High-resolution Pan-STARRS and SMA Observations of IRAS 23077+6707: A Giant Edge-on Protoplanetary Disk

Pan-STARRS (Chambers et al. 2016) is a system of two optical 1.8 m telescopes (PS1 and PS2), and the observations of IRAS 23077+6707 presented here were performed with the PS1 Gigapixel Camera, covering a field of view of ≈7 deg², a pixel scale of 0257, and median seeing values of 1''–13 for the grizy_P1 broadband filters (spanning λ_mean = 4866–9633 Å). Stacked images of IRAS 23077+6707 (R. A. = 23^h09^m43645 and $\mathrm{decl}.\,=\,+67^\circ 23^{\prime} 38\buildrel{\prime\prime}\over{.} 94$) were retrieved from the first data release of PS1 (Flewelling et al. 2020; STScI 2022). ⁷ These were then used to produce the RGB-composite image shown in Figure 1 (left) where the i-, r- and g-bands (λ_mean = 7545, 6215, and 4866 Å) are represented by red, green, and blue colors respectively, and with equal image intensity weighting.

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IRAS 23077+6707 shows the typical morphological features of an edge-on protoplanetary disk, namely two elongated bright reflection nebulae (tracing the upper layers of the disk atmosphere), separated by a dark lane (tracing the disk midplane) that obscures the light emitted by the central star. The scattered-light emission of the disk dark lane has an angular extent of ≈14'' and a position angle of 338° (from north, counterclockwise). Further, two faint filaments that likely trace the arcs of the disk's flared upper layers extend northwards by almost 9''. We see no evidence of a jet nor any obvious signatures of envelope material, which are both typically associated with early-stage Class 0 and Class I young stellar objects (YSOs; see Williams & Cieza 2011 for a review), thus likely suggesting a more evolved evolutionary stage for IRAS 23077+6707.

IRAS 23077+6707 exhibits three prominent asymmetries that we derive in detail in the Appendix. First, its western lobe is brighter than its eastern lobe by a factor of 6, typical for disks with sky-projected inclinations that are not perfectly edge on (due to preferential grain scattering), for which a simple visual comparison with the models of Watson et al. (2007; see their Figure 4, in which the disks shown in the top row display brightness variations for different disk inclinations) implies the disk is likely inclined to the plane of the sky by ≈80°. Second, in the eastern (fainter) lobe, the southern region is brighter than the northern one by a factor of 3, whereas in the western (brighter) lobe, the northern and southern region brightnesses are within 1%. Lateral asymmetries can be a result of foreground extinction preferentially dimming the northeast of the disk. However, they could also imply a misaligned, inner disk or a warp that would cast shadows on the observed disk (see Section 3.3).

Third, while the northern extent has two prominent filaments, the southern side hosts none. These filaments extend northwards beyond the edge of the dark lane by ≈9''. The publicly available Digitized Sky Survey images of IRAS 23077+6707 also resolve the disk and likewise demonstrate similar asymmetries (in DSS1, DSS2-blue, DSS2-red, and DSS2-infrared), which we thus deem a physical feature of the disk (Minkowski & Abell 1963; Lasker et al. 1996). The presence of such a filament asymmetry could imply that the outermost disk layers (or possibly a disk wind) are truncated or perhaps angled backwards, plausibly due to interactions with a nearby star and/or the interstellar medium or from asymmetric ionization/wind launching from the central star.

2.2.1. Data Reduction and Calibration

2.2.2. Visibility Modeling and Imaging

"Corrected" data for IRAS 23077+6707 were extracted using the CASA mstransform task, in which we time averaged our data by 20 s. For the continuum emission, we rebinned these corrected data to four channels per 2 GHz spectral window (appropriate for continuum analysis), whereas for line emission analysis, we split these corrected data into the 12 separate SMA spectral windows (with no channel averaging) and transformed these to the local standard of rest frame (LSRK).

