ALMA OBSERVATIONS OF THE ORION PROPLYDS

Circumstellar disks are the birthsites of exoplanets. Hubble Space Telescope (HST) images of the Orion Nebula Cluster (ONC) revealed a hostile environment; many of the disks that orbit low mass stars are being photoevaporated by the intense UV radiation from the most massive nearby star, θ¹ Ori C (spectral type O6; O'Dell & Wen 1994; McCullough et al. 1995; Bally et al. 1998a; Smith et al. 2005; Ricci et al. 2008). These disks are surrounded by tear-drop-shaped structures with bright heads facing θ¹ Ori C and tails pointing radially away. These distinctive circumstellar morphologies led to the nomenclature "proplyds," an acronym for protoplanetary disks, that is now regularly applied to low-mass stars and their disks in the centers of massive star forming regions (O'Dell & Wen 1994). The Orion proplyds were found to suffer photoevaporative mass-loss rates of $\dot{M} \approx 10^{-7}$ M_☉ yr⁻¹ (Churchwell et al. 1987; Henney & O'Dell 1999), high enough to disperse the amount of disk mass required to form a planetary system like our own in under 1 Myr. That dissipation timescale is too short compared to the core accretion requirements for giant planet formation (e.g., Hubickyj et al. 2005), and is in apparent conflict with the inferred ages of the ONC stars (∼2 Myr; Reggiani et al. 2011; Da Rio et al. 2009).

Measurements of the masses that remain in the Orion proplyds are crucial for characterizing the photoevaporation process and assessing their potential for planet formation. The most straightforward way to estimate a disk mass is from a measurement of the thermal dust continuum luminosity at long wavelengths, where the emission is optically thin (cf. Beckwith et al. 1990). Although molecular gas likely comprises the vast majority of the mass budget in disks, the dust dominates the opacity and is significantly easier to detect. The Berkeley-Illinois-Maryland Array, Owens Valley Radio Observatory, Plateau de Bure Interferometer, and Combined Array for Research in Millimeter Astronomy observed the Orion proplyds at wavelengths of 1.3–3.5 mm (Mundy et al. 1995; Bally et al. 1998b; Lada 1998; Eisner & Carpenter 2006; Eisner et al. 2008), but unfortunately these data provided limited constraints on the disk masses: contamination by free–free radiation from the ionized cocoons generated by the photoevaporation process often dominated the dust emission. Soon after the Submillimeter Array (SMA) was commissioned, it produced the first successful detections of the Orion proplyds at a submillimeter wavelength (880 μm), where the dust emission dominates (Williams et al. 2005). Those observations revealed that at least some of these disks still have sufficient mass (>10 M_jup) remaining to potentially form giant planets. A larger scale SMA survey of the Orion proplyds identified the erosion of the high end of the disk mass distribution due to photoevaporation by θ¹ Ori C (Mann & Williams 2009a, 2010). To date, however, these surveys have only been sensitive enough to detect the most massive disks (⩾8.4 M_jup) in the ONC, and therefore provide relatively biased information about disk evolution in the hearts of massive star forming regions.

To probe the full disk mass distribution in the ONC and further study the impact of external photoionizing radiation on disk properties, we carried out a much more sensitive pilot survey with the Atacama Large Millimeter/submillimeter Array (ALMA) that targeted 48 young stars, including 22 HST-identified proplyds. In this article, we present the results of the 856 μm (ALMA Band 7) continuum observations. The observations and data reduction are described in Section 2. Estimates of disk masses are presented in Section 3, and an examination of the dependence of disk mass on location in the ONC is discussed in Section 4.

Five individual pointings (hereafter Fields) in the ONC were observed with ALMA using the Band 7 (345 GHz) receivers on 2012 October 24, as part of the Cycle 0 Early Science operations (see Table 1). Figure 1 marks the pointing centers, with reference to the high mass members of the ONC. Twenty-two 12 m diameter ALMA antennas were arranged in a hybrid configuration that yielded (with robust weighting) images with a ∼05 angular resolution. This target resolution was chosen to distinguish individual disks toward the crowded Trapezium cluster, resolve the emission from three large proplyds, and filter out potentially confusing large-scale emission from the background molecular cloud. Each Field was observed for 136 s per visit. Fields 2–5 were visited six times over 6.5 hr to improve sampling in the Fourier plane, but Field 1 was observed twice as often as the others to achieve higher sensitivity for the disks nearest to θ¹  Ori C. The correlator was configured to observe simultaneously four 1.875 GHz-wide spectral windows, each divided into 3840 channels each with a width of 488.28 kHz; after online Hanning smoothing, the spectral resolution was 976.56 kHz. The spectral windows were arranged to cover the CO (3–2), HCN (4–3), HCO⁺ (4–3), and CS (7–6) emission lines. The focus here will be on the wideband (Δν ≈ 7.5 GHz) continuum emission, extracted by integrating over all line-free channels: the spectral line data will be presented elsewhere. At 856 μm, the mean wavelength of the four spectral windows, the effective field of view is 18'' (the FWHM primary beam of an individual antenna). Since the shortest baseline was 21.2 m in length, the maximum recoverable scale was 499.

