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1956Apj. . .124 . . .93M SPECTROPHOTOMETRY OF .93M . SPECTROPHOTOMETRY OF PLANETARY NEBULAE .124 . R. Minkowski and L. H. Aller Mount Wilson and Palomar Observatories Carnegie Institution of Washington, California Institute of Technology 1956ApJ. and University of Michigan Observatory Received February 13, 1956 ABSTRACT Measurements of the line and continuous spectra of several bright planetary nebulae by methods of photographic spectrophotometry have been carried out with the Cassegrain spectrographs at the 60- and 100-inch reflectors of the Mount Wilson Observatory. The results of extensive observations are re- ported for NGC 40, NGC 1535, NGC 2022, NGC 2392, and NGC 7662 Additional material has been ob- tained for IC 351, IC 1747, J 320, NGC 2165, and NGC 2440 In each object the slit was placed on a selected position in the nebula and the exposures carefully guided The line intensities, expressed on the scale /(H/3) = 100, are given for lines from the Baimer limit to X 9000 I. INTRODUCTION For a number of astrophysical problems it is desirable to have reliable measurements of emission-line intensities in typical gaseous nebulae. On the one hand, for theoretical studies, it is necessary to know the emission per typical unit volume in the radiation of hydrogen and other permanent gases. On the other hand, nebulae in which line intensi- ties have been accurately measured may serve as photometric standards to which other, and often much fainter, objects can be compared. Recent investigations (Filler and Aller 1954; MacRae and Stock 1954; Code 1956) have demonstrated the value of photoelectric techniques for the measurement of the intensities of the stronger lines. For the present, at least, the weaker lines must be measured by photographic photometry. It also is possible to select smaller regions in the nebula for detailed analysis. Previous photographic spectrophotometric studies (Berman 1930; Plaskett 1931; Page 1936, 1942; Aller 1941, 1951) have been confined to the stronger lines. Wyse (1942) attempted to measure the weaker lines with the aid of eye- estimates on scale plates. The most interesting planetary nebula from the standpoint of the richness of spectrum and range in excitation is NGC 7027 (Bowen, Minkowski, and Aller 1955). Similar spectrophotometric techniques have been applied to other nebulae. We selected a number of nebulae ranging from low to high excitation which were well placed for the long exposures required to record moderately faint lines and continua. For obvious reasons, most of them were rather bright. We included the ring objects NGC 1535, NGC 2022, NGC 2392, and NGC 7662 and the somewhat irregular struc- ture, NGC 40. A few additional observations were secured of IC 351, IC 1747, J 320, NGC 2165, and NGC 2440. The faintest object on our list, NGC 2022, was chosen be- cause of its high excitation. II. THE OBSERVATIONAL TECHNIQUE The observations discussed in this paper were all secured with Cassegrain spectro- graphs at the 60- and 100-inch reflectors at the Mount Wilson Observatory. At the 60- inch reflector we used a two-prism spectrograph with the 3-inch Schmidt camera, which gives a dispersion of about 170 A/mm at H7. At the 100-inch telescope we used the two- prism spectrograph with the Schmidt camera of 6-inch focal length, which gives a dis- persion of 110 A/mm at H7. This optical arrangement permitted us to obtain plates in 93 © American Astronomical Society • Provided by the NASA Astrophysics Data System .93M . 94 R. MINKOWSKI AND L. H. ALLER .124 . good definition from the near infrared to the Baimer limit. Since both spectrographs were equipped with auxiliary guiding eyepieces, it was possible to keep the slit placed accu- rately upon a selected portion of the nebula for the entire exposure. In this way, differ- 1956ApJ. ences of excitation in different parts of the nebula may be compared. No attempt has been made to obtain integrated intensities of the nebular images. Since the lines have an intensity range of over a thousand fold, it is necessary to secure observations ranging in exposure time from a few seconds to many hours. Table 1 gives the nebulae and comparison stars observed and the spectrographs em- ployed. The JS plate series refers to the 60-inch Cassegrain spectrograph; the E series to the 100-inch Cassegrain spectrograph. For the photographic and visual regions, the 103a-F emulsion was used; for the infrared, hypersensitized I-N plates were employed. The sixth and eighth columns give the range of exposure times on nebula and comparison stars for each plate secured. III. PHOTOMETRIC CALIBRATION To measure the intensities of the nebular lines and continua, it is necessary to estab- lish the relation between photographic blackening and intensity as well as the effects of plate sensitivity, atmospheric transparency, and telescope reflectivity plus spectrograph transmission. The first step is accomplished by a suitable photometric standardization; the second is carried