ALMA 0.88 mm Survey of Disks around Planetary-mass Companions

As shown in Figure 1, we do not convincingly detect any PMC disks in the 0.88 mm dust continuum (dashed circles labeled as "b"). Table 3 lists the measured flux levels of PMCs. The 3σ upper limits for the 0.88 mm flux density span 120–210 μJy assuming they are point sources. On the other hand, disks around the three host stars CT Cha A, DH Tau A, and FU Tau A are clearly seen. To our knowledge, this is the first millimeter detection of FU Tau A.

Table 3.  Coordinates and 0.88 mm Flux Densities of the Host Stars, PMCs, and Unknown Sources

Object	α	δ	Fν (mJy)
CT Cha	${11}^{{\rm{h}}}$04m0900	−76°27'193	94.0 ± 9.4
CT Cha b	${11}^{{\rm{h}}}$04m0834	−76°27'180	0.019 ± 0.052
1RXS 1609	${16}^{{\rm{h}}}$09m3030	−21°04'593	0.007 ± 0.041
1RXS 1609 b	${16}^{{\rm{h}}}$09m3037	−21°04'574	0.011 ± 0.045
ROXs 12	${16}^{{\rm{h}}}$26m2803	−25°26'482	0.078 ± 0.070
ROXs 12 b	${16}^{{\rm{h}}}$26m2805	−25°26'464	0.098 ± 0.071
ROXs 42B	${16}^{{\rm{h}}}$31m1501	−24°32'441	−0.001 ± 0.043
ROXs 42B b	${16}^{{\rm{h}}}$31m1493	−24°32'441	−0.002 ± 0.043
DH Tau	04h29m4156	+26°32'577	52.3 ± 5.2
DH Tau b	04h29m4168	+26°32'560	0.017 ± 0.041
FU Tau	04h23m3540	+25°03'024	0.53 ± 0.09
FU Tau b	04h23m3575	+25°02'592	0.033 ± 0.039
SMM J042335.9+250255	04h23m3591	+25°02'552	0.40 ± 0.09
SMM J160929.9−210457	16h09m2989	−21°04'570	1.86 ± 0.21
SMM J163115.2−243247	16h31m1518	−24°32'467	0.42 ± 0.10

Note. The coordinates are derived for the epochs of the ALMA observations (Table 1). Flux uncertainties of the detected sources include a 10% absolute flux calibration error.

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We use the software package pdspy (Sheehan et al. 2019) to fit an elliptical Gaussian to the visibilities and derive 0.88 mm continuum fluxes of 94.0 ± 9.4, 52.3 ± 5.2, and 0.53 ± 0.09 mJy for CT Cha A, DH Tau A, and FU Tau A, respectively. Similar values were also obtained using the CASA task uvmodelfit when the primary beam response is accounted for. Flux errors are generally dominated by a 10% uncertainty in the ALMA absolute flux calibration. Our results are consistent with previous 0.88 mm flux measurements of CT Cha A (104.78 ± 0.60 mJy; Pascucci et al. 2016) and DH Tau A (47 ± 4 mJy; Harris et al. 2012) at about a 1σ level. This discrepancy might simply arise from absolute flux calibration; free–free emission from ionized disk material is usually minor at 0.88 mm (e.g., Eisner et al. 2018).

Table 3 also lists the coordinates of the host stars and PMCs at the time of the ALMA observations as calculated from the Gaia DR2 coordinates, proper motions, position angles, and separations.

Several continuum sources in our maps seem to have no optical or infrared counterparts. Some of them could be dusty star-forming galaxies at high redshift, similar to the object spatially coincident with the debris disk of HD 95086 (Su et al. 2017). The object SMM J042335.9+250255 at $\alpha ={04}^{{\rm{h}}}$ 23^m3591 and δ = +25°02'552, ∼10'' southeast of FU Tau A, has a 0.88 mm flux of 0.40 ± 0.09 mJy. The source SMM J163115.2−243247 at α = 16^h31^m1518 and δ = −24°32'467, 35 southeast of ROXs 42B, has a flux of 0.42 ± 0.10 mJy. It shares a similar P.A. of about 138° to the background near-infrared source identified by Bryan et al. (2016) but with a wider separation from ROXs 42B (35 versus 3''; see Figure 1 of Bryan et al. 2016), an offset that cannot be accounted for by the star's proper motion (${\mu }_{\alpha }\,\cos \,\delta =-6.5\pm 0.2$ mas yr⁻¹ and ${\mu }_{\delta }=-23.2\pm 0.1$ mas yr⁻¹; Gaia Collaboration et al. 2018).