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Data calibration and image reconstruction were performed using standard procedures in the CASA package. The antenna-based complex gains were calibrated based on repeated observations of the quasar J0607–085. The absolute amplitude scale was determined from observations of Callisto, and the bandpass response of the system was measured from observations of the bright quasar J0522–364. The model of Callisto was that provided by Butler (2012). The mean and standard deviation of the 856 μm continuum flux of Callisto over all five scheduling blocks and all line-free channels in four spectral windows was 18.52 Jy and 0.53 Jy, respectively. Each field was Fourier-inverted separately, using the multi-frequency synthesis mode in the CLEAN task with an intermediate Briggs robustness parameter of 0.5 chosen to achieve the desired angular resolution of ∼05. A dirty image was generated out to a diameter of ∼34'' (only ∼2% of the maximum primary beam sensitivity), in an effort to include any bright disks that might have been located at the far margins of each field. These dirty images were then CLEANed with the Clark algorithm down to a threshold of ∼2 times the rms observed in emission-free locations near the pointing centers (boxes were used to focus the algorithm toward features that could be visually confirmed in the dirty images). After CLEANing, each field was corrected for primary beam attenuation and restored with a synthesized 051 × 046 beam. The 1σ rms levels obtained toward the center of each Field are listed in Table 1.

The 856 μm continuum maps of the surveyed ONC fields are shown in Figures 2 and 3, alongside the corresponding optical HST images (Bally et al. 2000; Smith et al. 2005). These ALMA images represent the highest resolution observations at submillimeter wavelengths toward the central OB stars in the ONC. We detected submillimeter emission from 21 of the 22 targeted HST-identified proplyds in this survey; the only proplyd not detected is 169–338 (see Table 2). Of those detections, 8 are new and 13 are recoveries of previous SMA detections. The centroid positions and 856 μm integrated flux densities for each target were measured by fitting elliptical Gaussians in the image plane. A suitable rms noise level in each field was determined from the emission-free regions within the primary beam.

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The observed emission is composed of a free–free (F_ff) contribution from the ionized cocoons that surround the photoevaporating disks and the thermal dust emission (F_dust) from the disks themselves, such that F_obs = F_dust + F_ff. The radio-submillimeter spectral energy distributions (SEDs) for the 22 disks detected at 3σ at 856 μm with ALMA are shown in Figure 4. The free–free contributions from the disk targets were extrapolated from centimeter wavelengths into the submillimeter regime using published Very Large Array flux densities from 1.3 cm to 6 cm (Garay et al. 1987; Felli et al. 1993a, 1993b; Zapata et al. 2004). Fits to the free–free emission (F_ff∝ν^−0.1) and dust emission (F_dust∝ν²) are overlaid on the SEDs to show their relative contributions and contrasting spectral dependences. The radio observations show the flat spectral dependence consistent with optically thin emission, but with a range, highlighted by the gray scale, which we attribute to variability (Felli et al. 1993b; Zapata et al. 2004). We avoided observations taken at wavelengths longer than 6 cm (5 GHz) in this analysis, in order to avoid the turnover frequency, where the free–free emission becomes optically thick and no longer follows a ν^−0.1 dependence.

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$F^{856\, \mu{\rm m}}_{\rm ff}$ is listed in Table 2, and represents the maximum level of free–free emission extrapolated to 856 μm from Figure 4. The highest levels of centimeter emission were used to account for the free–free contributions to result in the most conservative estimate of disk mass. After accounting for the free–free contamination, we estimated F_dust for each source (see Table 2); all 21 of the detected proplyds were detected in thermal dust emission in excess of the free–free emission (see Figure 4).

