THE HUBBLE SPACE TELESCOPE/ADVANCED CAMERA FOR SURVEYS ATLAS OF PROTOPLANETARY DISKS IN THE GREAT ORION NEBULA^*

The Orion Nebula (M42, NGC 1976) is a unique laboratory for studying the physical processes related to star and planet, formation. It harbors one of the richest and youngest clusters (Orion Nebula Cluster, ONC) in the solar neighborhood, spanning the full spectrum of stellar and substellar masses down to a few Jupiter masses (Lucas & Roche 2000). In 1979 several compact photoionized knots were first detected in the central region of the Orion Nebula as emission-line sources (Laques & Vidal 1979), and then important follow-up studies were made in radio (Garay et al. 1987; Churchwell et al. 1987) and via emission-line spectroscopy (Meaburn 1988; Meaburn et al. 1993; Massey & Meaburn 1993). Since the early 1990s, Hubble Space Telescope (HST) observations of the ONC have been fundamental for clarifying the main characteristics of these young stellar objects (YSO) and their accretion disks. After the pioneering surveys of O'Dell et al. (1993) and Prosser et al. (1994), performed with the spherically aberrated WF/PC, O'Dell & Wen (1994) used Wide Field Planetary Camera 2 to discover several externally ionized protoplanetary disks (proplyds), as well as a number of disks seen only in absorption against the bright nebular background (silhouette disks), both rendered visible by their location in or near the core of the H ii region. Following this discovery, other HST programs have increased the number of known objects (O'Dell & Wong 1996; McCaughrean et al. 1996; Bally et al. 1998, 2000). O'Dell (2001) and Smith et al. (2005), targeting areas out of the core, showed that these systems are ubiquitous across the Great Orion Nebula.

So far a total of ∼200 silhouette disks and bright proplyds have been revealed by the HST observations of the Orion Nebula, the large majority through narrowband filters centered on the Hα λ6563 emission lines, and occasionally through filters centered on the [N ii] λ6583, [O i] λ6300, [O iii] λ5007, and [S ii] λ6717 + 6731 lines.

In this paper we present an atlas of multicolor observations of circumstellar disks and resolved circumstellar emission obtained with the Wide Field Channel of the Advanced Camera for Surveys (ACS/WFC). These images are part of the HST Treasury Program on the ONC (Cycle 13, GO Program 10246, P. I.: M. Robberto), aimed at measuring with high precision the main stellar parameters of the cluster members. For this reason, the Treasury Program used broadband filters to obtain the most accurate photometry of each source, together with Hα-narrowband images to address the presence of circumstellar emission that may contaminate the photometry and the point-spread function of the broadband data. The combination of broadband and narrowband images opens a new window on the study of disks in the ONC. It also makes it possible to detect disks where the nebular background is too faint, thanks to the light of the central stars reflected by the circumstellar material at the disk's polar regions (reflection nebulae).

After a brief description of the observations (Section 2), we present the new ACS/WFC images of all circumstellar disks (Section 3). We then provide a complete catalog of circumstellar disks in the Orion Nebula, including also the few disks that were not detected in our programs for a variety of reasons (Section 4). Finally, after a brief description of the new proplyds, silhouette disks, and bipolar nebulae² (Sections 5 and 6) we present the images of 16 red, elongated, and diffuse objects which most probably represent galaxies seen through the background curtain provided by the Orion Molecular Cloud (OMC-1, Section 7). A few remarkable objects are being investigated and will be discussed in separate papers (see, e.g., Robberto et al. 2008a).

