Ultraviolet imaging of planetary nebulae with $\mathit{}$
Astrophys Space Sci
Ultraviolet imaging of planetary nebulae with GALEX
Luciana Bianchi 0 1
David Thilker 0 1
B L. Bianchi 0 1
0 Department of Physics and Astronomy, The Johns Hopkins University , Baltimore, MD 21210 , USA
1 This article belongs to the Topical Collection: UV Surveys, the Needs and the Means. Guest Editors: Ana I. Gómez de Castro , Noah Brosch
Over four hundred Galactic Planetary Nebulae (PNe) have been imaged by GALEX in two ultraviolet (UV) bands, far-UV (FUV, 1344-1786 Å, λeff = 1528 Å) and near-NUV (NUV, 1771-2831 Å, λeff = 2271 Å). We present examples of extended PNe, for which UV spectroscopy is also available, to illustrate the variety in UV morphology and color, which reflects ionization conditions. The depth of the GALEX imaging varies from flux ≈ 0.4/5 × 10−18 ergs cm−2 s−1 Å−1 −1 (FUV/NUV) for exposures of the order of ∼ 100 seconds, typical of the survey with the largest area coverage, to ∼ 0.3/8.3 × 10−19 ergs cm−2 s−1 Å−1 −1 (FUV/NUV) for ∼ 1500 sec exposures, typical of the second largest survey (see Bianchi in Astrophys. Space Sci. 320:11, 2009; Bianchi et al. in Adv. Space Res. 53:900, 2014). GALEX broad-band FUV and NUV fluxes include nebular emission lines and in some cases nebular continuum emission. The sensitivity of the GALEX instrument and the low sky background, especially in FUV, enable detection and mapping of very faint ionization regions and fronts, including outermost wisps and bow shocks. The FUV-NUV color of the central star provides a good indication of its Teff , because the GALEX FUV-NUV color is almost reddening-free for Milky Way type dust (Bianchi et al. in Astrophys. J. Suppl. Ser. 230:24, 2017; Bianchi in Astrophys. Space Sci. 335:51, 2011, Bianchi in Astrophys. Space Sci. 354:103, 2014) and it is more sensitive to hot temperatures than optical colors.
Astronomical data bases; surveys; Stars; white dwarfs; ISM; planetary nebulae; general; ISM; planetary nebulae; individual
1 Introduction
Planetary Nebulae (PN) are the evolved descendants of
intermediate mass stars, the major providers of important
chemical elements such as carbon and nitrogen. The
expanding layers of gas, shed in the previous Red Giant phases and
then ionized by the hot central star (CSPN), offer clues about
the progenitor’s evolution, in particular about the chemical
elements produced by nucleosynthesis and brought up to
outer layers, about the temperature and luminosity of the
stellar remnant, through their ionization, and about
massloss and wind momentum in subsequent phases, through
their complex expansion kinematics and density structure.
Studies of both the nebula and the central star benefit by
observations in the Ultraviolet (UV), where crucial
diagnostic transitions of important chemical elements, and trace
elements, abound
(e.g., Bianchi 2016, 2012)
. CSPNe, the
hottest stars known, emit most of their light at UV
wavelengths or shortwards. IUE and HST spectrographs have
collected UV spectra of a few hundred PNe, mostly of their
central stars. FUSE has provided high resolution spectra at
shorter UV wavelengths (905–1187 Å) for several objects.
The FUSE observations, although difficult to obtain and
limited to the brightest sources, have enabled major discoveries,
such as highly ionized neon in the wind of CSPNe
(Herald
et al. 2005; Herald and Bianchi 2009, 2011; Keller et al.
2011, among others)
, whose lines are a crucial diagnostics
for the hottest (Teff > 85000 K) CSPN; the brightest PNe in
the Magellanic Clouds were also observed by FUSE
(Herin portion of the image, and a plot of archival IUE spectra taken in the
bright part of the nebula, with the GALEX transmission bands
overplotted, suggesting that the FUV flux mostly originates from HeII emission
in the inner regions
ald and Bianchi 2004, 2007)
. For a review of the role of UV
observations in the understanding of CSPNe see Bianchi
uniquely complement ground-based and HST imaging in
optical emission lines, and spectroscopic information.
(2012).
A completely different, new type of information has
become available thanks to the deep sensitivity and wide
field of view of the GALEX instrument. In this work we
show examples of UV images of Planetary Nebulae, which
2 The data: UV imaging
Figures 1 to 3 show FUV and NUV imaging data of
selected PN obtained with GALEX. The instrument was first
described by
Martin et al. (2005)
, and its performance by
Morrissey et al. (2007)
. The characteristics of the data and
the sky surveys are described by
Bianchi (2009)
, Bianchi
et al.
(2011a, 2011b, 2014)
; in depth discussion of data
quality and an updated version of science-enhanced catalogs of
UV sources are presented in Bianchi et al. (2017, 2018).
GALEX imaged the field in two bands simultaneously:
FUV (1344–1786 Å, λeff = 1528 Å) and NUV (1771–
2831 Å, λeff = 2271 Å), with a field of view of 1.28/1.24◦
[FUV/NUV] diameter, and resolution of ≈ 4.2/5.3 [FUV/
NUV]. The images, reconstructed from photon counting (...truncated)