We measure the total 1.3 mm flux of IRAS 23077+6707 as F_{1.3 mm} = 44.0 ± 2.5 mJy by fitting a Gaussian model to IRAS 23077+6707's calibrated visibilities with the CASA package uvmodelfit (with ten iterations to ensure model convergence, where the error incorporates a standard SMA 5% flux calibration error in quadrature with the fitting uncertainty). This fit derives a position angle of 342° and a small phase offset (−02 in R.A. and 065 in decl.). By instead fitting the same visibility model to the upper and lower sidebands of the continuum visibilities independently (between 209.5–221.5 GHz and 229.5–241.5 GHz), we found best-fit fluxes of 37.2 ± 3.4 mJy and 52.6 ± 3.8 mJy. This results in a spectral index of α = 3.9 ± 1.3 (for $\alpha =\partial \mathrm{log}{F}_{\nu }/\partial \mathrm{log}\nu$), which is at the higher end of millimeter spectral indices measured for other inclined disks (e.g., Ribas et al. 2017; Villenave et al. 2020). Our value is consistent with either optically thin or optically thick emission, but due to the significantly shorter frequency range probed by our observations, our estimate of α carries substantial uncertainties, and a direct comparison to these previous studies is not yet possible. Higher-resolution observations with wider frequency baselines would be needed in order to discern between optically thin or optically thick dust in IRAS 23077+6707.

Standard imaging of the continuum data was conducted with the tclean algorithm to a 2σ threshold (∼1.0 mJy), using an image-centered 16'' × 8'' elliptical mask (fully covering the disk), and a Briggs robust parameter of 0. The remaining panels of Figure 1 show the resulting continuum emission overlaid as contours on the ¹²CO, ¹³CO, and C¹⁸O velocity-integrated intensity maps (or "moment 0 maps," which we will discuss further below), presenting an elongated morphology with a consistent position angle of ≈340° with the scattered-light emission, typical for an EOD imaged at relatively low resolution where the 1.3 mm emission traces the cooler midplane dust. We deem the low axis ratio of the continuum extent (major/minor axis disk extent) to be dominated by the 36 × 24 beam roughly aligned along the disk minor axis with a beam position angle (BPA) of 23°.

We subtract the continuum emission from each channel in our CO line measurement sets (for the spectral windows containing the ¹²CO, ¹³CO, and C¹⁸O) using the CASA package uvcontsub with a linear fit to all regions excluding the channels with CO emission (±50 MHz). We apply standard imaging on each CO line separately with the tclean algorithm (to a clean threshold of 2σ ≈ 0.1 Jy) using an image-centered 16'' × 8'' elliptical mask and a Briggs robust parameter of 0) to produce 60-channel-wide LSRK-frame data cubes, with the line emission spanning the central 10–20 channels. All cubes present spatially and spectrally resolved line emission at IRAS 23077+6707's location, with the resulting cubes having beams with sizes and position angles of 39 × 25, BPA = 21° (¹²CO), 40 × 26, BPA = 17° (¹³CO), and 41 × 27, BPA = 17° (C¹⁸O).

In the top right and bottom panels of Figure 1, we present the moment 0 maps for the CO emission lines, generated with bettermoments (Teague & Foreman-Mackey 2018) using a 2σ emission clip that is integrated only from those channels of the data cube that span the full line width (which ranges from −2.4 to 5.6 km s⁻¹; see Figure 2). All three gas lines are clearly detected, with disk emission (detected at a 3σ level) covering more than ∼14'' for both ¹²CO and ¹³CO and ∼12'' for C¹⁸O (i.e., three to four beams across the radial disk extent). Notably, while being significantly less abundant than ¹²CO, the extents of both the ¹³CO and C¹⁸O emissions along the radial and vertical directions are comparable to the ¹²CO emission, revealing this as a highly gas-rich system.