The images have been extracted from the large dataset (520 images) of ACS/WFC observations executed between 2004 November and 2005 April. The ACS/WFC survey has covered an area of about 450 arcmin², centered about 4' southwest of the Trapezium cluster. The filters and exposure times are listed in Table 1. The narrowband F658N filter transmits both Hα λ6583 and [N ii] λ6583, but it is conventionally referred to as the ACS Hα filter. Due to the dithering strategy adopted for the survey, most of the field has been exposed two times (or more, occasionally), so the total integration time is typically twice that reported in Table 1. Only at the edges of the ACS survey field was a single image obtained. Images have been combined using the Py-Drizzle algorithm, allowing for the removal of cosmic rays when two or more exposures were available. Sources lying at the outer edges of the nebula are therefore clearly recognizable by the presence of uncorrected cosmic rays in our images. The mosaics of ACS images have been registered against the Two Micron All Sky Survey (2MASS) catalog (Cutri et al. 2003) to derive absolute astrometry of the sources accurate to approximately 1/2 pixel (25 mas).

Each individual ACS image, both raw and drizzled, has been visually inspected for source identification, resulting in a master catalog of ∼3200 stellar or compact sources. Each source has been classified as either a single star, binary, photoevaporated disk, dark silhouette, or candidate galaxy. The last three classes constitute the sample presented in this paper. Note that since the ACS catalog targets stellar sources, it does not include Herbig-Haro objects, bow-shocks, and jets unless they are closely associated (within ≈1'') with a point source. Further details on the HST Treasury Program observing strategy and on data-reduction procedures are given in Robberto et al. (2008b), whereas the complete photometric catalog is given in Soderblom et al. (2008).

We found 219 sources that show distinct evidence of circumstellar matter. Of them, 178 are externally ionized protoplanetary disks seen in emission, five show jet emission in Hα with no evidence of either external ionized gas emission or dark silhouette disks, and 36 can be classified as dark silhouette disks. They are directly visible either in absorption against the nebular background, or revealed through the blocking of light coming from their central star, or by the presence of detached bipolar lobes.

Figures 2–5 show the ACS/WFC images of our sources in the five photometric bands used for our ACS observations. In particular, the ionized protoplanetary disks seen in emission are reported in Figure 2, while Figure 3 shows the dark silhouette disks, Figure 4 shows the reflection nebulae, and Figure 5 shows the jets with no external ionized gas emission or silhouette disk. Each frame is 100 × 100 ACS/WFC pixels, corresponding to ∼5'' × 5'', or ∼2000 × 2000 AU at the distance of the Orion Nebula, here assumed to be ∼420 pc (Menten et al. 2007).

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In each row, we report the images in the five photometric bands, in order of increasing wavelength: F435W, F555W, F658N, F775W, and F850LP. The gray scale goes from 2σ below the average sky level through 3σ above it, where both the average and σ have been estimated using an iterative algorithm to reject outliers. The color images at the end of each row were created in this way: the intensity of blue is the average of the fluxes measured in the F435W and F555W bands, the intensity of red is the average of F775W and F850LP, and the intensity of green is the flux measured only in the F658N filter.

All the FITS files from which these images have been taken are in the electronic version of the Astronomical Journal. In these drizzled images the pixel values are in counts per seconds, and an estimate in magnitudes of the photometry of a source can be directly extracted by

$$\begin{equation} {\it m_X}=-2.5\cdot {\rm log}\,F_X + ZP_X, \end{equation} \tag{ 1 }$$

where X is the passband of interest, F_X is the observed flux of the source in counts per seconds in the passband X, and ZP_X is the zero-point magnitude in the passband X for a certain photometric system. In Table 2 we list the zero-point magnitudes derived by the photometric calibration of the HST/ACS camera (Sirianni et al. 2005) in the VEGAMAG, ABMAG, and STMAG standard photometric systems for the filters used by our survey.

