THE ASTROPHYSICAL JOURNAL, 461 : L111–L114, 1996 April 20 q 1996. The American Astronomical Society. All rights reserved. Printed in U.S.A.
THE DOUBLE-SHELL STRUCTURE OF THE VARIABLE YOUNG PLANETARY NEBULA IC 4997 ´ M. TORRELLES, 2 LUIS F. MIRANDA, 1 JOSE
AND
CARLOS EIROA 3
Received 1995 December 22; accepted 1996 February 6
ABSTRACT VLA-A 3.6 cm continuum and H92a observations and long-slit optical spectroscopy of IC 4997 are presented. The radio continuum map shows that IC 4997 is a double-shell planetary nebula consisting of a faint, knotty outer shell of size 32"7 3 1"4, elongated at position angle 3548, and a bright, circular inner shell of 30"28 in diameter. The outer shell presents an hourglass–like morphology and exhibits a striking mirror symmetry with respect to its minor axis. The deduced mean electron densities are 31.2 3 10 4 cm 23 and 31.4 3 10 5 cm 23 in the outer and inner shells, respectively. The knots in the outer shell present electron densities of 33– 4 3 10 4 cm 23 , somewhat higher than the mean. The analysis of the Ha, [N II], and [S II] emission lines, spatially and spectrally resolved in the spectrum, allows us to deduce the basic kinematical properties of the two shells. The outer shell presents bipolar motions, expands at 312 km s 21 in the equatorial plane, and was formed several hundred years ago. The inner shell expands at 325 km s 21 and was ejected some decades ago. The Ha emission from the central star presents extremely wide wings that can be traced up to 35375 km s 21 . This value is much higher than those previously reported for IC 4997. Our results suggest that the two shells are interacting and that a dense region may have been formed between the two shells. The physical conditions in this dense region could vary as the inner shell expands, and, therefore, variations of the emitted spectrum from this region are expected. This interaction should be taken into account in order to explain the observed variability of the emission lines in the nebula. Subject headings: ISM: kinematics and dynamics — planetary nebulae: individual (IC 4997) geometrical structure of IC 4997 is poorly known. Tamura, Kazes, & Shibata (1990) show a radio continuum map at 18 cm in which IC 4997 appears elongated at position angle (P. A.) 3548, suggesting nonspherical mass motions. However, the angular resolution of this map (310) does not allow us to recognize much detail. In order to progress in the understanding of IC 4997 and to provide an observational basis for realistic models of the nebula, the detailed knowledge of its structure represents a crucial step. In addition, because IC 4997 is very young, the knowledge of its structure may provide interesting information about the first stages of PN evolution. In this work, we present 3.6 cm continuum and H92a observations at high spatial resolution (30"3) and long-slit optical spectroscopy of IC 4997. The radio continuum observations show the subarcsecond structure of IC 4997 for the first time. The optical spectroscopic data reveal details of the internal kinematics and mass ejection processes involved in the formation of the nebula.
1. INTRODUCTION
The planetary nebula (PN) IC 4997 has attracted attention since Liller & Aller (1957) and Aller & Liller (1966) reported on variability of the [O III] 4363/Hg line intensity ratio in the nebula (see also Feibelman et al. 1979; Purgathofer & Stoll 1981). More recent observations have shown that many emission lines from IC 4997 are also variable (Feibelman, Aller, & Hyung 1992). IC 4997 is a peculiar PN in many respects. The electron density N e is very high and presents an unusually wide range of values, from 310 4 cm 23 up to $10 7 cm 23 (Aller & Liller 1966; Feibelman et al. 1981; Hyung, Feibelman, & Aller 1994). Estimates of the electron temperature T e range from 310 4 K up to 31.8 3 10 4 K (Pottasch 1984; Hyung et al. 1994). H I absorption has been detected toward IC 4997 (Altschuler et al. 1986). The dust-to-gas mass ratio of 30.02 in the nebula is one of the highest among PNs (Lenzuni, Natta, & Panagia 1989). All these results agree to indicate that IC 4997 is a very young PN. However, the interpretation of the observations presents many problems. The variability of the [O III] 4363/Hg intensity ratio is probably caused by variations of T e , which would ultimately be due to changes in the stellar flux (Feibelman et al. 1979 and references therein; Ferland 1979; Purgathofer & Stoll 1981). Other authors consider that the expansion alone of the PN is enough to explain that variability (Aller & Liller 1966; Kiser & Daub 1982). The wide range of N e and T e has been interpreted and modeled by assuming the existence of strata, knots, and several shells, each of them with different physical conditions (Hyung et al. 1994). The interpretations are complicated by the fact that the physical and