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With GoFish (Teague 2019) and a simple 22'' field-of-view cut centered on the respective line emission, we extract integrated line fluxes for the CO lines of ${F}_{{}^{12}\mathrm{CO}}=35.4\pm 1.8\,\mathrm{Jy}\,\mathrm{km}\,{{\rm{s}}}^{-1}$, ${F}_{{}^{13}\mathrm{CO}}=14.0\pm 0.8\,\mathrm{Jy}\,\mathrm{km}\,{{\rm{s}}}^{-1}$, and ${F}_{{{\rm{C}}}^{18}{\rm{O}}}=8.6\pm 0.5\,\mathrm{Jy}\,\mathrm{km}\,{{\rm{s}}}^{-1}$ (which we report here and in Table 1 with an additional 5% flux uncertainty in quadrature with the GoFish fitting error). This results in flux ratios of ${F}_{{}^{12}\mathrm{CO}}/{F}_{{}^{13}\mathrm{CO}}\approx 2.5$ and ${F}_{{}^{13}\mathrm{CO}}/{F}_{{{\rm{C}}}^{18}{\rm{O}}}\,\approx 1.6$. As expected for disks in Keplerian rotation, the line profiles for all three CO isotopologues as shown in Figure 2 are double peaked, with velocity centroids of 1.6, 1.7, and 1.6 km s⁻¹ respectively. Out of these, the ¹²CO line profile is the only observed as asymmetric, with its redshifted (western) component being ≈2 × brighter than its blueshifted (eastern) component, which is most likely a consequence of the ¹²CO emission being optically thick, preventing us from observing as much CO on the disk rear side.

Table 1. Summary of Measured Properties for IRAS 23077+6707. All Reported Millimeter Fluxes Include a 5% Flux Uncertainty in Quadrature with Their Respective Fitting Errors

	Flux		R2σ	R3σ	Asymmetries
	(mJy)	(Jy km s−1)	(arcsec)	(arcsec)
Scattered-light (gri)	⋯	⋯	(3.8, 3.7, 3.5)	(5.7, 5.6, 5.3)	(1) East–west brightness asymmetry (≈6 × )
⋯	⋯	⋯	⋯	⋯	(2) North–south brightness asymmetry in east lobe (≈3 × )
⋯	⋯	⋯	⋯	⋯	(3) Two filaments extending northwards by ≈9''
cont.	44.0 ± 2.5	⋯	4.7	7.1	⋯
12CO	⋯	35.4 ± 1.8	4.9	7.3	(4) West–east (blue–redshifted) line flux asymmetry (≈2 × )
13CO	⋯	14.0 ± 0.8	4.7	7.0	⋯
C18O	⋯	8.6 ± 0.5	4.5	6.7	⋯

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In Figure 3, we further show position–velocity (PV) diagrams for all three CO J = 2 − 1 lines, extracted with pvextractor (Ginsburg et al. 2016), using 22'' long and 6'' wide cuts (which were chosen in order to encompass the 5σ contour of the ¹²CO emission along the disk minor axis), oriented along a position angle of 342° and referenced to each of their respective line velocity centers, as well as the center of each cut. All of these PV diagrams present the expected double-peaked ("dumbbell"-like) emission morphology of an inclined disk in Keplerian rotation.

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Our observations present compelling evidence that the giant bipolar reflection nebula, IRAS 23077+6707, is a protoplanetary disk that is viewed nearly edge on. While its size is extreme, with IRAS 23077+6707 hosting a protoplanetary disk that subtends the largest known angular scale on the sky, what is perhaps most remarkable is that this source has until now gone unreported in the scientific literature. We note that in the recent work of Angelo et al. (2023) it was reported that the number count of EODs falls far short of simulated disk populations in the galaxy. The serendipitous discovery of IRAS 23077+6707 therefore suggests that there may be many more EODs awaiting discovery.

Nevertheless, decades of all-sky mapping, alongside detailed local galaxy studies, have reported unresolved detections of dust and gas associated with IRAS 23077+6707 (for example, at moderate angular resolution with the 30 m telescope of the Institut de radioastronomie millimétrique, IRAM; the Wide-field Infrared Survey Explorer, WISE; and the Two Micron All Sky Survey, 2MASS; see Wouterloot & Brand 1989; Wouterloot et al. 1993; Skrutskie et al. 2006; Cutri et al. 2021). Using these all-sky data, as well as employing the spectral slope expression pioneered by Lada (1987) and Adams et al. (1987),

$$\begin{eqnarray}&&{\alpha }_{\mathrm{IR}}=\displaystyle \frac{\partial \mathrm{log}\nu {F}_{\nu }}{\partial \mathrm{log}\nu },\end{eqnarray} \tag{ 1 }$$

