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Massive White Dwarfs in Young Star Clusters Draft version January 22, 2021 Typeset using LATEX default style in AASTeX63 Massive White Dwarfs in Young Star Clusters Harvey B. Richer ,1 Ilaria Caiazzo ,2 Helen Du,1 Steffani Grondin,1 James Hegarty,1 Jeremy Heyl ,1 Ronan Kerr,3 David R. Miller1 And Sarah Thiele1 | 1University of British Columbia Vancouver, BC, Canada 2California Institute of Technology Pasadena, Ca, USA 3University of Texas at Austin Austin, Texas, USA (Received December 4, 2020; Revised January 6, 2021; Accepted January 15, 2021) Submitted to The Astrophysical Journal ABSTRACT We have carried out a search for massive white dwarfs (WDs) in the direction of young open star clusters using the Gaia DR2 database. The aim of this survey was to provide robust data for new and previously known high-mass WDs regarding cluster membership, to highlight WDs previously included in the Initial Final Mass Relation (IFMR) that are unlikely members of their respective clusters according to Gaia astrometry and to select an unequivocal WD sample that could then be compared with the host clusters' turnoff masses. All promising WD candidates in each cluster CMD were followed up with spectroscopy from Gemini in order to determine whether they were indeed WDs and derive their masses, temperatures and ages. In order to be considered cluster members, white dwarfs were required to have proper motions and parallaxes within 2, 3, or 4-σ of that of their potential parent cluster based on how contaminated the field was in their region of the sky, have a cooling age that was less than the cluster age and a mass that was broadly consistent with the IFMR. A number of WDs included in current versions of the IFMR turned out to be non-members and a number of apparent members, based on Gaia's astrometric data alone, were rejected as their mass and/or cooling times were incompatible with cluster membership. In this way, we developed a highly selected IFMR sample for high mass WDs that, surprisingly, contained no precursor masses significantly in excess of ∼6 M . Keywords: stars: clusters { massive { supernovae { white dwarfs { Galaxy: open clusters 1. INTRODUCTION arXiv:2101.08300v1 [astro-ph.SR] 20 Jan 2021 The maximum mass of a star that is capable of forming a WD is an important astrophysical quantity. It controls the SN II rate, the rate of formation of neutron stars and black holes and the chemical enrichment and star formation rate of galaxies. If the maximum mass is in fact lower than previously thought, the number of supernova explosions would be higher, as would the number of neutron stars and black holes formed. Additionally, the amount of heavy elements pumped back into the interstellar medium would increase along with the dust production and likely the star formation rate in a galaxy. A related quantity is the maximum possible mass of a WD, whose upper limit from theoretical considerations is thought to be 1.38 M (Nomoto 1987). However, presently, there are no WDs known with Corresponding author: Harvey B. Richer [email protected] 2 Richer et al. Gaia parallaxes for which we can say with a high degree of confidence that they have evolved via single star evolution that approach this limit. Single star evolution implies that in any phase of its life the star was not involved in mass transfer from another star or in a merger. The current record-holder for the most massive WD thought to be the product of single-star evolution is GD 50, at 1.28 M . Historically associated with the Pleiades, recent Gaia analysis seems to indicate that GD 50 is not associated with any star cluster but appears to be a member of the 150 Myr old AB Doradus moving group (Gagn´eet al. 2018). If GD 50 is excluded from the formal IFMR because of its uncertain origin (which makes it difficult to establish its precursor's mass) and the possibility that it may indeed be a merger remnant (Vennes et al. 1996), the most massive single-evolution WD in a star cluster with a well-determined Gaia parallax currently known becomes the Pleiades WD LB1497 at 1.05 M , whose precursor was 5.86 M (Cummings et al. 2018). This WD is well below the Chandrasekhar mass, and therefore the current sample does not come close to testing this upper limit. It is the goal of our current survey to try and locate more massive single-source WDs. On a related issue, there appears to be some discrepancy in the rate of supernovae explosions from massive stars. The current upper main sequence mass limit for WD production is thought to be about 8 M (Weidemann & Koester 1983); however, Horiuchi et al.