106 research outputs found

    Distances of Stars by mean of the Phase-lag Method

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    Variable OH/IR stars are Asymptotic Giant Branch (AGB) stars with an optically thick circumstellar envelope that emit strong OH 1612 MHz emission. They are commonly observed throughout the Galaxy but also in the LMC and SMC. Hence, the precise inference of the distances of these stars will ultimately result in better constraints on their mass range in different metallicity environments. Through a multi-year long-term monitoring program at the Nancay Radio telescope (NRT) and a complementary high-sensitivity mapping campaign at the eMERLIN and JVLA to measure precisely the angular diameter of the envelopes, we have been re-exploring distance determination through the phase-lag method for a sample of stars, in order to refine the poorly-constrained distances of some and infer the currently unknown distances of others. We present here an update of this project.Comment: 4 pages, 1 figure, 2 tables, to appear in the Proceedings of the IAU Symposium No. 336: Astrophysical Masers: Unlocking the Mysteries of the Univers

    OH masers in the Milky Way and Local Group galaxies in the SKA era

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    The intense line emission of OH masers is a perfect tracer of regions where new stars are born aswell as of evolved stars, shedding large amounts of processed matter into the interstellar medium. From SKA deep surveys at 18 cm, where the maser lines from the ground-state of the OH molecule arise, we predict the discovery of more than 20000 sources of stellar and interstellar origin throughout the Galaxy. The study of this maser emission has many applications, including the determination of magnetic field strengths from polarisation measurements, studies of stellar kinematics using the precisely determined radial velocities, and distance determinations from VLBI astrometry. A new opportunity to study shocked gas in different galactic environments is expected to arise with the detection of lower luminosity masers. For the first time, larger numbers of OH masers will be detected in Local Group galaxies. New insights are expected in structure formation in galaxies by comparing maser populations in galaxies of different metallicity, as both their properties as well as their numbers depend on it. With the full capabilities of SKA, further maser transitions such as from excited OH and from methanol will be accessible, providing new tools to study the evolution of star-forming regions in particular.Comment: Contribution to the conference on "Advancing Astrophysics with the Square Kilometre Array" for the SKA science book, Giardini-Naxos, Sicily, June 2014; in Proceedings of Science, 14 page

    HS 1857+5144 : a hot and young pre-cataclysmic variable

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    Aims. We report the discovery of a new white dwarf/M dwarf binary, HS 1857+5144, identified in the Hamburg Quasar Survey (HQS). Methods. Time-resolved optical spectroscopy and photometry were carried out to determine the properties of this new cataclysmic variable progenitor (pre-CV). Results. The light curves of HS 1857+5144 display a sinusoidal variation with a period of Porb = 383.52 min and peak-to-peak amplitudes of 0.7 mag and 1.1mag in the B-band and R-band, respectively. The large amplitude of the brightness variation results from a reflection effect on the heated inner hemisphere of the companion star, suggesting a very high temperature of the white dwarf. Our radial velocity study confirms the photometric period as the orbital period of the system. A model atmosphere fit to the spectrum of the white dwarf obtained at minimum light provides limits to its mass and temperature of Mwd 0.6−1.0 M and Twd 70 000−100 000 K, respectively. The detection of He II λ4686 absorption classifies the primary star of HS 1857+5144 as a DAO white dwarf. Combining the results from our spectroscopy and photometry, we estimate the mass of the companion star and the binary inclination to be Msec 0.15−0.30 M and i 45◦−55◦, respectively. Conclusions. We classify HS 1857+5144 as one of the youngest pre-CV known to date. The cooling age of the white dwarf suggests that the present system has just emerged from a common envelope phase ∼105 yr ago. HS 1857+5144 will start mass transfer within or below the 2–3 h period gap

    Variability of water masers in evolved stars on timescales of decades

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    For several decades (1987-2015) we have been carrying out observations of water masers in the circumstellar envelopes (CSE's) of Mira variables, Red Supergiants (RSG's) and Semi-Regular Variables (SRV's) with the Medicina 32-m and Effelsberg 100-m antennas. The single-dish monitoring observations provide evidence for strong H2O maser profile variations, which likely are connected to structural changes in the maser shells. Such variations include strong flares in intensity lasting several (tens of) months and systemic velocity gradients of maser components developing over years, as well as other secular variations which are superimposed on periodic variations following the stellar light variations. When complemented with interferometric observations, it is possible to derive the 3-D distribution of the maser spots, and their lifetime, as we have done for RX Boo (Winnberg et al. 2008) and U Her (Winnberg et al. 2011; Brand et al. in prep.)

    'The language of the naked facts': Joseph Priestley on language and revealed religion

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    Joseph Priestley (1733-1804) is usually remembered for his experiments in natural philosophy and celebrated for his isolation of the gas we now call oxygen. However, Priestley had a wide range of interests and published extensively on education, history, politics, political philosophy, language, theology and religion. He dedicated his life to elucidating a coherent set of epistemological, metaphysical and theological principles which he believed explained the human mind, the natural world and the nature of God and revelation. Recent studies of Priestley have emphasised the difficulties that arise from isolating the various aspects of his thought and the fruitful outcome of uncovering the many connections between his diverse areas of study. With this in mind, the present dissertation aims to elucidate the relationship between two aspects of Priestley’s thought that have not previously been studied together. It examines his theory of language and argument alongside his work on theology and the evidences of revelation. Chapter One provides an overview of Priestley’s epistemology, focusing on his work on induction, judgment and assent. Chapter Two looks at Priestley’s analysis of the role of the passions in our assent to propositions and the progressive generation of the personality, while paying particular attention to the origins of figurative language. Chapter Three examines Priestley’s theory of language development including the relationship between figurative language and the extension of vocabulary and the close connection between language and culture. Chapter Four demonstrates that Priestley’s discussion of the evidences of revealed religion is structured around his theory of assent and judgment. It also explains how assent to revelation is essential for the generation and transcendence of the ‘self’. Chapter Five brings all the themes of the dissertation together in a discussion of Priestley’s rational theology and examines his analysis of figurative language in scripture