We have searched the original literature and the SIMBAD³ database to cross-identify each object that we found. For 170 of them there is a previous HST classification as proplyd, silhouette disk, or compact non-stellar object. In particular, our catalog includes all the sources identified by O'Dell & Wen (1994), O'Dell & Wong (1996), Bally et al. (2000), O'Dell (2001), and Smith et al. (2005). For 34 of these objects we could not confirm their nature as proplyds or silhouette disks. Our observations missed three sources (158-314, 163-322, and 163-323), hidden by the bleeding trail of saturated bright stars, and the silhouette disk 216-0939 (Smith et al. 2005), which is located outside the field covered by the HST Treasury Program. We have excluded these sources from our main catalog of disks, listing them separately in Table 4. Regarding the other 30 sources, the HST/ACS images (Figure 7) show that some are close binary systems with no visible circumstellar emission, while some are Herbig-Haro objects. We have listed these objects in Table 5, in which Column 11 points out the object's type as it appears from our images. However, it is important to note that some of these objects may still have low-ionization circumstellar emission (e.g. from [O iii] emission line), since the ACS filters would not pick this up. On the other hand, our images provide the first identification for 63 objects, of which 29 are proplyds, seven are silhouette disks, six are bipolar nebulae, five are jets with no external ionized gas emission or silhouette disk, and other 16 are probably galaxies.

In Figure 1 we show a map with all the circumstellar disks and the extended objects found in the HST/ACS images. The 63 newly discovered objects are shown in red. It is remarkable that new circumstellar disks have also been discovered in the well-explored inner part of M42. Namely, 18 new proplyds and three disks seen only in silhouette have been found in a 6 × 6 arcmin region around θ¹Ori-C. This demonstrates how important it is to search for these objects with a multi-wavelength strategy. In several cases, only the presence of a star in our reddest filter images (F850LP) allows us to unambiguously recognize the presence of circumstellar disks or a reflection nebula too faint to be detected in the Hα filter.

It is evident from Figure 1 that almost all the disks seen only in silhouette (the circles in the figure) and reflection nebulae (the squares) have been observed in the outskirts of the Orion Nebula. This is for the following two reasons: (1) in the outer regions the ultraviolet photon flux is low because of the larger distance from the O- and B-type stars of the Trapezium cluster (the few silhouette disks seen in the inner part of the nebula most probably lie in the foreground); (2) in regions of low nebular background, it is easier to spot the presence of a disk through the scattering from the polar regions than by direct imaging of the dark silhouette against the background. This is how circumstellar disks are commonly imaged in Tau associations (e.g., Koresko 2002). Conversely, most of the circumstellar disks associated with the emission of externally ionized gas are observed close to the Trapezium stars. Some of them are also detected in the outer regions, indicating that stars other than θ¹Ori (the Trapezium) are affecting the structure and evolution of protoplanetary disks in the ONC.

In Table 3, we list all the sources sorted according to their right ascension and declination, derived from the absolute astrometric solution of our survey. Cross-references to previous HST surveys follow the SIMBAD convention, where the O'Dell & Wen (1994) and O'Dell & Wong (1996) lists are merged together and labeled under the "OW" prefix (in this catalog we also included the four circumstellar disks observed by O'Dell 2001), whereas the Bally et al. (2000) and Smith et al. (2005) lists are merged with the "BOM" prefix. We also list the corresponding entry in the Prosser et al. (1994) catalog, in the optical survey of Jones & Walker (1988), in two near-infrared sources catalogs (Ali & DePoy 1995; 2MASS, Cutri et al. 2003), and in the X-ray source catalog of the Chandra Orion Ultradeep Project (COUP; Getman et al. 2005). The last two columns of Table 3 report the main characteristics of the objects derivable from the images. In particular, Column 11 defines the type of each object (either an ionized disk seen in emission or a dark disk seen only in silhouette or a reflection nebulae with no external ionized gas emission), while Column 12 points out the presence of jets, reflection nebulae, binary stellar systems, nearly edge-on, or face-on circumstellar disks.

