Rocky Extrasolar Planetary Compositions Derived From Externally Polluted White Dwarfs

Abstract

We report Keck High Resolution Echelle Spectrometer data and model atmosphere analysis of two helium-dominated white dwarfs, PG1225-079 and HS2253+8023, whose heavy pollutions most likely derive from the accretion of terrestrial-type planet(esimal)s. For each system, the minimum accreted mass is ∼10 22 g, that of a large asteroid. In PG1225-079, Mg, Cr, Mn, Fe and Ni have abundance ratios similar to bulk Earth values, while we measure four refractory elements, Ca, Sc, Ti and V, all at a factor of ∼2-3 higher abundance than in the bulk Earth. For HS2253+8023 the swallowed material was compositionally similar to bulk Earth in being more than 85% by mass in the major element species, O, Mg, Si, and Fe, and with abundances in the distinctive proportions of mineral oxides -compelling evidence for an origin in a rocky parent body. Including previous studies we now know of four heavily polluted white dwarfs where the measured oxygen and hydrogen are consistent with the view that the parents bodies formed with little ice, interior to any snow-line in their nebular environments. The growing handful of polluted white dwarf systems with comprehensive abundance measurements form a baseline for characterizing rocky exoplanet compositions that can be compared with bulk Earth.Subject headings: Planets and satellites: composition -Stars: individual: (HS2253+8023, PG1225-079, G 241-6) -Stars: abundances -white dwarfs required mass influx to explain observed polluted white dwarf (WD) accretion rates. Objects that journey within the Roche radius of the WD will experience tidal shredding, forming a disk of dust and/or gas (Jura 2003(Jura , 2008, and subsequently accrete into the star's, otherwise pure, hydrogen and/or helium atmosphere. This natural mechanism offers the potential to measure the distilled elemental constituents of a planetary parent body or bodies -a unique and powerful tool, indeed.There are a number of benefits of working with helium-dominated WDs. One is that, compared to hydrogendominated WD atmospheres, the lower opacity helium environments more readily display tiny concentrations of numerous contaminant elements, e.g. the trace elements vanadium and scandium are detected in PG1225 at 10 -11 orders of magnitude lower abundance than the helium background they are diluted in. Secondly, detailed model atmosphere analysis of helium-atmosphere WDs indicates the hydrogen in these stars must be accreted (Voss et al. 2007;Dufour et al. 2007); thus when helium is the background constituent, one obtains a measurement of the polluting hydrogen abundance, which is important for understanding the potential accretion of water ice Jura & Xu 2010). A simplifying aspect of the helium-atmosphere WDs is that in our T ef f range of interest (< 20,000 K), all have outer convection zones where the elements are homogeneously mixed. Finally, another benefit of working with helium-rich WDs is that with relatively large convective zones (relatively long settling times), the pollutants are not disappearing so fast from our view. Thus, a ...

Detailed Compositional Analysis of the Heavily Polluted DBZ White Dwarf SDSS J073842.56+183509.06: A Window on Planet Formation?

“…We note, finally, that our observations for J0738+1835 show no sign of emission cores in the CaII H&K lines, as observed for PG 1225−079 (Klein et al 2011). Klein et al (2011) proposed that NLTE effects could possibly cause such emission since these line cores are formed high up in the atmosphere.…”

“…Klein et al (2011) proposed that NLTE effects could possibly cause such emission since these line cores are formed high up in the atmosphere. However, comparing the thermodynamic structures of PG 1225−079, calculated with Klein et al (2011)'s parameters, with that of J0738+1835 argues against this interpretation (not to mention that NLTE effect are not expected in such a low effective temperature white dwarf). Indeed, we find that the atmospheric pressure in J0738+1835 is lower than that of PG 1225−079, while its gas temperature is higher at every depth point in the atmosphere.…”

Remnant planetary systems around bright white dwarfs

“…This accretion results in a spectroscopically detectable pollution of the otherwise pristine WD atmosphere. Photospheric abundance analyses of polluted WDs show that the accreted metals originate from tidally disrupted minor bodies similar in composition to that of bulk Earth (Zuckerman et al 2007;Dufour et al 2010;Klein et al 2011;Xu et al 2014). Depleted levels of hydrogen and carbon rule out accretion of the interstellar medium (ISM) as a source of photospheric pollution (Jura 2006).…”

Rates of E1, E2, M1, and M2 transitions in Ni II

“…Low-ionisation stages of these species are highly abundant in the astronomical environment, being commonly observed in a multitude of astrophysical sources including white dwarfs (Klein et al 2011), evolved stars (Richardson et al 2011), γ-ray bursts (Cucchiara et al 2011;Vreeswijk et al 2007), luminous blue variables (LBVs) (Davidson et al 2001;Hartman et al 2004), Seyfert galaxies (Véron-Cetty et al 2006), quasi-stellar objects (Fynbo et al 2010), and damped Lyα systems (DLAs; Dessauges-Zavadsky et al 2006). There is therefore an on-going demand for accurate transition data among levels of Ni II.…”