A 3 Gyr White Dwarf with Warm Dust Discovered via the Backyard Worlds: Planet 9 Citizen Science Project

TL;DR

This paper reports the discovery of a 3-billion-year-old white dwarf with a warm dust disk, challenging existing models of planetary system evolution and tidal disruption timelines around white dwarfs.

Contribution

It presents the first detection of a white dwarf with a dusty disk at nearly 3 Gyr post-formation, extending the known timescale for tidal disruption events.

Findings

White dwarf LSPM J0207+3331 has a luminous dust disk with 14% IR luminosity.

The dust disk cannot be explained by a simple thin disk model, suggesting a second dust ring.

Infrared excesses can be confirmed by co-moving IR sources, reducing confusion noise.

Abstract

Infrared excesses due to dusty disks have been observed orbiting white dwarfs with effective temperatures between 7200 K and 25000 K, suggesting that the rate of tidal disruption of minor bodies massive enough to create a coherent disk declines sharply beyond 1~Gyr after white dwarf formation. We report the discovery that the candidate white dwarf LSPM J0207+3331, via the Backyard Worlds: Planet 9 citizen science project and Keck Observatory follow-up spectroscopy, is hydrogen-dominated with a luminous compact disk (L$_{\rm IR}$/L$_{\star}$=14%) and an effective temperature nearly 1000K cooler than any known white dwarf with an infrared excess. The discovery of this object places the latest time for large scale tidal disruption events to occur at $\sim$3 Gyr past the formation of the host white dwarf, making new demands of dynamical models for planetesimal perturbation and disruption around post main sequence planetary systems. Curiously, the mid-IR photometry of the disk cannot be fully explained by a geometrically thin, optically thick dust disk as seen for other dusty white dwarfs, but requires a second ring of dust near the white dwarf's Roche radius. In the process of confirming this discovery, we found that careful measurements of WISE source positions can reveal when infrared excesses for white dwarfs are co-moving with their hosts, helping distinguish them from confusion noise.

Tables

Table 1. Table 1: Photometry and Astrometry of LSPM J0207+3331

Band	Mag
GALEX NUV	21.92$\pm$0.62
$g$	17.86$\pm$0.02
$r$	17.49$\pm$0.02
$i$	17.34$\pm$0.02
$z$	17.34$\pm$0.02
$J_{2MASS}$	16.6$\pm$0.1
$H_{2MASS}$	16.3$\pm$0.2
$K_{s,2MASS}$	15.9$\pm$0.3
$W1$	15.08$\pm$0.03
$W2$	14.20$\pm$0.03
$W3$	12.2$\pm$0.3

Equations

$$gaia.parallax>5$$

$$\mathrm{AND}parallax/parallax\_error>20$$

$$\mathrm{AND}{\mathrm{G}}_{\mathrm{abs}}>5(\mathrm{G}-{\mathrm{G}}_{\mathrm{RP}})+10$$

$$\mathrm{AND}{\mathrm{G}}_{\mathrm{abs}}<16.3$$

$$\mathrm{AND}{\mathrm{G}}_{\mathrm{abs}}>9.5$$

$$\mathrm{AND}\mathrm{G}-{\mathrm{G}}_{\mathrm{RP}}>0.33$$

$$\mathrm{AND}\mathrm{G}-{\mathrm{G}}_{\mathrm{RP}}<0.62$$

$$\mathrm{AND}W1<16$$

Full text

white dwarfs — planetary systems — circumstellar matter — stars:individual(LSPM J0207+3331)

1 Introduction

White dwarfs with infrared excesses hold critical clues to the long term evolution and chemistry of minor bodies in orbit around stars outside the Solar System. About 1-4% of white dwarfs have detectable infrared excesses due to geometrically flat and optically thick dust disks (von Hippel et al., 2007; Debes et al., 2011; Barber et al., 2014; Bonsor et al., 2017). These disks are believed to be caused by the tidal disruption of rocky bodies that then accrete onto the surface of the host white dwarf (Debes, & Sigurdsson, 2002; Jura, 2003). Since white dwarfs are sensitive probes to the accretion of material, the atomic abundance of the dust is revealed through spectroscopy of the white dwarf photosphere (Zuckerman et al., 2007). Nearly 25-50% of white dwarfs show dust accretion, implying that most white dwarfs have some flux of rocky bodies entering within their tidal disruption radius over their cooling lifetime (Koester et al., 2014; Hollands et al., 2018). While very old white dwarfs show photospheric evidence of dust accretion, dusty infrared excesses have previously only been seen around white dwarfs with T${}_{\rm eff}>$7000 K or a cooling age of $\sim$1 Gyr (Farihi et al., 2008; Barber et al., 2014).