2. OBSERVATIONS
Radio observations.—The observations were made with the Very Large Array (VLA) of the National Radio Astronomy Observatory (NRAO)4 in the A configuration during 1995 July 6, with the same spectral line mode (63 channels of 97.7 kHz resolution plus a continuum channel [channel 0] containing the central 75% channels) and procedure for calibration and imaging as described in Miranda, Torrelles, & Eiroa (1995). C3 286 and 19231210 were used as flux and phase calibrators, respectively. Continuum emission at 3.6 cm from IC 4997 was detected through the continuum channel (see § 3). We failed to detect H92a emission, with an upper (3 s) level of 0.6 mJy beam 21 with 18 km s 21 of velocity resolution. Optical spectroscop y.—A long-slit spectrum of IC 4997 was
1 Departamento de Astrofı ´sica, Facultad de Ciencias Fı´sicas, Universidad Complutense de Madrid, E-28040 Madrid, Spain;
[email protected]. 2 Instituto de Astrofı ´sica de Andalucı´a, CSIC, Ap. Correos 3004, C/Sancho Panza s/n, E-18080 Granada, Spain;
[email protected]. 3 Departamento de Fı ´sica Teo ´rica, CXI, Universidad Auto ´noma de Madrid, Cantoblanco, E-28049 Madrid, Spain;
[email protected].
4 The NRAO is operated by Associated Universities Inc., under cooperative agreement with the National Science Foundation.
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obtained on 1993 July 14 with the coude´ spectrograph of the 2.2 m telescope at Calar Alto Observatory.5 A TEK 24 mm CCD was used as detector. The spectral region 6542– 6750 Å was observed at a dispersion of 38.6 Å mm 21 . The slit was centered on the object and oriented at P. A. 548, the major nebular axis. The achieved spectral resolution (FWHM) is 317 km s 21 . Seeing was 31"4. No flux calibration was carried out. 3. RESULTS
Figure 1 (Plate L19) presents the radio continuum map at 3.6 cm of IC 4997 in which a wealth of detail can be distinguished at subarcsecond scales. The nebula is resolved into two shells: a faint, extended outer shell, and a bright, very compact inner shell. The outer shell, of 32"7 3 1"4 in size, is elongated at P. A. 548 and presents an hourglass–like morphology. Several knots with sizes of 30"3– 0"5 are observed in the outer shell. A line through P. A. 548 divides the outer shell into two different halves distinguished by their intensity. The northwest half contains the brightest knots, whereas the southeast half contains fainter knots. In addition, the outer shell exhibits a striking mirror symmetry with respect to its minor axis (P. A. 1448). The inner shell has been resolved by the beam into a circular shell, with a deconvolved size of 30"28. We note that the center of the inner shell is displaced by 30"15 toward the northwest with respect to the centroid of the outer shell. The total flux at 3.6 cm is 360 mJy, from which 347 mJy corresponds to the outer shell and 313 mJy to the inner shell. We can obtain several parameters for the shells by assuming a distance of 2.5 kpc (Cahn, Kaler, & Stanghellini 1992), assuming a T e of 310 4 K, and using the formulation given by Mezger & Henderson (1967) for optically thin emission. In this way we obtain that the mean N e in the outer shell is 31.2 3 10 4 cm 23 . The brightest knots show a somewhat higher N e , 33– 4 3 10 4 cm 23 . In the inner shell, N e is 31.4 3 10 5 cm 23 . On the other hand, the values of the ionized mass in the outer and inner shell become 33.1 3 10 22 M J and 32.6 3 10 24 M J , respectively. The optical spectrum of IC 4997, integrated along the slit, is shown in Figure 2. The emission lines detected in the spectrum have been spectrally and spatially resolved in several components, the parameters of which have been obtained by using a multicomponent Gaussian fit. The Ha emission-line profile is shown in Figure 3. The Ha emission is dominated by three components: a narrow high-velocity component (V LSR 3 2102 km s 21 , DV(FWHM) 3 25 km s 21 ); a strong and narrow lowvelocity component (V LSR 3 235 km s 21 , DV(FWHM) 3 51 km s 21 ); and a very broad component (V LSR 3 251 km s 21 ) distinguished by its extremely wide wings. These results agree with those obtained by Feibelman et al. (1992). The blue wing of the broad component extends beyond the observed spectral range. However, the red wing can be traced up to 36620 Å (at the 4 s level above the continuum; Fig. 2). By assuming symmetry with respect to the central velocity, the total velocity width of the broad component is 35375 km s 21 . This value is much higher than the value of 31000 km s 21 reported by Feibelman et al. (1992) and, to the best of our knowledge, than any value observed in PNs. In the two-dimensional spectrum 5 The Calar Alto Observatory is operated jointly by the Max-Planck-Institut fu ¨r Astronomie (Heidelberg) and the Spanish Comisio ´n Nacional de Astronomı´a.