Table 2. 22 Orion Proplyds Targeted in ALMA Early Science Observations

Field	Proplyd	α (J2000)	δ (J2000)	F856 μm	$F^{856\,\mu{\rm m}}_{\rm ff}$	Fdust	Mdisk	d (θ1C)	Maj, Min, P.A.	Notes
	Name	(h m s)	(deg m s)	(mJy)	(mJy)	(mJy)	(Mjup)	(pc)	(AU, AU, deg)
1	157–323	5:35:15.74	−5:23:22.49	2.8 ± 0.3	1.0 ± 0.1	1.8 ± 0.2	0.89 ± 0.12	0.022	..., ..., ...
1	158–323	5:35:15.84	−5:23:22.47	15.6 ± 0.3	10.4 ± 0.9	5.1 ± 0.9	2.50 ± 0.46	0.019	113, 22, +55
1	158–326	5:35:15.85	−5:23:25.56	10.6 ± 0.3	5.0 ± 1.6	5.6 ± 1.5	2.72 ± 0.74	0.020	144, 66, +57
1	158–327	5:35:15.79	−5:23:26.56	22.9 ± 0.4	19.1 ± 1.4	3.8 ± 1.4	1.84 ± 0.70	0.021	147, 96, +45
1	161–314	5:35:16.11	−5:23:14.06	3.1 ± 0.6	0	3.1 ± 0.6	1.51 ± 0.29	0.020	272, 211, +4	New
1	161–324	5:35:16.07	−5:23:24.37	5.3 ± 0.3	4.7 ± 0.5	0.6 ± 0.5	0.30 ± 0.27	0.013	103, 36, +83	New
1	161–328	5:35:16.08	−5:23:27.80	6.1 ± 0.3	2.2 ± 0.1	3.9 ± 0.3	1.89 ± 0.17	0.015	193, 76, +67
1	163–317	5:35:16.29	−5:23:16.55	12.5 ± 0.3	7.6 ± 0.1	4.9 ± 0.3	2.35 ± 0.15	0.014	87, 68, +38	New
1	163–323	5:35:16.33	−5:23:22.56	6.4 ± 0.3	3.7 ± 0.5	2.7 ± 0.6	1.29 ± 0.29	0.004	154, 130, +61
1	166–316	5:35:16.62	−5:23:16.12	3.5 ± 0.4	2.0 ± 0.4	1.5 ± 0.6	0.73 ± 0.28	0.014	215, 128, +83	New
1	167–317	5:35:16.75	−5:23:16.44	26.0 ± 0.4	25.4 ± 1.3	0.6 ± 1.4	0.30 ± 0.66	0.015	121, 118, +64
1	168–328	5:35:16.77	−5:23:28.05	7.1 ± 0.3	3.1 ± 0.4	4.0 ± 0.5	1.91 ± 0.25	0.013	131, 92, −65
1	168–326	5:35:16.85	−5:23:26.22	23.5 ± 0.5	20.3 ± 2.4	3.2 ± 2.5	1.53 ± 1.18	0.012	222, 118, −42	New
2	169–338	5 35 16.88	−5 23 38.10	⩽0.8	0.3 ± 0.0	⩽0.5	⩽0.23	0.032	..., ..., ...
2	170–337	5:35:16.98	−5:23:37.05	23.7 ± 0.4	8.9 ± 2.2	14.8 ± 2.2	7.13 ± 1.08	0.031	86, 73, +54
2	171–334	5:35:17.07	−5:23:34.04	9.0 ± 0.4	3.8 ± 0.9	5.2 ± 1.0	2.49 ± 0.49	0.028	..., ..., ...	New
2	171–340	5:35:17.06	−5:23:39.77	33.0 ± 0.5	0.2 ± 0.0	32.8 ± 0.9	15.83 ± 0.42	0.037	102, 50, +53
2	173–341	5:35:17.34	−5:23:41.49	2.9 ± 0.9	1.0 ± 0.1	1.9 ± 0.5	0.92 ± 0.22	0.044	574, 37, −54*	New
2	177–341	5:35:17.68	−5:23:40.98	26.4 ± 0.5	10.9 ± 0.9	15.5 ± 0.9	7.48 ± 0.45	0.049	125, 100, +58
3	114–426	5:35:11.32	−5:24:26.52	7.0 ± 1.2	0	7.0 ± 1.2	3.38 ± 0.56	0.195	..., ..., ...	New
4	216–0939	5:35:21.58	−5:09:38.96	94.9 ± 1.6	0	94.9 ± 1.6	45.84 ± 0.77	1.605	525, 150, −7
5	253–1536	5:35:25.30	−5:15:35.40	162.9 ± 0.9	0	162.9 ± 0.9	78.66 ± 0.42	0.942	268, 95, +72