we determine IRAS 23077+6707 to have mid-IR spectral slopes of α_{2–12 μm} = − 0.21 and α_{2–22 μm} = 0.1 between the 2MASS K-band data point (2.16 μm, 39.2 ± 0.8 mJy) and either the WISE W3 (12 μm, 310.0 ± 3.1 mJy) or WISE W4 (22 μm, 318.0 ± 5.3 mJy) photometry. While these values would ordinarily place a disk in the spectral slope range defining a Class F YSO (i.e., an intermediate Class I–II source; see Greene et al. 1994; Williams & Cieza 2011), IRAS 23077+6707's edge-on morphology may suggest it is instead more evolved due to the emission being highly self-extincted at these near- and mid-infrared wavelengths (see Chiang & Goldreich 1999; Robitaille et al. 2006). Indeed, since Class II YSOs viewed edge on present SEDs more consistent with Class I and F sources, we deem IRAS 23077+6707's photometry as evidence for it instead being a Class II YSO and thus host to a mature protoplanetary disk, which is consistent with the scattered-light imaging that is clearly dominated by disk emission.

Figure 4 shows the radial intensity profiles of the scattered light in the left panel (each normalized to the peak of the g-band intensity) and the 1.3 mm continuum, as well as the CO J = 2 − 1 moment 0 maps in the right panel. Each profile reflects the emission of a single pixel lane along the disk major axis (i.e., midplane), either at a position angle of 338° (PS1) or 342° (SMA). ⁹ While the first approximate radii at which the radial profiles of our data are consistent with noise are 81, 77, 79, 91, 84, 87, and 85 (for the irg scattered-light, millimeter continuum, and progressively fainter CO lines respectively), convolution with either the PS1 PSF or SMA clean beams (with their respective FWHMs ¹⁰ shown in the upper left corners of Figure 4) suggests we are likely overestimating the disk's true angular extent. As such, we fit 1D Gaussian profiles using a least-squares method in order to provide an estimate for the scale on which dust and gas are plausibly present in the disk. These 1D Gaussian profiles offer a simple but reasonable approximation of the disk emission and thus allow us to estimate where the physical disk emission plausibly extends to, with these having deconvolved 2σ–3σ radii of 38–57, 37–56, 35–53, 47–71, 49–73, 47–70, and 45–67, respectively (see Table 1 for a summary of all measured values). We nevertheless note that the profiles, especially in the case of the scattered light, the continuum, and the ¹²CO emission, host long tails that are not well fitted by a simple Gaussian model, and consequently this Gaussian width approximation likely underestimates the disk's true physical size.

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In summary, we find it highly plausible that the total extent of the disk's (sub)millimeter emission is likely tracing dust and gas out to radii of at least ∼7'', far larger than any other reported protoplanetary disks. At the distance to the Cepheus star-forming region (spanning a range of 180–800 pc; see, e.g., Kun et al. 2008, 2009; Szilágyi et al. 2021; Kerr et al. 2023) to which IRAS 23077+6707 is likely associated, such large radii would suggest that IRAS 23077+6707's disk thus spans 1000 s of astronomical units in size.

What presents an interesting finding is that at the resolution of our observations, both IRAS 23077+6707's dust continuum and gas emission span very similar spatial scales. In other young planet-forming disks, the difference in gas-to-disk radii ratio is typically on the order of at least a factor of 2 or more (see Birnstiel & Andrews 2014; Ansdell et al. 2018; Facchini et al. 2019; Boyden & Eisner 2020; Trapman et al. 2023). While this suggests an immense reservoir of dust and gas present in IRAS 23077+6707, higher-resolution data are needed to test if this disk indeed corresponds to an outlier among previously studied protoplanetary disks.

In addition, IRAS 23077+6707 is 4.3 × brighter in ¹²CO (2–1), 5.7 × brighter in ¹³CO (2–1), and over 9.4 × brighter in C¹⁸O (2–1) than HD 34282 (SpT A3; see Merín et al. 2004), which is the brightest disk studied in a recent SMA survey of chemistry in Herbig Ae/Be disks (at similar resolution to our observations) performed by Pegues et al. (2023). In contrast, IRAS 23077+6707's 1.3 mm continuum emission is not only fainter than HD 34282 by a factor of ∼3 but also fainter than all other disks studied by Pegues et al. (2023). This may be a direct consequence of the different disk inclinations (e.g., i ≲ 60° for the disks in Pegues et al. 2023) and possibly the 1.3 mm continuum emission also being optically thick for IRAS 23077+6707.

