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Discovery of an OVI Emitting Nebula around the Hot White Dwarf KPD 0005+5106

A survey of diffuse interstellar sight lines observed with the Far Ultraviolet Spectroscopic Explorer has led to the serendipitous discovery of a high-ionization nebula around the hot white dwarf KPD 0005+5106. The nebula has an OVI 1032A surface brightness of up to 25,000 photons/s/cm^2/sr, making it the brightest region of extended OVI emission in our survey. Photoionization models using the incident white dwarf continuum successfully reproduce the observed OVI intensity. The OVI emission arises in the highly ionized inner region of a planetary nebula around KPD 0005+5106. This newly discovered nebula may be one member of a class of high-ionization planetary nebulae that are difficult to detect in the optical, but which can be easily identified in the ultraviolet.


INTRODUCTION
Planetary nebulae (PNe) come in a variety of shapes, irregular, bipolar, or spherical, and exhibit emission from forbidden lines such as [O III], [Ne III], and [Ne V] as well as from the Hα recombination line. In addition, numerous high-excitation PNe exhibit most of the other Balmer lines as well as He II, C III], and other forbidden lines such as [Ne IV], [Ar IV], [Mg V]. The typical radial extent is a few tenths of a parsec. Only a few PNe have radii exceeding 1.5 pc. Because of their small size and low surface brightness, PNe are notoriously difficult to discover (e.g., Kwitter et al. 1989;Werner et al. 1997;Rauch 1999).
We report the discovery of an O VI emitting nebula around KPD 0005+5106 with the Far Ultraviolet Spectroscopic Explorer (FUSE) that we identify as a high-ionization PN (HIPN) around this white dwarf (WD). This would make KPD 0005+5106 only the third WD (out of 162 WDs listed by Napiwotzki 1999) known to have a PN. Including KPD 0005+5106, only eight of the fifteen stars listed with T eff ≥ 120, 000 K (Napiwotzki 1999) have a known PN.

OBSERVATIONS
The FUSE instrument consists of four optical systems. Two employ LiF coatings and are sensitive to the wavelength range 990 − 1187Å. The other two use SiC coatings and are sensitive to wavelengths between 905 and 1100Å. Each spectrograph possesses three apertures that simultaneously observe different positions on the sky separated by a few arcminutes (for a complete description of FUSE, see Moos et al. 2000;Sahnow et al. 2000).
Because of the strong interstellar H 2 features in its spectrum, the WD KPD 0005+5106 was selected as a wavelength-calibration target and has been observed repeatedly throughout the mission. When the FUSE high-or medium-resolution aperture is centered on the star, the low-resolution (LWRS) aperture samples a 30 ′′ × 30 ′′ region 1.8 or 3.5 arcmin (respectively) away. Changes in the spacecraft roll angle from one observation to another place the LWRS aperture at different position angles around the star. Figure 1 shows the observed LWRS positions. In three additional background observations, the LWRS aperture was 2.2 arcmin from the star. Table 1 lists the observational data for each pointing: observation ID, equatorial coordinates of the LWRS aperture, date of the observation, and the polar coordinates for the LWRS aperture relative to KPD 0005+5106. Angle ϕ is measured counterclockwise from the north celestial pole. A map key identifies the LWRS positions in Figure 1. All observations were performed in time-tag mode, in which the spectrograph records the position and arrival time of each photon.

DATA REDUCTION
The individual exposures of each pointing were aligned using the measured positions of the O I airglow lines between 1027 and 1042Å. The data were screened for pulse heights in the range 2 − 25, shifted to a heliocentric wavelength scale, and combined, and the region around the O VI λλ 1032,1038 doublet was extracted. The spectra were binned by 16 pixels to increase the signal-to-noise ratio. A more detailed description of the data reduction can be found in our O VI emission survey paper (Otte, Dixon, & Sankrit 2004, in preparation).
In most cases, we used night-only data to avoid possible contamination by an unidentified airglow feature near the O VI λ1032 line. For M1070208, M1070223, and M1070224, we combined day and night data to obtain a more reliable measurement of the faint O VI emission; contamination by the airglow feature could be excluded in these cases (by comparing the day-plus-night and night-only data).
Due to their nearly identical positions, the data of observations M1070211 and M1070214 were combined into one spectrum, as were the data of observations M1070212 and M1070215. Although the overlap of their positions is less than for the other two combined observations, we combined M1070209 and M1070221 to gain signal-to-noise at the expense of spatial resolution.

RESULTS
All spectra were fit using the IRAF 1 routine SPECFIT (Kriss 1994) with emission-line models consisting of a convolution of a Gaussian and a 106 km s −1 wide flat-top profile representing the LWRS aperture. The spectrum with the brightest O VI λ1032 feature (S4054403, map key 3) is shown in Figure 2.
The intensities derived from our fits to the O VI emission lines are listed in Table 2. Due to its low signal-to-noise ratio, possible blending with the C II * λ1037 line, and vicinity to the O I λ1039 airglow line, the O VI λ1038 emission line was difficult to fit in many cases. To date, the intensities of diffuse O VI λ1032 emission detected with FUSE range from 1700 to 11,000 LU, with an average value of 4700 ± 2400 LU (Otte, Dixon, & Sankrit 2004, in preparation) 2 . With a peak intensity of 25,000 LU, the high-ionization nebula around KPD 0005+5106 is the brightest region of extended O VI emission yet found.