FIG. 2.—Spectrum of IC 4997 in the spectral range 6542– 6735 Å. The observed emission lines are [N II] 654816583, Ha, C II 6578, He I 6678, [S II] 671616731, and an unidentified emission feature at l obs 3 6705.82 Å. The spectrum is presented at two different intensity scales in order to show the very wide wings of the Ha emission. The dotted line represents the stellar continuum level extrapolated from the region 6680 – 6700 Å.
(not shown here), the three Ha components are observed at the stellar position (given by the position of the continuum intensity peak) and are spatially unresolved. Spatially extended Ha emission is also detected, but it presents a very complex kinematics, different from other emission lines (see below). Spectra at higher resolution are necessary for a detailed study and interpretation. Figures 4 a and 4b present a position-velocity map of the [N II] 6583 emission line and the [N II] 6583 emission line profile as observed at the stellar position, respectively. The total angular size (at 11% level of the intensity peak) of the [N II] emission is 36"4. The extended [N II] emission has been resolved into two velocity components that are observed all along the spatial extent of the emission line: a blueshifted component (V LSR 3 263 km s 21 at the stellar position) and a redshifted component (V LSR 3 238 km s 21 at the stellar position) with respect to the deduced systemic velocity V SYS ([N II]) 3 249 km s 21 (see Figs. 4 a and 4b). The radial velocity of the redshifted component increases toward P. A. 548, whereas the radial velocity of the blueshifted component decreases toward P. A. 548. These two velocity components were also detected but not spatially resolved by Tamura
FIG. 3.—Central part of the Ha emission line (see also Fig. 2). Radial velocities (LSR) (km s 21 ) of the two narrow components (see text) are indicated.
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DOUBLE-SHELL STRUCTURE OF IC 4997
L113 4. DISCUSSION
FIG. 4.—(a) Position-velocity contour plot of the [N II] 6583 emission line. The contours are logarithmic, separated by a factor 2 1/2 in intensity. Radial velocities (km s 21 ) with respect to the deduced systemic velocity of [N II] (V LSR 5 249.5 km s 21 ) are also indicated. (b) The [N II] 6583 emission-line profile as observed at the stellar position. The dotted lines represent the result of a three-component Gaussian line fit to the observed profile.
et al. (1990). In addition, a narrow high-velocity component (V LSR 3 2104 km s 21 , DV [FWHM] 3 25 km s 21 ) is observed in [N II]. This component is detected at the stellar position and is spatially unresolved. The spatiokinematical properties of this component are identical to those of the narrow highvelocity component observed in Ha, which suggests that both components arise in the same region. The [N II] emission also presents faint extended wings (DV 3 170 km s 21 at the continuum level), which indicates the presence of a broad component (V LSR 3 250 km s 21 ) that is probably related to the very broad component observed in the Ha emission (see above). The [S II] emission lines (not shown here) present a structure almost identical to that of the [N II] lines, except that extended wings are not observed. In particular, the total angular size (at 31% level) of the [S II] emission is 36"6, V SYS ([S II]) is 3 250 km s 21 , and the extended [S II] emission has been resolved into two velocity components that are separated by 327 km s 21 at the stellar position. A faint high-velocity component is also detected at V LSR 3 2105 km s 21 . The mean [S II] 6731/6716 line intensity ratio in the extended emission, obtained from the integrated spectrum, is 32.05, corresponding to N e 3 10 4 cm 23 . This value coincides with the value obtained by Hyung et al. (1994) in 1990 and 1991. In the high-velocity component, we obtain a value of 3500 –900 cm 23 . We note that the [N II]/[S II] intensity ratio of 37.1 in our spectrum coincides with that obtained by Hyung et al. (1994). However, the [N II]/He I, [S II]/He I intensity ratios in our spectrum (3.27, 0.46) are lower than those obtained in 1991 (3.87, 0.55) and in 1990 (4.14, 0.58) by these authors.
