Hambly, N.

Lam, M.

Rowell, N.

English

We discuss the Gaia Data Release 1 (September 2016) and preliminary work on maximising the benefit for cool white dwarf (WD) science in advance of the full parallax catalogue which will appear around one year later in DR2. The Tycho catalogue is used in conjunction with the all-sky ground based astrometric/ photometric SuperCOSMOS Sky Survey in order to identify candidate faint common proper motion objects to the Tycho stars. Gaia DR1 is supplemented by the Tycho-Gaia Astrometric Solution catalogue containing some 2 million parallaxes with Hipparcos-like precision for Tycho stars. While hotter, brighter WDs are present in Tycho, cooler examples are much rarer (if present at all) and CPM offers one method to infer precision distances for a statistically useful sample of these very faint WDs

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     Edinburgh Research Explorer                                      Get Ready for Gaia: Cool White Dwarfs in Common ProperMotion with Tycho StarsCitation for published version:Hambly, N, Rowell, N & Lam, M 2017, 'Get Ready for Gaia: Cool White Dwarfs in Common Proper Motionwith Tycho Stars', pp. 55. <http://adsabs.harvard.edu/abs/2017ASPC..509...55H>Link:Link to publication record in Edinburgh Research ExplorerDocument Version:Peer reviewed versionGeneral rightsCopyright for the publications made accessible via the Edinburgh Research Explorer is retained by the author(s)and / or other copyright owners and it is a condition of accessing these publications that users recognise andabide by the legal requirements associated with these rights.Take down policyThe University of Edinburgh has made every reasonable effort to ensure that Edinburgh Research Explorercontent complies with UK legislation. If you believe that the public display of this file breaches copyright pleasecontact openaccess@ed.ac.uk providing details, and we will remove access to the work immediately andinvestigate your claim.Download date: 04. Jan. 2021Get ready for Gaia: cool white dwarfs in common proper motionwith Tycho starsNigel Hambly, Nick Rowell and Marco LamInstitute for Astronomy, SUPA (Scottish Universities Physics Alliance), Schoolof Physics and Astronomy, University of Edinburgh, Royal Observatory,Blackford Hill, Edinburgh, UK; nch@roe.ac.ukAbstract. We discuss the Gaia Data Release 1 (September 2016) and preliminarywork on maximising the benefit for cool white dwarf (WD) science in advance of thefull parallax catalogue which will appear around one year later in DR2. The Tychocatalogue is used in conjunction with the all–sky ground based astrometric/photometricSuperCOSMOS Sky Survey in order to identify candidate faint common proper motionobjects to the Tycho stars. Gaia DR1 is supplemented by the Tycho–Gaia AstrometricSolution catalogue containing some 2 million parallaxes with Hipparcos–like precisionfor Tycho stars. While hotter, brighter WDs are present in Tycho, cooler examples aremuch rarer (if present at all) and CPM offers one method to infer precision distancesfor a statistically useful sample of these very faint WDs.1. IntroductionESA’s Gaia mission has been in routine operations for two years. The first intermediatedata release (GDR1) occurred in September 2016 and is based on the first 14 monthsof data gathered by the spacecraft. Such a time baseline is insufficient to disentanglereliably proper motion and parallax so the primary data product of GDR1 will be a mapof around one billion stars to V ∼ 20 with mean epoch positions and single passbandphotometry but no proper motions nor parallaxes. However, by using the Tycho datafrom the precursor Hipparcos mission it is possible to anchor the positions for over 2million brighter stars (V < 11.5) at a mean early epoch around 1991 and hence solve forproper motion and parallax. This yields trigonometric distances for 20× as many starsas in the Hipparcos catalogue with similar precision (Michalik et al. 2015). A bonuspart of GDR1 are these Tycho–Gaia Astrometric Solution (TGAS) sources. Data arereleased to the community via a central Science Archive system and associated partnerData Centers (e.g. CDS) with extensive exploitation facilities and documentation1.2. White Dwarfs in Hipparcos and TychoThere are 10 or so WDs in the Hipparcos catalogue (which is at best complete to V ∼ 9)with parallaxes measured at a 5σ level. The number of WDs present in the Tycho1http://archives.esac.esa.int/gaia/1arXiv:1702.02188v1  [astro-ph.SR]  7 Feb 20172 Hambly, Rowell, and Lamcatalogue (90% complete to V ∼ 11.5) is larger of course, so TGAS will deliver initself a major step forward for WD science via the addition of precision distances (andtherefore WD masses, radii, etc). However the TGAS WDs will be biased towards thehot, bright end of the WD luminosity function simply because of the bright magnitudelimit of that catalogue coupled with the extreme faintness of cooler WDs.3. White Dwarfs in common proper motion with Tycho starsOne method for expanding significantly the sample of cool WDs with measured dis-tances in GDR1 is to find those with high proper motion using existing ground–basedcatalogues, e.g. the digitised optical all–sky Schmidt photographic surveys, and as-sociate them with Tycho stars via common proper motion. This can be done withusefully high confidence