The bright source SMM J160929.9−210457 at α = 16^h09^m2989 and δ = −21°04'570, ∼6'' northwest of 1RXS 1609, was also detected in the 1.3 mm survey of Wu et al. (2017a). It has a flux density of 1.86 ± 0.21 mJy at 0.88 mm and 0.68 ± 0.17 mJy at 1.3 mm. The 1.3 mm flux density was derived using aperture photometry due to the irregular morphology, which made it difficult to reliably measure flux density using an elliptical Gaussian fit. Interestingly, it appears resolved into west and east components at 1.3 mm (Figure 2), which could represent two star-forming regions within a galaxy, or perhaps a pair of merging galaxies. Indeed, given flux-dependent number densities of such galaxies at 0.88 mm (Casey et al. 2014), ∼2.5 arcmin⁻² at 1.9 mJy and ∼25 arcmin⁻² at 0.4 mJy, we would expect to detect ∼1 and ∼10 galaxies, respectively, for six ALMA pointings (assuming an 18'' primary beam).

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Figure 3 shows the channel maps of the FU Tau system in the LSRK velocity frame. CO emission is seen from 6 to 10 km s⁻¹, however, it does not spatially coincide with either A or b. This suggests that it is probably not associated with both components, but is instead diffuse emission from the surrounding Barnard 215 dark cloud (Barnard et al. 1927). Although FU Tau A has active accretion, we do not find concrete evidence of gas emission from the disk in our data. The CO (2–1) emission detected by the IRAM-30 m telescope (Monin et al. 2013) is probably from this cloud emission as well, rather than a molecular outflow from FU Tau A.

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The dominant physical processes regulating the co-evolution of planets and their environments may leave observable imprints on the bulk properties of disks. Disk–host scaling relationships, along with their age and stellar-mass dependences, therefore have important implications for the observed exoplanet population (e.g., Andrews et al. 2013, 2018; Barenfeld et al. 2016; Pascucci et al. 2016; Ansdell et al. 2017). The behaviors of scaling relationships in the planet-mass regime provide clues about the physical properties of circumplanetary disks as well as the formation of satellites. Any deviations from the stellar relationships may imply that circumplanetary disks have different mass distributions, sizes, substructures, or evolutionary timescales. It may also imply that the co-evolution of satellites and circumplanetary disks are not self-similar to that of planets and protoplanetary disks.

Here we investigate the behavior of the disk-flux–host-mass (F_disk–M_⋆) relation toward the planetary-mass regime. We opt not to convert continuum emission to dust mass, as the observed millimeter spectral index of ${F}_{\mathrm{disk}}\propto {\lambda }^{-2.2}$ (e.g., Ribas et al. 2017) suggests that typical assumptions used to make the conversion (optically thin, opacity ∝λ⁻¹; e.g., Beckwith et al. 1990) may be incorrect. Recent radiative transfer disk modeling also finds that dust emission is generally optically thick for circum(sub)stellar disks (e.g., Ballering & Eisner 2019; Rab et al. 2019). In addition, small disks are likely warmer and optically thicker than large disks; as a result, dust masses derived from the optically thin assumption may be underestimated by a factor of 1–5 (e.g., Ballering & Eisner 2019). Finally, Liu (2019) and Zhu et al. (2019) demonstrated that an optically thick disk may look optically thin due to dust scattering.

Figure 4 shows the F_disk–M_⋆ relationship at 0.88 mm for young (<5 Myr; left panel) and older (5–20 Myr; right panel) systems. All the fluxes and upper limits in the figure have been scaled to a common distance of 140 pc via ${F}_{\mathrm{disk}}={F}_{\mathrm{disk}}\,\times {({\rm{distance}}/140{\rm{pc}})}^{2}$. PMC 3σ limits at 0.88 mm are either from this survey or previous ALMA observations (<200 μJy for GSC 6214-210 b, Bowler et al. 2015; <150 μJy for GQ Lup b, MacGregor et al. 2017; <78 μJy for 2M1207 b, Ricci et al. 2017). We also include the recent tentative detections of circumplanetary disks around PDS 70 b and c (73 ± 19 and 106 ± 19 μJy; Isella et al. 2019), with planet masses adopted from Keppler et al. (2018) and Haffert et al. (2019). Disk fluxes of stars and brown dwarfs are compiled from millimeter studies of star-forming regions, including Taurus (Andrews et al. 2013; Ricci et al. 2014; Ward-Duong et al. 2018), Chamaeleon I (Pascucci et al. 2016; Long et al. 2018), Lupus (Ansdell et al. 2016; Sanchis et al. 2020), and Upper Scorpius⁹ (Barenfeld et al. 2016; van der Plas et al. 2016). Sources with no Gaia DR2 distances or that are clearly foreground/background objects are excluded. Stellar masses are adopted from the literature, but dynamical masses derived from the Keplerian rotation of gas are used when available (Sheehan et al. 2019; Simon et al. 2019).