Only IRAS 23077+6707's most well-known counterpart, Gomez's Hamburger (SpT A0; see Ruiz et al. 1987), measures a similarly high integrated intensity for the CO gas, with 37.2 Jy km s⁻¹ for ¹²CO and 16.5 Jy km s⁻¹ for ¹³CO (Teague et al. 2020), while having a significantly higher continuum flux of 293 mJy compared to the 44 mJy we measure from our observations (despite the similarly high inclination of ∼86° for Gomez's Hamburger), which could indicate a higher gas-to-dust ratio in IRAS 23077+6707.

In summary, at this point it remains unclear what is driving these differences in the dust and gas properties of IRAS 23077+6707, and observations at comparable resolution and sensitivity to these previous disk studies are needed for more direct comparisons to similar Herbig Ae/Be systems.

The flux asymmetries that are observed in the PS1 image of IRAS 23077+6707, with the western lobe being brighter than its eastern one by a factor of 6, may be produced through different physical processes. Most likely, it is simply a geometric effect as a consequence of the disk not being observed perfectly edge on, as previously discussed in Section 2.1. However, also perfectly edge-on systems have been found to display brightness asymmetries that may be possibly rooted in physical processes.

For instance, the formation of brightness asymmetries may happen in the earliest stages of a YSO during the late infall of gas after the initial collapse phase of the forming star (e.g., Kuffmeier et al. 2021) or even during the Class II phase (see Ginski et al. 2021). Also, there is growing evidence that brightness asymmetries may have a planetary origin, either due to localized gravitational instabilities as a result of ongoing giant planet formation (e.g., Morales et al. 2019), misaligned inner disks casting time-variable shadows on the outer disk due to warps created by super-Jupiters (e.g., Nealon et al. 2018; Debes et al. 2023), or binary companions (Facchini et al. 2018).

Indeed, in the PS1 data, IRAS 23077+6707 shows striking similarity with the simulated scattered-light images presented by Nealon et al. (2019; see their Figure 6). These authors found that when a planet is massive enough to carve a gap and separate the inner from the outer disk (which is naturally the case for giant planets; see Lin & Papaloizou 1986), warps will form across the planetary orbit, resulting in a misalignment between the inner and outer disks (Xiang-Gruess & Papaloizou 2013; Nealon et al. 2018). Inner–outer disk misalignments of less than a degree are already able to cast shadows at larger stellocentric radii, resulting in brightness asymmetries that can be readily observed in scattered-light emission in addition to the brightness asymmetry induced by the nearly edge-on orientations.

With strong resemblance to the factor 3 brightness asymmetry along IRAS 23077+6707's eastern lobe, Villenave et al. (2024) have very recently identified a lateral brightness symmetry (with a switch in brightness at 12.8 and 21 μm) in the edge-on Class I system IRAS 04302+2247, which they attribute to a tilted inner disk. In addition, they uncover similar lateral asymmetries in optical and infrared scattered-light emission in 15 out of 20 EODs, suggesting that a significant fraction of protoplanetary disks may be hosts to tilted inner regions. It is thus likely that also the presence of the lateral asymmetry in IRAS 23077+6707's eastern lobe may hint at the presence of a misaligned inner disk.

Regardless, whichever precise mechanism is driving the asymmetry in IRAS 23077+6707's disk, only higher-angular-resolution scattered-light observations in optical and especially near- to mid-infrared wavelengths (e.g., in order to identify the switch in brightness due to an inner tilted disk—see Villenave et al. 2024; or the identification of an eccentric circumbinary disk producing similar brightness asymmetries, as, e.g., for IRAS 04158+2805—see Andrews et al. 2008 and Ragusa et al. 2021) will be able to provide constraints on the possible origins of the asymmetries observed in IRAS 23077+6707.