We report the discovery that the high proper motion object , LSPM J0207+3331, has a hydrogen dominated atmosphere with an infrared excess. We show through the construction of a multi-wavelength spectral energy distribution (SED) that it possesses a Teff=6120 K and hosts a strong infrared excess with LIR/L${}_{\star}\sim$0.13, consistent with a significant amount of dust filling the region interior to the Roche disruption radius of the white dwarf.

The independent identification that this object was a white dwarf and the discovery of the infrared excess were made by citizen scientists of the Backyard Worlds: Planet 9 citizen science project. This project is primarily designed to discover high proper motion WISE sources, with a focus on finding nearby brown dwarfs and new planets in the outer solar system (Kuchner et al., 2017). The project’s volunteers identify high proper motion sources by viewing flipbooks of four different epochs of unsmoothed WISE images (Meisner et al., 2018) at a Zooniverse website (www.backyardworlds.org). They have also developed their own online tools for interrogating the WISE data (Caselden et al., 2018).

In §2 we discuss the discovery and confirmation of LSPM J0207+3331 as a nearby white dwarf with an infrared excess. In §3 we derive the parameters for the host star and its dusty disk, and finally in §4 we discuss the significance of this discovery in the context of previously known dusty white dwarfs.

2 Discovery and Confirmation of an Infrared Excess Around LSPM J0207+3331

LSPM J0207+3331 was first flagged as an interesting candidate white dwarf by the Backyard Worlds: Planet 9 citizen scientist Melina Thévenot, who alerted the science team of its coordinates and its unusually red AllWISE $W1$-$W2$ color for a white dwarf: W1-W2=0.92$\pm$0.05, indicative of a significant IR excess consistent with a dusty disk (Hoard et al., 2013). Its AllWISE photometry is free from artifacts or obvious contaminants, with no significant variability in the Level 1b epoch data for either the WISE or NEOWISE photometry (Wright et al., 2010). Both photometry taken in 2010/2011 during WISE’s main mission and over the 3.5 years of NEOWISE epochs were consistent within the uncertainties, and thus we average the $W1$/$W2$ magnitudes between the missions (Mainzer et al., 2011, 2014). We also obtained GALEX NUV photometry (Bianchi et al., 2014), $g,r,i,z$ photometry from the Pan-Starrs PS1 catalog (Chambers et al., 2016), and NIR photometry from 2MASS (Cutri et al., 2003). A table of all photometry obtained is listed in Table 1.

In additon to photometry, we obtained medium-resolution near-infrared spectra of LSPMJ0207+3331 using the Near-Infrared Echellette Spectrometer (NIRES) on the Keck II telescope (Wilson et al., 2004). The data were obtained on October 27th 2018 in partly cloudy conditions. NIRES has a fixed configuration, and covers a wavelength range of 0.94-2.45 $\mu$m at a resolution of $\sim$ 2700, using a slit width of 0$\farcs$55. We obtained 12 exposures of 300s each, nodding 5″ along the slit between exposures for sky subtraction (3 complete ABBA sequences), at a mean airmass of 1.14 and with the slit aligned at the parallactic angle. We also obtained spectra for HD13869, an A0V star with $V_{\mathrm{mag}}$ = 5.249, at a similar airmass for telluric correction and flux calibration. Dark and flat field frames were obtained at the end of the night. The data were reduced using a modified version of Spextool (Cushing et al., 2004), following the standard procedure. Wavelength calibration was determined using telluric lines, with a resulting RMS scatter of 0.073 Å. The spectra for each A-B pair were extracted individually and then combined together with the other extracted pairs. The telluric correction procedure was carried out as detailed in Vacca et al. (2003).

The final SED of the system is shown in Figure 1 compared to a best-fit model of the white dwarf as discussed in §3. As can be seen, a clear excess occurs just longwards of the $H$ band, confirmed both by the NIRES spectrum and the lower signal-to-noise ratio 2MASS photometry. The excess increases beyond 5$\micron$ with a low SNR detection of LSPM J0207+3331 in $W3$, implying significant amounts of colder dust in the system. The inferred flux density at 12 $\micron$ is comparable to the peak flux density of the white dwarf.