(2011) have shown that, if every star above 8 M becomes a core-collapse SN II, the predicted rate for type II supernovae would be roughly double the observed rate. One way to resolve this problem would be if the lower mass limit for supernova production were significantly higher: about a 12 M lower limit to stars exploding as SN II would be needed for a Kroupa IMF (Kroupa & Weidner 2003) to bring the prediction in line with observations. We would then expect to see WDs that evolved from main sequence stars as massive as 12 M . At present, no WDs are known with progenitors anywhere near as massive as this, but it is not at all clear whether searches carried out thus far would have been sensitive to these massive WDs, as their high masses would have resulted in small WD radii and hence low luminosities. Additionally, this would have required examining star clusters as young as 20 Myrs for possible WD members, a search that has not been done systematically. In this survey paper, we remain agnostic about the mass limit for supernova production and we search for WDs in the Gaia DR2 database within open clusters whose turnoff masses are as high as 15 M . 2. THE WHITE DWARF SURVEY The Gaia DR2+ catalogue provides a 5-parameter astrometric solution (α, δ, µα, µδ, π) for more than 1.3 billion sources. The catalogue is only complete for G magnitudes between ∼12 and 17 with a general limiting G magnitude of approximately 21 (Gaia Collaboration et al. 2018). However, this value can be as bright as G = 18 in dense areas of the sky due to blending from other sources. Considering these restrictions, we primarily surveyed clusters from 10 Myr (turnoff mass ∼19.3 M ) up to 500 Myr (turnoff mass 2.9 M ) within 1 kpc of the Sun. A 1 M hydrogen atmosphere WD that has been cooling for ∼250 Myrs has an absolute G magnitude of ∼11.8 (B´edardet al. 2020), corresponding to an apparent G magnitude of 20.3 at 500 pc. Since young clusters are near the Galactic plane, at this distance we can generally expect about 0.7 mag of extinction, placing the WD close to the Gaia limit of detection. The current paradigm is that WDs form when a star with mass less than ∼8 M expels its outer layers after the red giant phase, leaving only a core remnant behind. Without fusion occurring, the degenerate core slowly radiates its thermal energy, causing the WD to become increasingly faint and cool. The limitations in the completeness of the Gaia catalogue, coupled with the typical faintness of WDs, results in a bias toward young and nearby open clusters as hosts for massive WDs. Young clusters with higher turnoff masses are more likely to produce higher mass WDs which have not yet cooled beyond Gaia's photometric limit. Additionally, the closer the cluster is to the Sun, the brighter the sources will appear. Nearby clusters can be older and still have observable massive WDs while distant clusters must be very young for potential high-mass WDs to be observable. The complete list of the 386 surveyed clusters was compiled from WEBDA data. Of this sample, 262 clusters are in the 10-500 Myr age range and are located within 1 kpc of the Sun, 111 have distances of 1-1.5 kpc and ages <220 Myr (young + distant) and 8 are in the 500 Myr-2.5 Gyr age range with distances < 400 pc (old + close). The first group is the best suited to look for WDs that might have evolved from massive main sequence stars, but we extended our search to the latter two groups because they also might still contain massive WDs that are observable in Gaia. The distribution of the ages and distances of these clusters are shown in Figure1. Also included are the Hyades and NGC 2099 (M37) who do not fit our age restrictions but contain known massive WDs we wished to rediscover. Four extremely young clusters IC 5146, Lynga 14, NGC 6383, Ruprecht 119, with ages <10 Myr, were also added as we wanted to remain agnostic to the upper mass limit for WD formation and any WDs present in these clusters could be derived from high-mass progenitors at the upper limit of the IFMR. Massive White Dwarfs in Young Star Clusters 3 We queried the Gaia DR2 archive for each cluster using equatorial cluster centre coordinates as the target and 2× the angular diameter as the search radius, using data from the DAML02 database (Dias et al. 2002). In each cluster, we retrieved the top 500,000 stellar results. Choosing the search radius as twice the literature diameter is an arbitrary choice; we expect that it should allow for the majority of cluster members to be included, accounting for a possible underestimate in the literature diameter while not searching too widely and flooding the data with non-cluster stars.
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