Notes. Column 1: field location; Column 2: proplyd name; Column 3 and 4: phase center coordinates; Column 5: integrated continuum flux density, corrected for ALMA primary beam attenuation, with 1σ statistical error; Column 6: extrapolated contribution of free–free emission at 856 μm using the highest centimeter flux (see Figure 4); Column 7: derived dust continuum flux density from the disk; Column 8: disk mass from ALMA observations (error does not include uncertainties in the flux scale of ∼10%); Column 9: projected distance from θ¹ Ori C; Column 10: disk size: deconvolved semi-major axis, semi-minor axis, position angle; "..." means that the source is effectively a point source. *Note that the disk size for 173–341 is quite uncertain, given its faintness; Column 11: new detections of submillimeter disk emission.

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It is worth noting that we did not correct for background cloud emission as was done for the SMA observations. The SMA synthesized beam size was relatively large in the Mann et al. work (25 ∼ 1000 AU) compared with the ALMA beam (05 ∼ 200 AU). The emission probed by the ALMA data is sufficiently compact compared to the beam size that it is not likely to be contaminated severely by background emission. Moreover, the many additional ALMA Cycle 0 baselines provide much better spatial frequency coverage, leading to greater image fidelity and allowing a better separation of the disk emission from the background cloud.

Disk mass estimates and upper limits were then derived from the estimated F_dust values assuming the standard optically thin isothermal relationship (e.g., Beckwith et al. 1990),

$$\begin{equation} M_{\rm disk} = \frac{F_{\rm dust}d^2}{\kappa _{\nu }B_{\nu }(T)}, \end{equation} \tag{ 1 }$$

where d = 400 pc is the distance to Orion (Sandstrom et al. 2007; Menten et al. 2007; Kraus et al. 2007, 2009), κ_ν = 0.034 cm² g⁻¹ is the Beckwith et al. (1990) dust grain opacity at 856 μm with an implicit gas-to-dust mass ratio of 100:1, and B_ν(T) is the Planck function. We assume a typical disk temperature of 20 K, as in previous disk surveys of Taurus and Ophiuchus by Andrews & Williams (2005, 2007) and the ONC by Mann & Williams (2009a, 2010), for ease of comparison. Disk continuum emission can deviate from the optically thin limit if the column densities are especially large, an issue that was discussed in detail by Andrews & Williams (2005). The typical disk structure that could produce the observed 856 μm flux densities in Orion would imply the brighter disks could have up to ∼10%–25% of their emission being optically thick (see Figure 20 of Andrews & Williams 2005), resulting in an underestimation of their disk masses.

An overall disk mass sensitivity for the survey was determined by measuring the fraction of sources that could be detected at ⩾3σ as a function of M_disk, depending on the varying levels of free–free emission and target locations within each field. We find that the observed fields are 100% complete for M_disk ⩾ 1.2 M_jup (1 M_jup = 9.5 × 10⁻⁴ M_☉) and 50% complete for M_disk ⩾ 0.4 M_jup. For comparison, the SMA survey at 880 μm was 100% complete for M_disk ⩾ 8.7 M_jup (Mann & Williams 2010), ∼ 7× higher than the ALMA data.

We were able to determine disk masses for all 21 proplyd detections, as there was sufficient dust emission in excess of the free–free contamination (see Table 2). In addition to the 22 HST-identified proplyds surveyed, we observed 14 sources from the ACS Survey of the HST Treasury Program (Robberto et al. 2013; Ricci et al. 2008), 10 sources from the CTIO/Blanco 4 m near-infrared survey by Robberto et al. (2010), and 2 massive stars, θ¹ Ori C and θ¹ Ori F (see Table 3). Only two of the non-proplyd sources were detected, including a newly discovered disk around 113–438 and the recovery of the disk around 253–1536b, which was originally discovered through SMA imaging (Mann & Williams 2009b). The ALMA observations place stringent (3σ) upper limits of 0.1–0.6 M_jup on the disk masses for the undetected targets (see Table 3). These disks, if they exist, must be not only low in mass, but are likely smaller than ∼015 (∼60 AU; Bally et al. 2000; Vicente & Alves 2005) to be unseen in the HST images. No dust emission was detected toward the massive stars in this survey, θ¹ Ori C (spectral type O6) and θ¹ Ori F (spectral type B8). In computing upper limits of ∼0.12 M_jup for these targets, we adopted a higher dust temperature of 40 K (see Beuther et al. 2002; Sridharan et al. 2002). The disk-to-stellar mass ratio for these massive stars is ≲0.12 M_jup/40 M_☉ ∼ 3 × 10⁻⁶, significantly lower than the typical range of 10⁻¹ to 10⁻³ for T Tauri and HAeBe stars, implying that either massive stars do not form with disks or that their disks have much shorter lifetimes (Williams & Cieza 2011).