The radio continuum map shows that IC 4997 is a doubleshell PN. The shells present a different size, a different morphology, and different physical conditions. The inner shell has N e higher by a factor 310 than the outer shell. Although our data do not allow us to estimate T e , it is highly probable that the inner shell has a higher T e than the outer shell. In this case, the inner shell must be the origin of both high-ionization emission lines as well as of those lines indicating high N e . On the other hand, both low-ionization emission lines as well as those lines indicating a relatively low N e must arise in the outer shell. It is important to note that emission lines of [Fe VII] and [Ar IV] have been detected in the spectrum of IC 4997 (Feibelman et al. 1992). In addition, the ultraviolet N III] lines indicate N e 3 10 8.8 –10 9.3 cm 23 (Hyung et al. 1994, and references therein). If these lines are of nebular origin, there must exist a region in the nebula, probably related to the inner shell, in which the physical conditions are much more extreme than those revealed by our radio continuum observations (probably due to a beam dilution effect). An interesting question is the origin of the different components observed in the emission lines. The broad component observed in Ha and [N II] can be attributed to the WolfRayet–type central star (Feibelman et al. 1992). The extended [S II] emission probably arises in the outer shell. This is supported by the fact that similar values of N e are obtained from the [S II] data and from the radio continuum emission of the outer shell. In addition, the fact that N e is very high in the inner shell and, therefore, strong [S II] is not expected also supports this idea. The same can be said for the extended [N II] emission, which presents spatiokinematical properties almost identical to the [S II] emission. The narrow Ha components (see § 3) must arise in compact regions that have not been spatially resolved in the spectrum; in principle, both could arise in the inner shell. However, N e in the narrow highvelocity component (500 –900 cm 23 ) is completely different from that in the inner shell (1.4 3 10 5 cm 23 ), and the narrow low-velocity component is not detected in [N II] and [S II] as it is in Ha. These results suggest that the narrow high-velocity component, observed in Ha, [N II], and [S II], may arise in a compact region that is not related to any of the shells. The spatiokinematical properties suggest that this region moves along the line of sight and/or is located very close to the central star. The strong and narrow low-velocity component, observed in Ha only, can be attributed to the inner shell. The [N II] and [S II] emissions indicate a bipolar flow for the outer shell. The flow axis is probably oriented near P. A. 548, where the outer shell is elongated (Fig. 1). This axis must be tilted with respect to the line of sight so that the southwest regions of the nebula point toward the observer and the northeast regions point away. At a given spatial position, the radial velocity (absolute value) of the two components is different, but the difference is small (Fig. 4). This suggests that the inclination angle of the flow axis, with respect to the plane of the sky, is small, too. Therefore, the radial velocity difference observed at the stellar position can be considered as being due to expansion of the outer shell in its equatorial plane. We obtain an equatorial expansion velocity of 312 km s 21 from [N II] and 313 km s 21 from [S II]. By assuming an equatorial radius for the outer shell of 30"7 from the 3.6 cm emission, the kinematical age is 3675 yr. This result is compatible with IC 4997 being a young PN.
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In the case of the inner shell, the radio continuum map suggests a spherical symmetry. The FWHM velocity width of the strong narrow high-velocity Ha component indicates an expansion velocity of 325 km s 21 . By assuming a constant expansion velocity (but see below), the kinematical age of the inner shell is 365 yr. We note that the systemic velocity of the inner shell (235 km s 21 ) is different from that of the outer shell (249 km s 21 ). Remarkably, this result is compatible with the displacement between the centers of both shells observed in the radio continuum map (Fig. 1). These shifts in space and velocity could be due to orbital motion in a binary system. Our results suggest that the inner shell was formed around 1930. However, as mentioned by Hyung et al. (1994), the 1919 –1960 spectra of IC 4997 do not show evidences for any particular ejection. Nevertheless, we note that the kinematical age has been obtained with a distance of 2.5 kpc, which may be erroneous. If smaller (greater) distances are assumed, the kinematical age would be smaller (larger) accordingly. For