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«Astrometry with Hubble Space Telescope: A Parallax of the Central Star of the Planetary Nebula NGC 68531 G. Fritz Benedict1, B. E. McArthur1, L.W. ...»

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Astrometry with Hubble Space Telescope: A Parallax of the

Central Star of the Planetary Nebula NGC 68531

G. Fritz Benedict1, B. E. McArthur1, L.W. Fredrick12, T. E. Harrison13, M. F. Skrutskie12, C. L.

Slesnick12, J. Rhee12, R. J. Patterson12, E. Nelan5, W. H. Jefferys6, W. van Altena7, T. Montemayor1,

P. J. Shelus1, O. G. Franz2, L. H. Wasserman2, P.D. Hemenway8, R. L. Duncombe9, D. Story10, A. L.

Whipple10, and A. J. Bradley11

Received ; accepted Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555 McDonald Observatory, University of Texas, Austin, TX 78712 Lowell Observatory, 1400 West Mars Hill Rd., Flagstaff, AZ 86001 Space Telescope Science Institute, 3700 San Martin Dr., Baltimore, MD 21218 Department of Astronomy, University of Texas, Austin, TX 78712 Department of Astronomy, Yale University, PO Box 208101, New Haven, CT 06520 Department of Oceanography, University of Rhode Island, Kingston, RI 02881 Department of Aerospace Engineering, University of Texas, Austin, TX 78712 Jackson and Tull, Aerospace Engineering Division 7375 Executive Place, Suite 200, Seabrook, Md. 20706 Spacecraft System Engineering Services, PO Box 91, Annapolis Junction, MD 20706 Department of Astronomy, University of Virginia, PO Box 3818, Charlottesville, VA 22903 Department of Astronomy, New Mexico State University, Las Cruces, New Mexico 88003 –2– ABSTRACT We present an absolute parallax and relative proper motion for the central star of the planetary nebula NGC 6853 (The Dumbell). We obtain these with astrometric data from FGS 3, a white-light interferometer on HST. Spectral classifications and VRIJHKT2 M and DDO51 photometry of the stars comprising the astrometric reference frame provide spectrophotometric estimates of their absolute parallaxes. Introducing these into our model as observations with error, we find πabs = 2.10 ± 0.48 mas for the DAO central star of NGC 6853. A weighted average with a previous ground-based USNO determination yields πabs = 2.40 ± 0.32. We assume that the extinction suffered by the reference stars nearest (in angular separation and distance) to the central star is the same as for the central star. Correcting for color differences, we find AV = 0.30 ± 0.06 for the central star, hence, an absolute magnitude MV = 5.48−0.16. A recent +0.15 determination of the central star effective temperature aided in estimating the central star radius, R∗ = 0.055 ± 0.02R, a star that may be descending to the white dwarf cooling track.

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Planetary nebulae are a visually spectacular and relatively short-lived step in the evolution from asymptotic giant branch (AGB) stars, to a final white dwarf stage. Iben & Renzini (1983) first showed that the ejection of most of the gaseous envelope in asymptotic giant branch (AGB) stars occurs at the tip of the thermal pulse phase, in the form of a massive, low-velocity wind. As summarized by Stanghellini et al. (2002), the remnant central star (CS) ionizes the gaseous ejecta, while a fast, low mass-loss rate CS wind shapes the PN. PN morphology depends on a complicated combination of phenomena, some occurring within the nebular gas, which evolves in dynamic timescale, and others caused by the evolution of the stellar progenitors and of the CS. Morphology may also depend on the physical status of the interstellar environment of the PN progenitor. Intercomparison of PN can aid our understanding of the complicated astrophysics of this stage of stellar evolution, particularly if distances are known. Many indirect methods of PN distance determination exist (Ciardullo et al. 1999 and Napiwotzki 2001). Agreement among these methods is seldom better than 20%. Direct parallax measurments of PN central stars rarely have precisions smaller than the measured parallax, a notable exception being Harris et al. (1997), who provide ∼ 0.5 mas precision parallaxes for 7 planetary nebulae CS nearer than 500 pc.