The Gaia photometry and parallax of the system also implies that LSPM J0207+3331 is a cool white dwarf (Gaia Collaboration et al., 2018). The parallax, 22.44$\pm$0.20 mas, implies a distance of 44.6$\pm$0.4 pc assuming a linear propagation of the parallax uncertainty. Based on Gaia photometry+parallax alone, LSPM J0207+3331 is consistent with a mass of 0.62 $M_{\odot}$ and a cooling age of $\sim$3 Gyr (Gentile Fusillo et al., 2019).

Following the initial recognition of its large proper motion in Backyard Worlds, we determined its WISE proper motion for WISE-only detections of LSPM J0207+3331 via a Monte Carlo fitting (Theissen et al., 2017) of the individual epoch positions, resulting in $\mu_{\alpha}cos(\delta)$=180$\pm$12 mas and $\mu_{\delta}$=-55$\pm$11 mas (See Figure 2). This is consistent within the uncertainties to the reported proper motion ($\mu_{\alpha}cos(\delta)$=170.0$\pm$0.3 mas and $\mu_{\delta}$=-25.4$\pm$0.3 mas) of the object in the Gaia DR2 catalog (Gaia Collaboration et al., 2018) and the LSPM catalog (Lépine, & Shara, 2005).

The combination of a common proper motion to the WISE mid-IR source and the presence of IR excess in the NIRES spectrum (with maximum angular extent of 1$\farcs$5), confirms that the excess is physically associated with LSPM J0207+3331 and located $<$67 AU from the star.

3 Determining the Origin of the Infrared Excess

Before modeling the infrared excess, we used existing observations to constrain the atmospheric composition, effective temperature (Teff) and gravity of LSPM J0207+3331. Figure 3 shows a weak detection of the hydrogen Paschen $\beta$ line at 1.28$\micron$ , making LSPM J0207+3331 a DA white dwarf. We then fit the NUV, visible, and $J$ photometry to DA white dwarf cooling models111http://www.astro.umontreal.ca/b̃ergeron/CoolingModels/ to infer the white dwarf properties (Holberg, & Bergeron, 2006; Kowalski, & Saumon, 2006; Tremblay et al., 2011; Bergeron et al., 2011). To obtain a best fit, we minimized a $\chi^{2}$ metric over our using the reported Gaia parallax. We estimated statistical uncertainties within a 95% confidence interval. The best-fit Teff=6120${}^{+48}_{-57}$ K, while the best-fit gravity is 8.16$\pm 0.03$. Based on the cooling models we used, this implies M=0.69${}^{+0.01}_{-0.02}$ M⊙ and a cooling age of 3$\pm$0.2 Gyr. Based on the best fit temperature, mass, and gravity, we infer a radius of 0.011 R⊙, and a bolometric luminosity of 1.6$\times$10*-4* L⊙.

We next attempted to fit the observed infrared excess with a face-on optically thick disk that fills the available orbital space as is traditional for most white dwarf disk modeling (Jura, 2003; Rocchetto et al., 2015; Dennihy et al., 2017). For LSPM J0207+3331, this implies a maximum LIR/L⋆ of $\sim$0.13, assuming an inner disk temperature of 1800 K and an outer radius consistent with a Roche disruption radius of 0.69 R⊙. This model failed to fit the observed data, overproducing flux between the NIR and $W1$ and underpredicting the observed flux at $W2$ and $W3$. Adjusting either the inner radius of the disk or the inclination forced a decent fit to the short wavelength data, but still failed to explain the fairly bright observed mid-IR flux beyond 4$\micron$. Similarly, increasing the outer radius of the disk to be consistent with a larger Roche radius as would be assumed for lower density material such as ice or porous rubble, did not solve the poor fit to the data.

As an alternative approach , we posit a hybrid combination of an optically thick face-on disk and a single temperature blackbody as a plausible fit to the observed photometry. For the interior optically thick disk, we assume an inner temperature of 1400 K and outer temperature of 550 K, corresponding to an inner and outer radius of 0.047 R⊙ and 0.21 R⊙ respectively. We next assume a second component corresponding to a cooler optically thin dust distribution with a bulk temperature of 480 K. To fit the observed dust SED requires an effective surface area of 1021 cm2. The two components thus have a combined LIR/L*⋆*=0.14. The proposed model is compared to the observations in Figure 4, assuming that the NIRES spectra have a systematic 5% uncertainty on the relative flux calibration. This model simultaneously fits the short wavelength emission as well as the longer wavelength emission. While background contamination in the mid-IR is possible, the common proper motion of the WISE $W1$ and $W2$ sources with LSPM J0207+3331 suggests that contamination is unlikely. Mid-IR spectroscopy of this object with the James Webb Space Telescope should better constrain the structure and composition of the disk as well as test whether there is a need for multiple components to the disk .

