Estimating The Binary Fraction Of Central Stars Of Planetary Nebulae

Author : Brent Miszalski
Publisher :
ISBN 13 :
Total Pages : 204 pages
Book Rating : 4.:/5 (758 download)

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Download or read book New Galactic Planetary Nebulae and the Role Binary Central Stars written by Brent Miszalski and published by . This book was released on 2009 with total page 204 pages. Available in PDF, EPUB and Kindle. Book excerpt: The Galactic population of planetary nebulae (PNe) offers great potential in improving our understanding of many astrophysical problems on both large and small scales. They are revealed out to large distances by their bright emission line spectra from which their radial velocities and chemical abundances can be measured. As members of the old stellar population, PNe are particularly abundant towards the Galactic bulge where their kinematics are a valuable, relatively unbiased tracer of the dynamics of the region. Chemical abundance variations may also be traced by PNe to place constraints on chemodynamical models of the Galaxy. On much smaller scales their central stars (CSPN) are a powerful window into the poorly understood later stages of binary stellar evolution. The capacity of PNe to perform these studies is critically dependent on the size of the population. The current Galactic population of PNe was recently doubled by the Macquarie/AAO/Strasbourg Halpha (MASH) PNe catalogue. A supplement to MASH, the MASH-II catalogue, is presented with more than 360 new Galactic PNe found after a thorough search of all 233 AAO/UKST SuperCOSMOS Halpha Survey fields in digital format. Novel semi-automated data processing and multi-wavelength visualisation techniques are developed to maximise the sensitivity of the search. MASH-II PNe are notable for being either small, star-like PNe of relatively high surface brightness, or very large, extremely low surface brightness PNe. Over 90% of the catalogue is confirmed spectroscopically during extensive observing campaigns and the catalogue is available via the vizier catalogue service at the Centre de Donn\'ees Astronomiques de Strasbourg (CDS). This thesis is based on the exploitation of the MASH and MASH-II PNe catalogues that have provided the largest and most representative sample of PNe towards the Galactic bulge. This offers a unique opportunity to contribute towards two different, largely unexplored research domains: (i) The kinematics of the bulge, and (ii) The role of binary central stars of PNe. Radial velocities of hundred of Pne towards the Bulge were measured from ANU 2.3-m longslit spectroscopy and from deep spectroscopy conducted with the AAT 2dF/AAOmega and VLT FLAMES multi-object spectroscopy facilities. Multiple measurements were recorded for many PNe resulting in a more accurate catalogue of about 1200 PNe within the longitude smaller than 30° region reaching a very high completeness of 95%. The kinematic study enabled a slope of 104 km/s/kpc for the rotation curve of the bulge that is in excellent agreement with 100 km/s/kpc determined from M-giants. General kinematic profiles were calculated and compared well with other tracer populations to bring new constraints on a dynamical model of the bulge. A completely new and powerful approach is conceived to discover large numbers of binary CSPN. The concept was employed to analyse the time-series photometry of nearly 300 Galactic bulge PNe from the OGLE-III microlensing survey. A total of 21 periodic binary CSPN candidates were found after careful elimination of 27 PN mimics identified using deep spectroscopy. The orbital period distribution is dominated by periods less than one day which indicates these binaries must have been produced via the common-envelope (CE) phase of binary stellar evolution. These discoveries have effectively doubled the population of close binary CSPN whose potential in advancing our knowledge of CE evolution has yet to be realised. Gemini GMOS spectroscopy of the 14 members of the OGLE sample produces 10 bona fide binary CSPN, 2 likely binary CSPN and 2 unlikely associations. There remains three candidates in the centre of small nebulae which leave little doubt of their bona fide status pending future spectroscopy, while four other candidates lie in larger nebulae awaiting confirmation. Cool giant companions are revealed in at least two binary CSPN and in one instance UV photometry proves the existence of the primary invisible in the optical spectrum. This suggests cool central stars may be more common than previously thought and more exotic scenarios explaining their presence can be ruled out. The observed orbital period distribution is found to be biased towards shorter periods than predicted by CE population synthesis models. Only one model in the literature matches the distribution reasonably well, but more recent models could not reproduce its predictions. A close binary fraction of at least 10--20% is estimated for PNe. After consideration of selection effects and other limitations of the survey, our estimate is found to be more robust than the previous estimate obtained from previous a survey conducted over 20--30 years with uncertain biases. Of particular interest is elucidating the role of binarity in the shaping of nebular morphologies. The close binary fraction imposes that at least 10--20% of PNe have been heavily shaped by a close companion, however no clear morphological properties have been identified amongst PNe with close binary CSPN. Nearly 30% of a carefully selected sample of 30 post-CE nebulae are found to have canonical bipolar morphologies. A very plausible bipolar fraction of at least 60% is reached once the inclination and other effects are considered. This is the strongest indication yet that the morphologies of post-CE nebulae largely satisfy theoretical expectations of a high density contrast established during the CE phase. Low ionisation structures (LIS) are common amongst post-CE nebulae suggesting they have a binary origin. LIS seem confined to either the orbital plane as radially distributed knots or filaments, or to the polar regions as (mostly) low surface brightness jets triggered by a dynamo effect. A binary origin may also be responsible for LIS around emission-line nuclei whereby one or more CE phases created the identifiable morphology and dual-dust chemistry. A likely binary formation scenario for LIS includes the distribution of neutral clumps of dust and H2 during the CE phase into the orbital plane that are then photo-ionised by winds during the PN phase. If the binary scenario holds then Nitrogen abundances for PNe with LIS would be rendered meaningless since the [NII] emission observed is not a result of enriched stellar material, but rather reflects shocked emission generated with models that assume standard abundances.