For the object name, we used the coordinate-based nomenclature of O'Dell & Wen (1994): objects with coordinates α = 5:35:AB.C, δ = −5:2X:YZ are labeled ABC-XYZ. If the right ascension is 5:34:AB.C, then a 4 is added, i.e. 4ABC at the beginning of the R.A. group. Similarly, if the declination is −5:1X:YZ, it becomes 1XYZ. This coordinate-based method is affected by astrometric errors, as better measures, or just measures at different wavelengths, may require a change of name. This is the approach followed by Bally et al. (2000), who renamed a few sources originally labeled by the O'Dell team on the basis of their improved astrometry. Unfortunately, this generates ambiguity and is a potential source of error when data are retrieved from archives. For this reason, we decided to maintain the nomenclature of the objects given in their discovery papers, i.e., in the case of different names in the OW and BOM catalogs, we used the OW name. Our coordinates thus take the lowest priority, and were used only for the new objects discovered by the HST Treasury Program to give them a name.

Among the 235 circumstellar disks and other extended objects presented in this paper, 118 have been observed by Ali & DePoy (1995) in the near-infrared; only 49 are listed in the 2MASS catalog (Cutri et al. 2003). The COUP survey shows 137 objects, i.e., 58% of all the objects. The COUP fraction rises to 63% if we do not consider the extended objects described in Section 7. The high fraction of circumstellar disks revealed in X-rays is particularly interesting since, even if the X-ray luminosities are in general relatively small, this high-energy radiation effectively penetrates deeper through the disks, ionizing otherwise neutral molecular gases and even melting solid particles. Together with the ionizing flux from the brightest cluster members, X-rays from low-mass stars may thus have profound effects on their associated circumstellar disks and therefore on planet formation (Feigelson et al. 2007).

We detected 29 previously unknown proplyds, whose images have been reported together with all the other Orion proplyds in Figure 2. In Table 3 they can be recognized as those ionized disks seen in emission (flag "i" in Column 11) that have not been observed in the OW and BOM catalogs (i.e., with no designations in Columns 4 and 5, except for the proplyd 280-931, observed by Bally et al. 2001 not included in the BOM catalog in SIMBAD).

Among these 29 new proplyds, three show evidence of emission from jets mainly in the Hα filter, and all of them are located in the outskirts of the Orion Nebula (4468-605, 099-339, and 351-349). Also, five other objects in the M42 outer regions show evidence of jet emission in Hα (4364-146, 4466-324, 006-439, 078-3658, and 353-130, see Figure 5). This is probably due to the low level of background nebular emission in those regions that makes the faint jets easier to detect than in the inner region of M42.

Compared with many of the previously known proplyds with bright cusps observed in the M42 core, these new objects are fainter. For the proplyds located in the outer regions of the Orion Nebula, this lack of ionized gas is due to the distance from the ionizing sources located in the M42 core. The lack of bright cusps in proplyds located in the inner region can be explained by several factors: their physical distance from the M42 core may be larger than the projected one due to the position of these objects with respect to the line of sight, these disks may have a smaller amount of mass compared with the brighter proplyds, the ongoing photoevaporation processes in the disk's surface may be in an early or late phase, so that the ionization front is not much developed.

In the following, we briefly describe two of the new proplyds with bright ionization fronts, located ∼10' away from the ONC core.

• 064-3335 (Figure 2(b), row 1): this proplyd is located ∼10' south of the ONC core. Its ionization cusp has a diameter of roughly 35 with a P.A. of ∼300°. Other than the bright cusp, observed in all the five ACS filters, two filaments extending for ∼800 AU from the center of the proplyd are visible mainly in the Hα filter, almost along the cusp axis direction.
• 066-3251 (Figure 2(b), row 2): this proplyd is located ∼10' south of the ONC core and is very close to 064-3335 (the distance between the two proplyds is 43''). The ionization cusp, oriented with a P.A. ∼320°, has a diameter of roughly 3''. With respect to 066-3251 this bright star is located 225'' in a direction P.A. ∼315°. In the southern side of the object a long outflow extends for about 30'', corresponding to ∼600 AU.

In this section, we provide a short description of the seven dark silhouette disks and the six reflection nebulae discovered by the new HST/ACS images.