Another possible explanation for the large $W2$ and $W3$ fluxes that doesn’t require cold dust remains. For highly magnetic white dwarfs, the accretion of material onto the white dwarf surface also results in infrared emission from cyclotron radiation (Debes et al., 2012; Parsons et al., 2013).

Assuming blackbody absorbing grains, we calculate that an optically thin distribution of dust will have an equilibrium temperature of 480 K at $\sim 0.94~{}R_{\odot}$, conveniently near the Roche radius of the white dwarf assuming a bulk density of water ice, or just outside it if the dust is composed of refractory material with higher density.

4 Discussion

The discovery of LSPM J0207+3331 as a cool white dwarf with a signficant infrared excess makes new demands of models that seek to explain dust around white dwarfs. The first demand is that this discovery pushes the latest observable infrared excess to $\sim$3 Gyr after white dwarf formation, a factor of three larger than previous discoveries indicated. Current models for delivering planetesimals inside the tidal disruption radius of a white dwarf struggle to deliver the requisite mass of material to create an observable infrared excess beyond the first Gyr of a white dwarf’s lifetime, especially if the white dwarf is not part of a binary. (Bonsor, & Wyatt, 2010; Debes et al., 2012; Frewen, & Hansen, 2014; Veras et al., 2014a, b, 2015, 2017; Mustill et al., 2018; Smallwood et al., 2018).

Second, dynamical models must now reproduce the observed frequency of detectable disks as a function of cooling age out to 3 Gyr. We have queried the Gaia DR2 catalog along with AllWISE cross-matches for white dwarfs with similar $G-G_{RP}$ colors and distances $<$200 pc (implying a range of Teff between 5000 K and 7000 K) with $W1$ and $W2$ detections to estimate the possible frequency of disks similar to LSPM J0207+3331. The full ADQL constraints we used were:

[TABLE]

We note that we do not apply many of the quality filters suggested to select white dwarfs by Gentile Fusillo et al. (2019), instead relying on high significance parallaxes $\pi/\sigma\pi>20$. Out of 346 candidate white dwarfs, only five had $W1-W2>0.3$ including LSPM J0207+3331. The candidates are: LSPM J1345+0504 (2 faint galaxies within 5″), 2MASS J21403597+7739195 (WD+M), 2MASS J18333593+5812176 (WD+M), and WISEA J032245.51+390445.0 (WD+M). This census implies a dust disk frequency of $\sim$0.2%, nearly an order of magnitude smaller than the frequency of dust disks around younger WDs.

Third, the infrared excess seen for this disk requires a second, colder ring of dusty material that could potentially signal the presence of a gap in the system, or a component of dust that extends beyond the outer edge of the inner disk. If the second ring is confirmed, it would be the first example of a two-component ring system around a dusty white dwarf. If the dust disk has a gap near 0.94 R⊙, this implies the possibility of a body that continuously clears dust from the system, since the PR drag timescale is so short.

Finally, the presence of an optically thick disk at late white dwarf cooling times affords an interesting test of dust disk accretion. The accretion rate from optically thick disks is likely mediated by PR drag and is thus dependent on the luminosity and temperature of the white dwarf (Rafikov, 2011). Models of disk accretion predict a slowly decreasing minimum accretion rate for a detectable disk, and older white dwarfs appear less likely to accrete at a high enough rate to yield an observable disk (Hollands et al., 2018). Based on the predictions of this model, LSPM J0207+3331 should have an accretion rate larger than $\sim$3$\times$107 g s*-1*, a prediction that can be tested by taking an optical spectrum of this white dwarf to look for metal lines from accreted dust.

We would like to thank the many Zooniverse volunteers who have participated in the Backyard Worlds: Planet 9 project. We would also like to thank the Zooniverse web development team for their work creating and maintaining the Zooniverse platform and the Project Builder tools. Additionally we thank the anonymous referee for their suggestions in improving the manuscript. This research was supported by NASA ADAP grant NNH17AE75I. Some of the data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. This research has made use of the VizieR catalogue access tool, CDS, Strasbourg, France. The original description of the VizieR service was published in A&AS 143, 23. This research has also made use of ”Aladin sky atlas” developed at CDS, Strasbourg Observatory, France. Keck:2 (NIRES) WISE