As the last object on the Hubble Space Telescope (HST) Astrometry Science Team list of astrophysically interesting stars, we have determined the absolute parallax of the CS of NGC 6853 (The Dumbell, M27) using FGS 3. Napiwotzki (1999) classifies the central star as a white dwarf of type DAO. Our extensive investigation of the astrometric reference stars provides an independent estimation of the line of sight extinction to NGC 6853, a significant contributor to the uncertainty in the absolute magnitude, MV, of its CS. We present the results of extensive spectrophotometry of the astrometric reference stars, required to correct our relative parallax to absolute; briefly discuss data acquisition and analysis; and derive an absolute parallax for the CS of NGC 6853. Finally, from a weighted average of our new result and that of Harris et al. (1997) we calculate an absolute magnitude for the CS and apply it to derive a stellar radius.

Bradley et al. (1991) and Nelan et al. (2001) provide an overview of the FGS 3 instrument and Benedict et al. (1999) and Benedict et al. (2002a) describe the fringe tracking (POS) mode astrometric capabilities of FGS 3, along with the data acquisition and reduction strategies used in the present study.

We time-tag our data with a modified Julian Date, mJD = JD − 2444000.5, and abbreviate millisecond of arc, mas, throughout.


2. Observations and Data Reduction

Figure 1 shows the distribution of the seven reference stars and the CS relative to the brightest regions of the PN. This image, produced by compositing Johnson B, V, and I bandpass frames obtained with the McDonald Observatory 0.8m telescope and Prime Focus Camera, reveals a particular difficulty with these data. The PN emission can contaminate the ancillary photometry and spectroscopy required to generate reference star spectrophotometric parallaxes (Section 3). Eight sets of astrometric data were acquired with HST, spanning 2.59 years, for a total of 140 measurements of the NGC 6853 CS and reference stars. Each data set required approximately 33 minutes of spacecraft time. The data were reduced and calibrated as detailed in Benedict et al. (2002a), Benedict et al. (2002b), and McArthur et al. (2001). At each epoch we measured reference stars and the target multiple times, this to correct for intra-orbit drift of the type seen in the cross filter calibration data shown in figure 1 of Benedict et al. (2002a).

Table 1 lists the eight epochs of observation and highlights another particular difficulty with these data.

We obtain observations at each of the two maximum parallax factors; hence the two distinct spacecraft roll values in Table 1. These are imposed by the requirement that HST roll to keep its solar panels fully illuminated throughout the year. This roll constraint generally imposes alternate orientations at each time of maximum positive or negative parallax factor over a typical 2.5 year campaign, usually allowing a clean separation of parallax and proper motion signatures. In this case two guide star acquisition failures on two attempts at roll∼ 101◦ (along with a bookkeeping error in scheduling the makeup observation), left us with the less than satisfactory temporal segregation of orientations shown in Table 1. Only the first two epochs occur at maximum negative parallax factor.

3. Spectrophotometric Absolute Parallaxes of the Astrometric Reference Stars

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The luminosity class is generally more difficult to estimate than the spectral type (temperature class).

However, the derived absolute magnitudes are critically dependent on the luminosity class. As a consequence we obtained additional photometry in an attempt to confirm the luminosity classes. Specifically, we employ the technique used by Majewski et al. (2000) to discriminate between giants and dwarfs for stars later than ∼ G5, an approach also discussed by Paltoglou & Bell (1994).

3.1. Photometry

Our band passes for reference star photometry include: BVRI (CCD photometry from a 0.4m telescope at New Mexico State University), JHK (from a pre-release of 2MASS1 ), and Washington/DDO filters M, DDO51, and T2 (obtained at McDonald Observatory with the 0.8m and Prime Focus Camera). The 2MASS JHK have been transformed to the Bessell & Brett (1988) system using the transformations provided in Carpenter (2001). The RI are transformed (Bessell, 1979) to the Johnson system from Kron-Cousins measures. Tables 3 and 4 list the visible, infrared, and Washington/DDO photometry for the NGC 6853 reference stars, ref-2 through ref-8.

3.2. Spectroscopy and Luminosity Class-sensitive Photometry

The spectra from which we estimated spectral type and luminosity class come from the New Mexico State University Apache Peak Observatory2. The dispersion was 1.61 ˚/pixel with wavelength coverage A 4101 – 4905 ˚. Classifications used a combination of template matching and line ratios. The brightest A targets had about 1500 counts above sky per pixel, or S/N ∼ 40, while the faintest targets had about 400 counts per pixel (S/N ∼ 20). The spectral types for the higher S/N stars are within ±1 subclass.

