H2 emission in the low-ionization structures of the planetary nebulae NGC 7009 and NGC 6543

Abstract

Despite the many studies in the last decades, the low-ionization structures (LISs) of planetary nebulae (PNe) still hold several mysteries. Recent imaging surveys have demonstrated that LISs are composed of molecular gas. Here, we report H 2 emission in the LISs of NGC 7009 and NGC 6543 by means of very deep narrow-band H 2 images taken with NIRI@Gemini. The surface brightness of the H 2 1-0 S(1) line is estimated to be (0.46-2.9)×10 −4 erg s −1 cm −2 sr −1 in NGC 7009 and (0.29-0.48)×10 −4 erg s −1 cm −2 sr −1 in NGC 6543, with signal-to-noise ratios of 10-42 and 3-4, respectively. These findings provide further confirmation of hidden H 2 gas in LISs. The emission is discussed in terms of the recent proposed diagnostic diagram R(H 2 )=H 2 1-0 S(1)/H 2 2-1 S(1) versus R(Brγ)=H 2 1-0 S(1)/Brγ, which was suggested to trace the mechanism responsible for the H 2 excitation. Comparing our observations to shock and ultraviolet (UV) molecular excitation models, as well as a number of observations compiled from the literature showed that we cannot conclude for either UV or shocks as the mechanism behind the molecular emission.

Molecular Hydrogen Microstructures in Planetary Nebulae

“…Shocks cannot be ruled out in the case of the young PN K 4-47, for which high knots' radial velocities (∼100 km s −1 ) have been measured [22]. The low line ratios (1-2 and 0.1, respectively) measured for the knots in NGC 7662 and NGC 6543 with nebular ages from 1000 to 2000 yrs, indicate UV-fluorescent H 2 emission [17].…”

“…The high R(H 2 ) (∼10) and R(Brγ) (10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20) values found in the knots of K 4-47 as well as in the CKs of NGC 6270 and NGC 7293 show the two extreme phases (early and late phases in PN evolution), in which H 2 emission has a thermal origin (see Figure 6 in [17]). Shocks cannot be ruled out in the case of the young PN K 4-47, for which high knots' radial velocities (∼100 km s −1 ) have been measured [22].…”

“…Deep and high sensitive H 2 imagery of PNe with LISs obtained with the Near InfraRed Imager and Spectrograph (NIRI) at Gemini North have also detected H 2 emission-lines from LISs (K 4-47 and NGC 7662, [16]; NGC 6543 and NGC 7009, [17]) ( Figure 2). Note that, the higher spatial resolution of the NIRI images ( [17]), revealed H 2 emission from the inner LISs of NGC 6543.…”

“…NGC 7009 is characterized by low (western knot) and high (eastern knots) R(H 2 ) values [17]. This implies a different origin for the H 2 emission of each knot.…”

SATELLITE: Application to Planetary Nebulae IFU Data

“…The pairs of knots or LISs in this nebula [17,18]) are substantially brighter in the low-ionization lines (e.g., [N II] λλ6548,6584, [S II] λλ6716,6731, [O I] λλ6300,6363, [N I] λ5200, among others) than their surrounding gas (see also, [1,19]). Moreover, the recent detection of a H 2 v = 1-0 2.12 µm S(1) ro-vibrational line emanating from the LISs in NGC 7009 [20] or generally in PNe [21,22] has shown a strong link between the emission from low-ionization lines and molecular hydrogen. Possible explanations to this link are either a low velocity shock interaction with the nebular shell or the photo-evaporation of dense molecular knots resembling the spectrum of mini-photodissociation regions (PDRs, see [23]).…”