• 090-326 (Figure 3(a), row 2): this silhouette disk is located ∼10' southeast of the ONC core. It has a P.A. of roughly 50°, major and minor axes of ∼03 and 015 in the Hα filter (approximately 120 × 60 AU), respectively, from which an inclination angle of ∼60° can be derived assuming a circular thin disk. However, since the central star is obscured in the F435W, F555W, and Hα filters, the disk may have larger inclination angle and thickness. In the bluer filters a faint emission is detected in the southeast side all around the disk. The emission from the disk edges can be either due to reflection nebular light, or due to a very mild level of ionization of the disk surface in a region of the Orion Nebula where the UV flux from the O- and B-spectral type stars is unable to support a fully developed proplyd.
• 230-536 (Figure 3(b), row 8): this small silhouette disk (approximately 05 × 025 in the F435W filter, corresponding to about 200 × 100 AU), located ∼5' southeast of the ONC core has an inclination angle of roughly ∼60° as derived from the apparent axes ratio. This justifies the detection of the red pre-sequence star in the Hα filter. Its P.A. is ∼160°.
• 280-1720 (Figure 3(c), row 1): this silhouette disk, located ∼8' northwest of the ONC core is seen nearly face-on in the F435W and Hα filters. The size is approximately 075 × 075 in the Hα filter, corresponding to about 300 × 300 AU.
• 281-306 (Figure 3(c), row 2): this small silhouette disk, located ∼12' southeast of the Trapezium cluster is visible face-on only in the Hα filter. The diameter of the disk is ∼04 or about 160 AU.
• 332-405 (Figure 3(c), row 5): this silhouette disk, located ∼5' east of the ONC core is seen in absorption in the F435W, F555W, and Hα filters. The major and minor axes are approximately 075 × 025, corresponding to about 300 × 100 AU in the F555W filter, and implying an inclination angle of about 70°. The disk P.A. is ∼120°.
• 346-1553 (Figure 3(c), row 6): this silhouette disk, located in the M43 region, ∼12' northeast of the ONC core, is seen in absorption only in the broadband filters F435W and F555W. The disk appears to be face-on with a diameter of ∼05 in the F435W, or ∼200 AU at the distance of Orion Nebula.
• 473-245 (Figure 3(c), row 8): this spectacular silhouette disk with reflection nebula is located ∼10' east of the ONC core. The disk is seen nearly edge-on (the two sides of the bipolar emission appear to be very symmetric), flared, with a P.A. of ∼60°, and a major axis in the Hα of roughly 075, corresponding to a physical diameter of 300 AU.
• 4538-311 (Figure 4, row 1): the disk, located ∼6' east of the ONC core, appears as an equatorial dark lane at the midplane of bipolar nebula. The emission is nearly symmetric, with the northeast side slightly wider and brighter than the southwest one, suggesting that it originates from the surface of a disk seen almost edge-on with the northeast face tilted toward us. The nebula is detected only in the F775W and F850LP filters, with some emission from the northwest side also visible in the F555W, close to the noise floor of our image. This indicates that the pre-main-sequence star hidden by the disk is very red. The P.A. of the disk is approximately 150°.
• 016-149 (Figure 4, row 2): this object, located ∼3' northeast of the ONC core, appears only in the F775W and F850LP filters as a bipolar nebula. In this case the morphology is highly asymmetric, with the southwest side much brighter and more extended than the northeastern one. The asymmetry, together with the low contrast of the equatorial dust lane, suggests that the disk is seen with low inclination angle, i.e., far from being face-on. However, the fact that the northeastern lobe appears as a point-like source brighter than the more-extended southwestern one suggests that the former source may rather be a red star, whose radiation scattered by circumstellar matter is seen as the southwestern lobe.
• 046-3838 (Figure 4, row 3): this source, ∼15' south of the ONC core, shows an extended region (diameter ∼5'') with bright emission especially in the F775W and F850LP filters, and a dark tail in the direction P.A. ∼350°. Since 046-3838 is rather weak in Hα compared to the fluxes observed in F555W, F775W, and F850LP, this source is most probably a reflection nebula with a red central star.
• 051-3541 (Figure 4, row 4): located ∼15' south of the ONC core, this bipolar source appears highly symmetric and relatively bright. It is detected mainly in the F775W and F850LP filters with a conspicuous equatorial dust lane at P.A. of about 100°, suggesting the presence of an almost edge-on disk around a red pre-main-sequence star. The lobes appear nearly round and more extended than those of 4538-311, for instance, suggesting either a very strong disk flaring or the presence of circumstellar material at the disk's polar regions.
• 193-1659 (Figure 4, row 6): this well-developed bipolar nebula is located ∼7' north of the ONC core. The asymmetrical brightness of the two lobes (seen only in the F775W and F850LP filters) and the detection of a red pre-main-sequence star away from the center of the bipolar nebula suggest that either the circumstellar disk that blocks the star light or the circumstellar matter that reflect it is non-symmetric. The P.A. of the disk is approximately 100°.
• 294-757 (Figure 4, row 8): this red source, observed only in the F775W and F850LP ∼7' southeast of the ONC core, is a bipolar nebula with equatorial dark lane clearly detected in the F775W image. This suggests the presence of a nearly edge-on disk with P.A. ∼70° (southeastern side facing toward us).