instance, if we assume that the formation of the inner shell coincides with the reversal of the [O III] 4363/Hg intensity ratio observed from 1962 (Feibelman et al. 1979; Kiser & Daub 1982), the distance would be 31300 pc. On the other hand, we have assumed a constant expansion velocity for the inner shell. This is not necessarily true. It is reasonable to assume that large amounts of kinetic energy are being transferred from the stellar wind of the Wolf-Rayet–type central star to the inner shell. If the temperature of the central star varies (Feibelman et al. 1979; Ferland 1979), the wind velocity might be variable. In fact, this is suggested by the very different total velocity width of the Ha wings measured by Feibelman et al. (1992) and by us (see above). Therefore, variations of the expansion velocity of the inner shell are possible. In any case, the formation of the inner shell corresponds to a much more recent event of mass ejection than the event that formed the outer shell. The expansion velocity of the inner shell is higher than that of the outer shell in the equatorial plane. These results suggest that the two shells are interacting
and that a dense region of compressed material may have been formed between the shells (see also Hyung et al. 1994). We do not know what the physical conditions in this region can be, but it is plausible that N e and T e are higher than in the shells themselves. As the inner shell expands under the action of the stellar wind, more material is swept up, and N e and T e may vary in the dense region. The change of the physical conditions would result in variations of the emission lines emitted from the dense region. We are not claiming that the variability observed in IC 4997 may be attributed to that interaction only, but shell interaction could be another aspect to be added to variations in the stellar flux, which has already been suggested to explain the variability of the emission lines in IC 4997. 5. CONCLUSIONS
Radio continuum 3.6 cm observations and long-slit optical spectroscopy have revealed the structure and internal kinematics of IC 4997. The nebula consists of two shells: an extended, knotty hourglass outer shell, and a very compact, probably spherical, inner shell. The electron density and, very probably, the electron temperature are different in the two shells; both density and temperature are higher in the inner shell. The formation of the inner shell corresponds to a recent event of mass ejection that has occurred much later than the mass ejection corresponding to the formation of the outer shell. The inner shell expands at a higher velocity than the outer shell, under the action of a very energetic stellar wind. This wind has been detected through the unusually and extremely wide wings of the Ha emission. The two shells may be interacting. This interaction could contribute somehow to the variations of the emission lines observed in IC 4997. J. M. T. acknowledges partial financial support from DGICYT grant PB92-0900 and by Junta de Andalucı´a (Spain). C. E. and L. F. M. are supported partially by DGICYT grants PB91-007 and PB94-165.
REFERENCES Aller, L. H., & Liller, W. 1966, MNRAS, 132, 337 Altschuler, D. R., Schneider, S. E., Giovanardi, C., & Silverglate, P. R. 1986, ApJ, 305, L85 Cahn, J. H., Kaler, J. B., & Stanghellini, L. 1992, A&AS, 94, 339 Feibelman, W. A., Aller, L. H., & Hyung, S. 1992, PASP, 104, 339 Feibelman, W. A., Boggess, A., McCracken, C. W., & Hobbs, R. W. 1981, ApJ, 246, 807 Feibelman, W. A., Hobbs, R. W., McCracken, C. W., & Brown, L. W. 1979, ApJ, 231, 111 Ferland, G. J. 1979, MNRAS, 188, 669
Hyung, S., Feibelman, W. A., & Aller, L. H. 1994, ApJS, 93, 465 Kiser, J., & Daub, C. T. 1982, ApJ, 253, 679 Lenzuni, P., Natta, A., & Panagia, N. 1989, ApJ, 345, 306 Liller, W., & Aller, L. H. 1957, S&T, 16, 222 Mezger, P. G., & Henderson, A. P. 1967, ApJ, 147, 490 Miranda, L. F., Torrelles, J. M., & Eiroa, C. 1995, ApJ, 446, L39 Pottasch, S. R. 1984, Planetary Nebulae (Dordrecht: Reidel) Purgathofer, A., & Stoll, M. 1981, A&A, 99, 218 Tamura, S., Kazes, I., & Shibata, K. M. 1990, A&A, 232, 195
PLATE L19
FIG. 1.—Gray-scale and contour 3.6 cm continuum maps of IC 4997. Gray levels are indicated in mJy beam 21 (top scale). Contour levels are 25, 22, 2, 5, 8, 11, 14, 17, 20, 23, 26, 29, 32, 35, 52, 72, 92 times 7 3 10 25 mJy beam 21 , the rms noise in the map. The beam size is indicated at the top left-hand corner (0"26). The map shows the structure of IC 4997 in considerable detail. Two shells can be distinguished: a faint, extended knotty outer shell, and a bright, very compact inner shell. The outer shell exhibits a bipolar hourglass morphology with a remarkable mirror symmetry with respect to its minor axis (P.A. 1448). The inner shell appears circular. MIRANDA, TORRELLES, & EIROA (see 461, L112)