The giant silhouette disk 114–426 was detected for the first time at submillimeter wavelengths with these ALMA observations. This disk has been one of the most puzzling objects in Orion, since it is the largest and most prominent optical disk in the entire ONC, but it had never been detected at long wavelengths (Bally et al. 1998b; Eisner & Carpenter 2006; Eisner et al. 2008; Mann & Williams 2010). An 1100 AU disk seen nearly edge-on in HST images (see Figure 3), 114–426 is found to have a surprisingly low flux of 7 mJy, over an order of magnitude less than the other giant silhouette disk in Orion, 216–0939 (∼95 mJy; see Table 2). The nature of this interesting disk will be the subject of a forthcoming article from our team (J. Bally et al., in preparation).

3.1. Disk Masses and Distance from θ¹ Ori C

Figure 5 shows the disk masses (and flux densities at 856 μm) in the ONC as a function of their projected distance from the massive star θ¹ Ori C, including the previous SMA results from Mann & Williams (2010) to fill in the intermediate distances not yet probed with ALMA. The 3σ upper limits for both surveys are indicated as gray arrows. All of the known proplyds in the central field, i.e., within 9'' of θ¹ Ori C (∼0.02 pc), are included. This plot reveals clearly that disk masses tend to be substantially lower when they are located closer to θ¹ Ori C. Using the correlation tests described by Isobe et al. (1986), including the Cox Hazard Model, Generalized Kendall's Tau, and Spearman's Rho tests, that make use of the combined censored data set quantitatively confirm this trend, where the probability of no correlation between disk mass and projected distance to θ¹ Ori C is <10⁻⁴ for all three tests. This provides a strong confirmation of the same relationship that was tentatively noted in the previous SMA survey data, particularly for the most massive disks (Mann & Williams 2010).

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We observed the 856 μm continuum emission toward 48 young stars in the ONC using ALMA in Cycle 0, including 22 HST-identified proplyds. With an overall 3σ survey sensitivity limit of ∼1.2 M_jup, we detected 23 disks (∼48%), including 9 that had not been detected previously. Aside from the disks around 253–1536b and 113–438 (see Table 3), these detections coincide with the optically discovered disks from HST observations, highlighting the sensitivity of the space telescope to ONC disks due to their contrast with the bright nebular background. The giant silhouette disk 114–426 was detected for the first time, and has a low flux density of 7 mJy, and an estimated mass of 3.9 M_jup (this disk will be the subject of a forthcoming article; J. Bally et al., in preparation). Using this ALMA survey and the results of previous observations with the SMA, we find clear, statistically significant evidence for a marked decrease in the 856 μm disk luminosities of the Orion proplyds that have smaller projected separations from the massive star θ¹ Ori C. In the assumption that the emission is optically thin, and the dust temperature and opacity are the same for all the disks, this implies that the masses of the Orion proplyds decrease for those disks located near θ¹ Ori C.