Classifications for the lower S/N stars are ±2 subclasses. Table 6 lists the spectral types and luminosity classes for our reference stars. The estimated classification uncertainties are used to generate the σMV values in that table.

The Two Micron All Sky Survey is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology The Apache Point Observatory 3.5 m telescope is owned and operated by the Astrophysical Research Consortium.

–6– The Washington/DDO photometry can provide a possible confirmation of the estimated luminosity class, depending on the spectral type and luminosity class of the star (later than G5 for dwarfs, later than G0 for giants). Washington/DDO photometry is less helpful as a discriminator in this case than it has proved for our previous targets (e.g., Benedict et al. 2002a, Benedict et al. 2002b, McArthur et al. 2001).

As seen in Figure 1 the nebular emission can contaminate the aperture photometry, depending on the filter bandpass. This contamination occurs predominantly in the M filter, because its effective bandpass (4500 ˚) includes the strong emission lines at λ5007 ˚ [O III] and λ4861 ˚ Hβ. This results in broadband A A A M-T2 colors which are slightly bluer than they would be in the absence of the nebula, and significantly bluer M-DDO51 indices (M-51 in Figure 2). Additionally, the nebular emission contaminates the sky annulus, which leads to larger measured magnitude errors than would be the case in the absence of the nebula. We list in Table 4 the Washington-DDO photometry. Unfortunately both ref-2 and ref-6 fell on bad columns in the photometer CCD. Figure 3 shows the Washington-DDO photometry along with a dividing line between dwarfs and giants (Paltoglou & Bell 1994 ). The boundary between giants and dwarfs is actually far ’fuzzier’ than suggested by the solid line in Figure 3 and complicated by the photometric transition from dwarfs to giants through subgiants. This soft boundary is readily apparent in Majewski et al. (2000) figure 14. In the absence of contaminating nebular flux objects just above the heavy line are statistically more likely to be giants than objects just below the line. After correcting for interstellar extinction, our reference stars lie on the dividing line to the left, where giant/dwarf discrimination is poorest. Nebular emission lines could have depressed ref-3 and ref-8 (giants, as determined from spectroscopy) below the giant-dwarf dividing line.

3.3. Interstellar Extinction

To determine interstellar extinction we first plot these stars on several color-color diagrams. A comparison of the relationships between spectral type and intrinsic color against those we measured provides an estimate of reddening. Figure 3 contains J-H vs H-K and J-H vs V-K color-color diagrams and reddening vectors for AV = 1.0. Also plotted are mappings between spectral type and luminosity class V and III from Bessell & Brett (1988) and Cox (2000) (hereafter AQ2000). Figure 3, and similar plots for the other measured colors, along with the estimated spectral types, provides an indication of the reddening for each reference star.

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& Brett (1988) and AQ2000. Specifically we estimate AV from four different ratios, each derived from the Savage & Mathis (1977) reddening law: AV /E(V-R) = 5.1; AV /E(J-K) = 5.8; AV /E(V-K) = 1.1; and AV /E(V-I) = 2.4. We excluded AV /E(B-V) due to the higher errors in that color index. The resulting AV are collected in Table 5. The errors are the standard deviation of the means for each star. We also tabulate AV per unit 100 pc distance for each star. Colors and spectral types of the NGC 6853 reference stars are consistent with a field-wide average AV =1.46±0.35, far less than the maximum reddening, AV 4.66 determined by Schlegel et al. (1998).

3.4. Adopted Reference Frame Absolute Parallaxes

We derive absolute parallaxes with MV values from AQ2000 and the AV derived from the photometry.

Our parallax values are listed in Table 6. Individually, no reference star parallax is better determined σπ = 18%. The average absolute parallax for the reference frame is πabs = 1.0 mas. As a check than π we compare this to the correction to absolute parallax discussed and presented in YPC95 (section 3.2, fig.

◦ 2). Entering YPC95, fig. 2, with the NGC 6853 galactic latitude, l = -3. 7, and average magnitude for the reference frame, Vref = 14.3, we obtain a correction to absolute of 1.1 mas. We will use the 1.0 mas correction derived from spectrophotometry. When such data are available the use of spectrophotometric parallaxes offers a more direct way of determining the reference star absolute parallaxes.

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