In Figure 6, 16 red and elongated objects are shown. These sources are listed in Table 6. They are all visible only in the F775W and F850LP filters and in the outer regions of the Orion Nebula (see Figure 1). These two facts suggest that these objects might be background galaxies seen through sparse regions of the reddening Orion Molecular Cloud OMC-1. An alternative plausible interpretation is that of reflection nebulae turned on by red pre-main-sequence stars (two similar objects in which nearly edge-on reflecting disks are visible are 294-757 and 016-149). In this hypothesis, their location in the Orion Nebula outskirts, far from the ionizing O- and B-type stars in the Trapezium cluster, is consistent with the non-detection of light from ionized plasma, which would have been observed in the Hα filter as well. To understand the nature of these objects spectra are needed.

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In this paper, we have shown the HST/ACS images of the 178 proplyds, 28 disks seen only in silhouette, eight reflection nebulae without external ionized plasma, five jets without either external ionized plasma or silhouette disks, and 16 other extended objects observed by the HST Treasury Program on the ONC. For every object we have reported all the images taken through the five photometric filters used by the HST Treasury Program (F435W, F555W, F658N, F775W, and F850LP).

The fact that most of these objects are associated with X-ray sources observed by the COUP is particularly interesting, since high-energy photons could play an important role in the star- and planet-formation processes.

Among all the objects reported, 63 have been discovered by these images: 29 proplyds, seven silhouette disks, six reflection nebulae with no external ionized plasma, five jets with no external ionized plasma or silhouette disk, and 16 other elongated objects.

Searching in the literature we found that four objects previously reported as circumstellar disks have not been detected by HST/ACS images either because they are hidden by the saturation bleeding trails of a close bright star or because they are located out of the HST/ACS Treasury Program field of view. For the 30 other sources previously reported as extended objects HST/ACS images reveal no circumstellar emission around them.

A brief description of all the newly discovered proplyds, disks seen only in silhouette, and reflection nebulae with no external ionized plasma has been carried out in Sections 5 and 6.

Finally, we have discussed possible interpretations for the nature of the 16 extended objects. Because of their location far away from the Trapezium cluster (≳10') and because of their red color, they are probably background galaxies reddened by the Orion Molecular Cloud OMC-1, but the alternative hypothesis of reflection nebulae turned on by red pre-main-sequence stars cannot be ruled out only by our observations.

We thank the referee, William Henney, for his useful comments that greatly improved the manuscript. Support for program 10246 was provided by NASA through a grant from the Space Telescope Science Institute (STScI), which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. The work of L.R. at STScI was done under the auspices of the STScI Summer Student Program.