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To explore if PMC disks largely follow the overall trend for protoplanetary disks, we perform Bayesian linear regression to the stellar and brown dwarf disks (including nondetections) using the linmix package (Kelly 2007) and extrapolate the F_disk–M_⋆ relationship from the stellar to planetary regime. The best-fit relations with and without the Gaussian-distributed intrinsic scatter terms are plotted as the dark and light shaded regions. In the left panel, the slope, intercept, and scatter of log(F_disk) are ${1.54}_{-0.21}^{+0.21}$, ${1.88}_{-0.13}^{+0.14}$, and ${0.74}_{-0.07}^{+0.08}$ dex, respectively, where the uncertainties represent the 95% confidence intervals. We note that as shown in the Appendix and in Figure A1, Taurus disks seem to have a shallower F_disk–M_⋆ relation than that of the Chamaeleon I and Lupus disks. However, it is not clear whether this is of astrophysical origin or simply because few disks with M_⋆ < 0.1 M_☉ have been detected in Chamaeleon I and Lupus. The inhomogeneity of stellar-mass determinations could also potentially affect our linear regression analysis (e.g., Ward-Duong et al. 2018), but we do not expect this to have a strong impact on our results. For the Upper Sco disks in the right panel, the slope, intercept, and scatter are ${2.37}_{-0.71}^{+0.73}$, ${1.58}_{-0.43}^{+0.44}$, and ${0.67}_{-0.13}^{+0.19}$ dex, consistent with Ansdell et al. (2017).

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Figure 4 shows that young PMCs have distance-scaled 3σ limits of 100–300 μJy at 0.88 mm. Given that the majority of these PMCs have evidence of disks or mass accretion, yet appear absent between the best-fit relation and the upper envelope in the stellar regime (represented by the 95% confidence interval of the intrinsic scatter), it remains possible that the (presumably) brightest PMC disks are in fact underluminous and may have different bulk properties from their stellar counterparts. More sensitive observations are needed to test this hypothesis. Alternatively, as individual disks in young binary systems of ≲300 au separations have on average ∼5 times lower millimeter fluxes than that of single stars (e.g., Harris et al. 2012; Akeson et al. 2019), young PMC disks may instead be in agreement with the F_disk–M_⋆ relationship of binary stars. Deep ALMA continuum imaging with rms ≲10 μJy (≳5 hr on-source at 0.88 mm) will be needed to definitively test whether young PMC disks indeed deviate from the scaling relation of single stars.

In contrast, Barenfeld et al. (2019) recently showed that in the more evolved Upper Scorpius star-forming region (age ∼10 Myr; Pecaut et al. 2012), the millimeter fluxes of disks around single and binary stars are statistically indistinguishable. This implies that binarity unlikely plays an important role in subsequent disk evolution, and single and binary stars may follow the same F_disk–M_⋆ relationship at ages of >5 Myr. Current ALMA observations, however, are not able to determine if older PMC disks are discrepant from the Upper Sco correlation (right panel of Figure 4). A sensitivity of ≲1 μJy will be required, but this will be very challenging to achieve given the current capabilities of ALMA.

Nondetections from this survey as well as other programs have demonstrated that PMC disks appear faint in dust continuum emission. As the radial drift of dust grains around substellar objects can be much faster than around stars (e.g., Pinilla et al. 2013; Zhu et al. 2018), the dust disks of these companions are likely very compact and optically thick (e.g., Wu et al. 2017a; Rab et al. 2019). As optically thick emission scales with disk area, our 0.88 and 1.3 mm surveys imply that these dust disks have radii <0.5 au (see Wu et al. 2017a). It is also possible that dust grains in PMC disks have been highly depleted due to this rapid inward drift. This could potentially hinder satellite formation unless there is a dust trap in the disk to form satellite seeds (e.g., Dra̧żkowska & Szulágyi 2018).

While a compact and/or dust-depleted disk is faint in radio continuum, Rab et al. (2019) find that the gas disk may still extend to a one-third Hill radius, which is the characteristic scale of companion disks under dynamical interaction (e.g., Ayliffe & Bate 2009) and is typically ≳10 au for wide-orbit PMCs at hundreds of astronomical units from their hosts. The synthetic observations in Rab et al. (2019) further show that such a large gas-rich disk can be detected by ALMA with ≳5 hr on-source time in Bands 6 and 7. PMC disks may therefore exhibit both extremely high gas-to-dust mass and size ratios due to a combination of tidal truncation, fast radial drift, and/or a lack of dust traps. As a comparison, for the Lupus circumstellar disks the mass ratio is mostly 1–10 (Miotello et al. 2017), and the size ratio is ∼2 on average (Ansdell et al. 2018).

Interestingly, free-floating planetary-mass objects, which might have formed like single stars, may be able to retain larger and brighter dust disks. For instance, the isolated 13 M_Jup object OTS 44 has a 1.3 mm flux of ∼100 μJy (Bayo et al. 2017), which translates to ∼230 μJy at 0.88 mm with a spectral index of 2.2. This implies that the radial drift timescale is intrinsically longer than that of PMC disks, or there are disk substructures to trap dust grains at a wider radius. Future sensitive ALMA observations can examine if free-floating planets, as a population, also follow the stellar F_disk–M_⋆ relationship.