out most expeditiously by photographic suitable comparison stars. Because of the small size of the plates used in the spectrographs, it was necessary to impress the photometric standards on a larger piece cut from the same plate and then to develop nebular and calibration plate together. We photographed the calibration standards with the 32-inch camera and the coudé spectrograph, adjusting the exposure times to be comparable with those at the telescope. Thus calibration exposures ranging from a minute to many hours had to be secured. The change in slope of the calibration- curve with exposure time is very small. In a single calibration exposure it is difficult to obtain the proper densities for all relevant wave lengths. Fortunately, the development procedures were kept the same for all plates. To compare the standard stars, of known energy distribution, with the nebulae, which are several magnitudes fainter, we adopted the following device. A small piece of ground quartz was placed over the slit of the spectrograph. We then ran the image of the com- parison star out of focus by a prearranged amount and placed a portion of the luminous ring of the out-of-focus image on the slit. Thus we provided a diffuse illumination of the slit with a source of known energy distribution. The transmission of the quartz diffuser as a function of wave length was calibrated photoelectrically in the laboratory by Edison Pettit, under conditions closely simulating those obtained at the telescope. In addition, the calibration was checked photographical- ly at the telescope. For this purpose, stellar spectra obtained with the diffuser were com- pared with spectra of the same stars obtained without the diffuser and widened by a rapid trailing back and forth of the focal stellar image on the slit with a frequency ade- quate to avoid the intermittency effect. The standard energy-distribution stars which served for comparison stars include a Lyrae, which was observed by Williams (1938), Williams and Hall (1941), Kienle, Strassl, and Wempe (1938), and Barbier and Chalonge (1940), as well as e Persei, ß Cephei, and rj Ursae Majoris, which have been observed by Kienle, Strassl, and Wempe, by Barbier and Chalonge, or by both groups. The energy distribution in Vega was measured by photographic photometry by R. Williams for XX 3700-9000 and by photoelectric photometry for the near infrared by Williams and Hall. The most frequently used comparison star, e Persei, has been meas- ured by Kienle’s group and by Barbier and Chalonge, whose results are in good agree- ment with one another. Redward of X 3700 the energy distribution corresponds to a color temperature of 26000° K; shortward of X 3700 the color temperature is 23000° K. The © American Astronomical Society • Provided by the NASA Astrophysics Data System .93M . TABLE 1 .124 . Spectrophotometric Observations of Planetary Nebulae Comparison Exposures 1956ApJ. Plate Emul Exposures Star NGC 40 OMO1^ + 72° 11 JS 753 I-N 420m ß Cephei 60“ JS 754 103a-F 420m ß Cephei 60“ JS 755 103ö-F 420m ß Cephei 60“ JS 756 103a-F 150m JS 757 103a-F 64m, 64m* ß Cephei 15“ JS 758 103a-F 100m JS 759 103a-F 480m ß Cephei 60“, 15“ JS 761 103#-F 60m, 60m* ß Cephei 60“ JS 762 103a-F 70m e Persei 60“ JS 763 103a-F 180m ß Cephei 60“,15“ JS 766 103a-F 90m ß Cephei 60“ e Persei 60“ IC 351 3 44 3 +34 55 JS 775 103a-F 91m NGC 1535 4 11 9 -12 52 JS 769 103a-F 225m e Persei 60“ JS 771 103a-F 322m e Persei 60“ JS 774 103a-F 60m, 20m, 7m e Persei 60“ J 320 2 8 + 10 39 JS 775 103a-F 60m NGC 2022 39 3 + 9 02 E 2283 103a-F 435m 7] Ursa Ma- 60“ jor JS 769 103a-F 110m e Persei 60“ JS 770 103a-F 510m e Persei 60“ JS 772 103a-F 145m IC 2165 6 19 4 -12 57 JS 774 103a-F 180m e Persei 60“ NGC 2392 7 26 3 +21 02 E 2236 103a-F 120m e Persei 10“ 8m, 2m e Persei 60“ E 2237 103a-F 60m, 30m, 15m, 4r E 2238 lOSa-F 180m, 90m e Persei 60“ E 2268 103a-F 480m € Persei 60“ E 2270 103ö-F 360m e Persei 60“ E 2269 I-N 520“ e Persei 60“ E 2282 I-N 430“ e Persei 60“ E 2284 I-N 120“ e Persei 60“ JS 762 103+F 120“ e Persei 60“ JS 764 103a-F 194“ e Persei 15“, 60“ JS 765 103ö-F 60“, 60“f € Persei 60“ JS 766 103a-F 336“ e Persei 60“ ß Cephei 60“ JS 768 103a-F 60“ e Persei 60“ JS 776 103ö-F 410“ e Persei 60“ NGC 2440 7 39 7 -18 05 JS 771 103a-F 116“ e Persei 60“ NGC 7027 21 05 2 +42 01 E 1729 I-N 360“ a Ly rae 60“ E 1730 I-N 390“ a Lyrae 60“ E 1731 I-N 330“ a Lyrae 60“ e Persei 60“ E 1746 I-N 67“, 25“ a Lyrae 60“ E 1747 I-N 10“, 150“ e Persei 60“ NGC 7027 21 05 2 +42 01 E 1748 I-N 360“ a Lyrae 60“ e Persei 60“ E 1751 103ö-F 120“ a Lyrae 60“ 4m a Lyrae 10“, 1“ E 1752 103tf-F 80s, 25s, 10s, 40“ E 1753 103a-F 13“ e Persei 60“, 10“ E 1754 103a-F 13“ a Lyrae 10“ 4m a Lyrae 2“, 20s E 1755 103a-F 80s, 25s, 10s, 5s E 1757 103a-F e Persei 10“, 3“ * A 2-mag diaphragm was placed in the optical system for this exposure f A 1-mag diaphragm was placed in the optical system for this exposure © American Astronomical Society • Provided by the NASA Astrophysics Data System .93M .
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