ABSTRACT
OBJECTIVE: To explore the association between clinical training characteristics and trainees' level of confidence in performing ultrasound scans independently. METHODS: A cross-sectional e-survey was distributed to members of the national societies of junior obstetricians/gynecologists in Denmark, Sweden and Norway (n = 973). Multiple linear regression models were used to explore the effect that amount of time spent in specialized ultrasound units and clinical experience had on trainees' confidence in performing ultrasonography independently. Exploratory factor analysis was used to identify factors that contributed to trainees' confidence in performing ultrasonography. Trainees' ultrasound confidence was finally compared with their expected levels of performance. RESULTS: Of the 682 respondents (response rate 70.1%), 621 met the inclusion criteria. Clinical experience and time spent in specialized ultrasound units were predictors of trainees' confidence in performing ultrasonography independently (P < 0.001). Trainees required more than 24 months of clinical experience and 12-24 days of training in specialized ultrasound units in order to feel confident about performing transvaginal and transabdominal ultrasound scans independently. Three factors were related to ultrasound confidence: technical aspects, image perception and integration of scan into patient care. There were significant differences between trainees' level of confidence and their expected levels of performance (P < 0.001). CONCLUSIONS: Clinical experience and time spent in specialized ultrasound units were predictors of trainees' confidence in performing ultrasonography independently. Discrepancies between trainees' confidence and their expected levels of performance raised concerns about the adequacy of current ultrasound training programs.
