A Study of the NGC 6914 Star Formation Region Johanna Malinen, Marius Maskoliunas, Minja M¨Akel¨A, and Paul Anthony Wilson
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Astronomy & Astrophysics manuscript no. g4˙article c ESO 2009 June 18, 2009 A study of the NGC 6914 star formation region Johanna Malinen, Marius Maskoliunas, Minja M¨akel¨a, and Paul Anthony Wilson Address(es) of author(s) should be given Preprint online version: June 18, 2009 ABSTRACT Aims. We have studied the star formation region NGC6914, specifically two chosen fields in the northern and southern parts. Methods. We combined near infrared, optical, and mm observations to look for manifestations of outflow activity and therefore sites of young stars. Imaging and spectroscopy was used to search for HH objects and radio observations to study the structure of the cloud. Results. We found a new Herbig-Haro object and an infrared jet. Key words. Herbig-Haro objects – NGC6914 – young stars 1. Introduction NGC 6914 is a reflection nebula located in the Great Cygnus Rift at a distance of approximately 1000 pc according to Racine (1968) and Shevchenko et al. (1991). It appears to be situated in the foreground of the Cygnus X region which has a distance of approximately 1.7 kpc. NGC 6914 is an active star forming area and several young stars are found in the region. Other known objects include a small reflection nebula PP95 (Parsamian & Petrossian 1979) and a nearby large HH object, HH 475 (Aspin & Reipurth 2000). However, the NGC 6914 region is still largely unexplored. For a review see Reipurth & Schneider (2008). The objective of this study is to explore NGC 6914 at several wavelengths. We concentrated on two areas: a region centered ′ ′ around the coordinates 20h24m31.3s, +42◦21 48” (J2000) and a region with two interesting stars, PP95 (20h24m29.6s, +42◦14 02”) ′ and V1585 Cyg (NW star) (20h24m23.3s, +42◦14 30”), about 6 arc minutes south. We call these regions the Northern and Southern region, respectively. 2. Near IR 2.1. Observations We have observed the Northern region in the near-infrared with the NOTCam instrument at NOT, La Palma, on June 10th 2009. The field of view is 4x4 arc minutes with a resolution of 0.234 arc seconds per pixel for NIR imaging. JHKs imaging was done in five frames with 5 arc second dithering and a total integration time of 3x3.6s. H2 (2.121 µm) narrowband images were done in beamswitching mode with a guide star chosen 4’ in the East-West direction and a dithering step of 10 arc seconds. The ON and OFF fields have both seven frames with a total integration time of 100s per frame. Beamswitching was used, because we were exploring extended emission, but we found that dithering could be used as the emission was not too extended. [FeII] at 1.645 µm was imaged in a 3x3 grid with a dithering step of 10 arc seconds and a total exposure time of 60s per frame. A He IC filter at 2.184 µm was used as ”off-H2” to determine if there is continuum-like emission close to the wavelength of H2, the frames were taken with the same exposure time, dithering anf grid as the [FeII] frames. Flats for all filters were taken during twilight before the observations. The seeing was ≈ 1.6 − 1.8 arc seconds during most of the observations and improving to ≈ 1.2 arc briefly before the observation run ended. An unresolved issue with the pointing was noticed after the observations: the wideband images are offset south from the nar- rowband images by almost an arc minute, with the narrowband images being closer to the original pointing coordinates. 2.2. Reduction The flats were reduced already at the NOT telescope. The notcam package in IRAF was used to reduce the data. The basic steps in reducing NIR data are masking the bad pixels, removing cosmic rays, sky subtraction and flat fielding. The wide- and narrow-band data were both reduced the same way. The H2 images were reduced both with using the OFF field for sky subtraction and only combining the dithered images of the ON field. The procedure for reducing beamswitched data produced a clear raster pattern on the image (most likely from incomplete masking of field stars); using the actual ON frames for measuring the sky produced better results although the dithering step was quite small (5”) for the sky subtraction. 2 Johanna Malinen et al.: A study of the NGC 6914 star formation region h m s ◦ ′ ” Fig. 1. The Ks band image. The coordinates in the center are 20 24 31.0 , +42 21 43 (J2000), and the field of view is 4x4 arc minutes. The bright reflection nebula is marked in the image. 