given a sufficiently high lower proper motion cut such that inany given part of the sky the likelihood of finding by chance two nearby stars with thesame high proper motion is negligibly small. Figure 1 illustrates this using the Super-COSMOS Science Archive2 RECONS proper motion survey plus lower proper motionsupplement (Hambly et al. 2004; Rowell & Hambly 2011) down to a lower proper mo-tion limit of 80 mas/yr. Wide binary candidates with separations of up to 16.7 arcmin(1000 arcsec, corresponding to maximal separations of order 100,000 AU at 100 pc)can be identified in this way and their position in reduced proper motion / colour spaceshows some of them to be likely cool WDs. In Figure 2 the image and adjacent thumb-nails show one example, an R = 19.7 object with R–I = 1.3 having a proper motionwithin 1.5σ of that of TYC 2734–750–1 (central in the main picture) which is morethan 8 magnitudes brighter in R. Using common proper motion it will be possible toinfer accurately the distances to a large sample of cool WDs prior to the availability ofdirect trigonometric parallaxes in GDR2.4. White dwarfs in GDR2 and subsequent releasesOf course the major leap forward for the science of all WD fields of study will befrom GDR2 onwards with the availability of full five–parameter astrometric solutionsand (some) radial velocities (Jordan 2007). Even then full 6D kinematics will not beavailable for the majority of isolated cool WDs because of the unknown and generallyunmeasurable radial velocity component. Moreover accurate model–independent agesare difficult to establish for such objects which leads to, for example, poor coverage inthe Initial–Final Mass Relation. If, however, a cool WD can be associated with a normalstar via common proper motion then the distance and radial velocity will nearly alwaysbe more accurately measured (or indeed may only be measurable) for the system usingthe brighter component. If the system can be identified kinematically with a cluster,association or moving group of known age and metallicity etc. then so much the betterand hence the technique of ‘benchmarking’ systems (Gomes & Pinfield 2013; Zhaoet al. 2012) identified via common proper motion will remain a valuable tool for WDscience in the Gaia era.An interesting aspect of this work is in the implementation details of the CPMsearch algorithm. Catalogue pairing is, of course, a solved problem in computer science2http://ssa.roe.ac.ukGaia, Tycho and cool white dwarfs 3222120191817161514131211109876-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5R - IH_R (=5 + 5log10[mu] + R)AllTycho starFigure 1. Reduced proper motion diagram for Tycho2 catalogue stars (blue) and1.5σ common proper motion objects from SuperCOSMOS (red). The usual dwarf,subdwarf and WD locii trace from the upper left to the lower right in this RPM–colour space. A search radius of 1000 arcsec was used along with a lower propermotion cut of 80 mas/yr. All the red objects share common proper motion with aTycho star.via ‘plane sweep’ algorithms that deliver true O(N log M) performance (see Devereuxet al. 2005 and references therein) for small search radii. For the larger search radiineeded in this application, we have further refined the implementation using the ‘zonedjoin’ technique developed by Gray et al. (2004) where the input catalogue(s) are firstsplit into separate Declination zones of extent equal to the maximum search radiusrequired to identify CPM wide binaries (in this case 1000 arcsec). This is particularlyimportant when scaling up to a self–join of a billion–row scale catalogue, as will be thecase when GDR2 is released.4 Hambly, Rowell, and LamFigure 2. Example of a candidate wide binary identified via common proper mo-tion between TYC 2734–750–1 (R = 11.3) and a cool WD (R = 19.7; R–I = 1.3).The wide angle image is 30 arcmin on each side while the thumbnails are 1 arcmin.Thumbnails on the left are from the POSS–E plate scan while those on the right arefrom the 2nd epoch red survey plate (exposed some 40 years later).Acknowledgments. This research has made use of data obtained from the Super-COSMOS Science Archive, prepared and hosted by the Wide Field Astronomy Unit,Institute for Astronomy, University of Edinburgh, which is funded by the UK Scienceand Technology Facilities Council.ReferencesDevereux, D., Abel, D. J., Power, R. A., & Lamb, P. R. 2005, Astronomical Data AnalysisSoftware and Systems XIV, 347, 346Gomes, J. I., & Pinfield, D. J. 2013, European Physical Journal Web of Conferences, 47, 06002Gray, J., Szalay, A. S., Thakar, A. R., Fekete, G. F., O’Mullane, W., Heber, G., Rots, A. H.,2004, There Goes the Neighbourhood: Relational Algebra for Spatial Data Search, Mi-crosoft Research Technical Report 2004–32Hambly, N. C., Henry, T. J., Subasavage, J. P., Brown, M. A., & Jao, W.-C. 2004, AJ, 128, 437Jordan, S. 2007, 15th European Workshop on White Dwarfs, 372, 139Michalik, D., Lindegren, L., & Hobbs, D. 2015, A&A, 574, A115Rowell, N., & Hambly, N. C. 2011, MNRAS, 417, 93Zhao, J. K., Oswalt, T. D., Willson, L. A., Wang, Q., & Zhao, G. 2012, ApJ, 746, 144

Get Ready for Gaia: Cool White Dwarfs in Common Proper Motion with Tycho Stars

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Get Ready for Gaia: Cool White Dwarfs in Common Proper Motion with Tycho Stars

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