DISCUSSION
With a surface temperature T eff = 120, 000 K, KPD 0005+5106 is the hottest known DO (i.e. helium-rich) WD. The stellar spectrum has been observed at X-ray, ultraviolet, and optical wavelengths. Model fits yield stellar parameters of log(g) = 7, E(B − V ) = 0.13, and a distance of about 270 pc (Werner, Heber, & Fleming 1994;Werner et al. 1996). The star possesses a soft X-ray corona (O'Dwyer et al. 2003), and appears to undergo mass ejection (Werner et al. 1996;Sion et al. 1997).
In the log(g) − log(T eff ) diagram, KPD 0005+5106 lies between the highest-gravity PG 1159 stars known to have PNe and the two DO WD PN candidates PG 0108+101 and PG 0109+111. The discovery of a PN around the DO WD PG 1034+001 (Hewett et al. 2003) extends the range of known PNe in the log(g) − log(T eff ) plane along the evolutionary tracks toward lower temperatures and higher surface gravities (log(g) = 7.5, T eff = 100, 000 K; Werner, Dreizler, & Wolff 1995). KPD 0005+5106 lies well within this extended parameter space. However, previous searches for an optical PN around this WD have been unsuccessful (Werner et al. 1997).
Photoionized O VI is expected from PN with hot central stars (T eff ≥ 120, 000 K) and has been detected in a few such objects (Chu, Gruendl, & Guerrero 2004). We used the photoionization code CLOUDY to determine whether photoionization of a putative PN around KPD0005+5106 could explain the detected O VI emission. For the ionizing source, we used a synthetic spectrum of the star calculated with the program TLUSTY (Hubeny & Lanz 1995), using the observed stellar abundances and parameters (Werner et al. 1996). The stellar model was scaled to the flux observed with FUSE. Simple PN expansion models by Osterbrock (1989) yield densities of a few cm −3 at the distances of the LWRS aperture positions for a ∼ 10, 000 yr old PN. We assumed solar abundances and a particle density of unity (i.e., n(H) = 1 cm −3 ) in the absorbing cloud. With no other free parameters, our model reproduces the observed O VI flux to within a factor of a few. Based on this agreement between model predictions and observations, we conclude that the O VI nebula around KPD 0005+5106 is photoionized. It may be one of a new class of HIPNe that are faint in standard optical lines.

The KPD 0005+5106 System
Our photoionization model predicts a [Ne V] λ3426 intensity seven times as bright as the O VI emission, whereas shock models yield [Ne V] intensities 10 − 100 times fainter than the O VI emission. This discriminant has been used to argue that the nebula LMC 62 in the Large Magellanic Cloud is photoionized (Herald & Bianchi 2004). Assuming E(B − V ) = 0.13, R V = 3.2, and extrapolating the extinction parameterization of Cardelli, Clayton, & Mathis (1989) to the Lyman limit, we find that the [Ne V] emission should be about 20 times brighter than the observed O VI λ1032 emission around this WD. The photoionization model predicts that the [Ne V] emission arises in a more extended zone than the O VI emission. Though the geometry of the emitting gas is unknown, [Ne V] imaging would allow us to confirm the existence of HIPNe around this and other hot WDs.
Another prediction of a radiation-bounded HIPN is the presence of a low surface brightness Hα and [O III] λ5007 nebula with a large angular diameter as has been observed around PG 1034+001 (Hewett et al. 2003), because lower ionization states are only possible at larger distances from the hot star. The PN search around KPD 0005+5106 by Werner et al. (1997) likely had too small a field of view to detect such an extended PN.

PN Populations and Evolution
As the central star of a PN contracts and its temperature rises, its radiation field becomes harder. Meanwhile, the expansion of the nebula causes its density to fall. The combination of these effects raises the ionization parameter (the ratio of photon density to particle density) at the inner face of the nebula, and with it the ionization state of the gas. This scenario has two implications: first, that every PN with a central star massive enough to reach a temperature greater than ∼ 120, 000 K will eventually pass through a high-ionization phase; second, that every hot white dwarf should exhibit a HIPN.
Observations of HIPNe around hot WDs may reveal important information about WD precursors and their evolution. One unresolved problem, for example, is the existence of helium-rich, hydrogen-deficient WDs that account for about 20% of all known degenerate stars (Sion et al. 1997). With the precursors' abundances imprinted in the diffuse gas, HIPNe could give clues to the evolutionary process that results in this bizarre type of WDs.
The photoionization model with unit hydrogen density and solar abundances, [O/H]= 7.4×10 −4 , predicts that the thickness of the O VI emitting zone is approximately 5×10 18 cm.
In this region, the fractional abundance O 5+ /O is about 0.11, which translates to a number density N O 5+ ∼ 8 × 10 −5 cm −3 . The O VI column density is thus about 4 × 10 14 cm −2 . Depending on the distribution of hot WDs in the Galactic disk, HIPNe could contribute a significant fraction to the total O VI column density in the plane. a 1 LU = 1 photon s −1 cm −2 sr −1 ; for O VI λ1032, 1 LU = 4.5 × 10 −22 erg s −1 cm −2 arcs −2 .
b Value is a 2 σ upper limit.