2.3. Results There is a reflection nebula visible in the lower right hand corner in all NIR images (Figure 1). Another fainter reflection nebula is seen ≈ 0.8 arc minutes north-east from the brighter one. Both have not been seen before and indicate the presence of newborn embedded stars. We find an emission feature that is visible only in the H2 and [FeII] images (Figures 2 and 3, respectively) and not in the off-H2 (Figure 4) or JHKs filters, which suggests that the area is shocked gas. There is also a bright outflow in the top edge of the H2 image that is faintly visible in the Ks image. We have thus discovered a new bright H2 jet in this region. We attempted to make JHKs photometry with SExtractor for the stars seen in all three wideband filters and we calibrated the images, but the scatter of the stars in a (J-H) versus (H-K) diagram was large, so we used 2MASS data to construct the two-color diagram (Figure 5) for the stars in the NOTCam field. The two stars located in the Southern region, PP95 and V1585 Cyg, are also plotted in Figure 5. There are a few IR excess stars and we see that out of the two specific stars, PP95 lies outside the reddening vectors, indicating that it has excess infrared emission, and thus warm circumstellar material. 3. Optical imaging 3.1. Observations We did observations at the NOT telescope in the optical region using three filters: Hα, [SII], and I. In optical we also made ”slitless spectroscopy” by putting an R band filter in front of a grism. Images were taken using the ALFOSC 6.4’x6.4’ CCD camera. Flat field images were made during twilight before the observation run. We used three flat field frames with each filter. Using the Hα filter we made three exposures with 300 seconds exposure time each. The same was made with [SII] filter images, three exposures with 300 seconds. We imaged the I band with an exposure time of 180 seconds. Also we made three images using a combination of a R filter and a grism with an exposure time of 600 seconds each. 3.2. Reduction The optical image reduction was made using IRAF. We combined ten bias images into a master bias. It is an average of all bias images and we use it to remove CCD additive effects from flat field and observed images. Then the flat frame images have been corrected with the master bias, and we combined them to the master flats. The master flat for each filter was made from three flat field images. Observed images were corrected with master flat frames separately, and after this we combined them to final images. Eventually we have a reduced image for each of the filters (Hα, [SII], I and R with a grism). Johanna Malinen et al.: A study of the NGC 6914 star formation region 3 h m s ◦ ′ ” Fig. 2. The H2 band image. The coordinates in the center are 20 24 31.0 , +42 21 43 (J2000), and the field of view is 4x4 arc minutes. The outflows we found are marked in the images. ′ Fig. 3. The [FeII] band image. The coordinates in the center are 20h24m31.0s, +42◦21 43” (J2000), and the field of view is 4x4 arc minutes. Two reflection nebulae seen in all filters are marked in the image. 4 Johanna Malinen et al.: A study of the NGC 6914 star formation region ′ Fig. 4. The He band image. The coordinates in the center are 20h24m31.0s, +42◦21 43.2” (J2000), and the field of view is 4x4 arc minutes. Fig. 5. A (J-H) versus (H-K) diagram made from 2MASS stars in the NOTCam field. The asterisk symbols mark stars that have 2MASS photometric quality better than BBB (i.e. the uncertainty for the JHKs magnitude is smaller than 0.155 in all three bands), crosses are other stars in the field with worse photometric quality. The colors of the stars PP95 and V1585 Cyg that lie in the Southern region are also plotted. Reddening vectors are from (Indebetouw et al. 2005). 3.3. Results The main point in our optical wavelength investigation was to detect Hα emission stars and Herbig-Haro objects. One way to do that is to compare images made at different wavelengths. For detection of Herbig-Haro objects we used [SII], Hα and I filters (Figures 7, 8, and 9, respectively). In I filter there is usually a small amount of continuum emission. We look how images from different filters appear. If an object appears in all three images it cannot be a Herbig-Haro object which are pure emission line objects. Comparing Johanna Malinen et al.: A study of the NGC 6914 star formation region 5 Fig. 6. A cropped grism+R band filter image of the new Hα star we have found.