The origins of that latter relationship could potentially be due to projection artifacts, initial conditions, and/or real evolutionary effects. The true separations between the proplyds and θ¹ Ori C are not known, but one can make a probabilistic argument that relates the projected separations to the true ones for an assumed distribution of orbital eccentricities around the ONC center of mass (cf. Torres 1999). For a uniform eccentricity distribution, the projected and true separations should be commensurate within a factor of ∼2; for a steeper eccentricity distribution, the projected separations represent a more biased tracer of the true values and could be considered lower limits. Such shifts in the abscissae of Figure 5 would not explain the lack of disks around the targets with very close projected separations from θ¹ Ori C, nor do they seem likely to be large enough to erase the overall trend (although they indeed may adjust the basic shape). An intrinsic correlation between the masses of disks and their stellar hosts (as found in the Taurus region by Andrews et al. 2013) could account for the observed trend in Figure 5 if the least massive stars are preferentially located near θ¹ Ori C. Unfortunately, the nature of the Orion proplyds makes a direct determination of their stellar masses exceedingly difficult. Hillenbrand & Hartmann (1998) argued that stellar mass segregation in the ONC works in the opposite sense, with the highest mass stars (⩾5 M_☉) concentrated toward the cluster center. If that were the case, we should have identified an anti-correlation between disk mass and distance from θ¹ Ori C, which is clearly not observed. High optical depths could be responsible for such a correlation, if the disks located near θ¹ Ori C are smaller than the distant disks, and most of their emission comes from optically thick regions. However, no correlation has been observed between disk size and distance from θ¹ Ori C (Vicente & Alves 2005). Furthermore, a small (∼50 AU; the resolution of HST), completely optically thick disk would be detectable by our sensitive ALMA observations, with a flux density of ∼55 mJy if viewed face-on, and an order of magnitude lower, ∼5.5 mJy, if viewed nearly edge-on, suggesting the submillimeter wave optical depths are not responsible for the observed correlation.

Instead, the evidence suggests that an externally driven disk evolution factor is likely responsible for the behavior in Figure 5. Tidal stripping by stellar encounters is not only too inefficient for substantial disk destruction in the ONC (Scally & Clarke 2001; Hollenbach et al. 2000), but, as Mann & Williams (2009b) argued, the conditions required for disk–disk interactions to deplete disk masses (e.g., Olczak et al. 2006) also implicitly involve very high photoevaporation mass-loss rates. Overall, the data suggest that photoevaporative mass-loss driven by the ultraviolet radiation from θ¹ Ori C is the most dominant process responsible for the observed relationship.

Theoretical models of disk photoevaporation indeed predict mass-loss rates that decrease with distance from the irradiation source (Johnstone et al. 1998; Störzer & Hollenbach 1999; Richling & Yorke 2000; Scally & Clarke 2001; Matsuyama et al. 2003; Adams et al. 2004). These models suggest that only low-mass (≲few M_jup) disks should exist within ∼0.01–0.03 pc of θ¹ Ori C because of the strong extreme-UV (EUV) irradiation at those distances (Johnstone et al. 1998; Störzer & Hollenbach 1999; Adams et al. 2004). At larger separations, ∼0.03–0.3 pc, less energetic far-UV photons dominate the radiation field, resulting in lower mass-loss rates and thereby preserving more massive disks for up to a few Myr (e.g., Adams et al. 2004).

This predicted behavior is consistent with the observations in the context of Figure 5. There is a clear lack of massive disks (≳3 M_jup) within 0.03 pc of θ¹ Ori C where EUV irradiation dominates, whereas we find a wide range of disk masses (similar to what is found in low-mass star formation regions) at larger projected separations in the less destructive FUV-dominated regime. Accordingly, the potential to form a planetary system like our own in the EUV-dominated region of the ONC seems unlikely, given the substantially depleted disk masses there. If these nearby disks have not formed planets already, they may be out of luck unless dust grains have grown very large in these disks, to sizes not probed by submillimeter wavelength observations. Resolved, multi-wavelength observations of the Orion proplyds are required to investigate how far planet formation has already progressed in these young disks. It is interesting to note, however, that the fraction of disks with masses that exceed the nominal "Minimum Mass Solar Nebula (MMSN)" model (∼10 M_jup; Weidenschilling 1977) in the more distant FUV-dominated region of the ONC is essentially the same as that found in the low-mass star formation environment of Taurus (∼10%; Andrews et al. 2013).⁹ Overall, these observations support the idea that the strength of the local EUV irradiation field has profound environmental consequences on the potential for giant planet formation in the centers of massive star-forming regions.

In ALMA Cycle 1, we expect to observe the disks around 300 stars in the ONC, including 160 HST-identified proplyds. This larger scale study will allow us to survey disks across a range of distances out to 1.6 pc from θ¹ Ori C, to probe different conditions in this massive star forming environment and uncover the overall disk fraction and the potential for forming planetary systems like our own.

This paper makes use of the following ALMA data: ADS/JAO.ALMA#2011.0.00028.S. ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada) and NSC and ASIAA (Taiwan), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.