An ALMA Survey for Disks Orbiting Low-Mass Stars in the TW Hya Association
David R. Rodriguez, Gerrit van der Plas, Joel H. Kastner, Adam C. Schneider, Jacqueline K. Faherty, Diego Mardones, Subhanjoy Mohanty, David Principe
AAstronomy & Astrophysics manuscript no. paper_v6 c (cid:13)
ESO 2018October 18, 2018
An ALMA Survey for Disks Orbiting Low-Mass Stars in the TW HyaAssociation
David R. Rodriguez , Gerrit van der Plas , , Joel H. Kastner , Adam C. Schneider , Jacqueline K. Faherty , , DiegoMardones , Subhanjoy Mohanty , and David Principe , Departamento de Astronomía, Universidad de Chile, Casilla 36-D, Santiago, Chile e-mail: [email protected] Millennium Nucleus Protoplanetary Disks, Chile Center for Imaging Science, School of Physics & Astronomy, and Laboratory for Multiwavelength Astrophysics, Rochester Instituteof Technology, 54 Lomb Memorial Drive, Rochester, NY 14623, USA Department of Physics and Astronomy, University of Toledo, 2801 W. Bancroft St., Toledo, OH 43606, USA Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road NW, Washington, DC 20015,USA Hubble Fellow Imperial College London, 1010 Blackett Lab., Prince Consort Road, London SW7 2AZ, UK Núcleo de Astronomía, Facultad de Ingeniería, Universidad Diego Portales, Av. Ejercito 441, Santiago, ChileOctober 18, 2018
ABSTRACT
We have carried out an ALMA survey of 15 confirmed or candidate low-mass ( < . M (cid:12) ) members of the TW Hya Association (TWA)with the goal of detecting molecular gas in the form of CO emission, as well as providing constraints on continuum emission due tocold dust. Our targets have spectral types of M4-L0 and hence represent the extreme low end of the TWA’s mass function. Our ALMAsurvey has yielded detections of 1.3mm continuum emission around 4 systems (TWA 30B, 32, 33, & 34), suggesting the presenceof cold dust grains. All continuum sources are unresolved. TWA 34 further shows CO(2–1) emission whose velocity structure isindicative of Keplerian rotation. Among the sample of known ∼ / disk systems, TWA 34, which lies just ∼
50 pc fromEarth, is the lowest mass star thus far identified as harboring cold molecular gas in an orbiting disk.
Key words. open clusters and associations: individual(TWA) — protoplanetary disks — stars: evolution — stars: pre-main sequence
1. Introduction
Beginning with the identification of the TW Hya Association(TWA; Kastner et al. 1997) several stellar associations with ages ∼ ∼ ∼ ∼ ∼
50 pc; Kastner et al. 1997),V4046 Sgr ( ∼
20 Myr, ∼
70 pc; Kastner et al. 2008), MP Mus( ∼ ∼
100 pc; Kastner et al. 2010), and T Cha ( ∼ ∼
110 pc; Sacco et al. 2014). All of these are roughly solar-mass(K type) stars; whether or not lower-mass (M type) stars can re-tain primordial disks to such relatively advanced ages remains tobe determined.Young M-dwarfs can exhibit strong levels of UV and X-rayradiation, either as a result of chromospheric and coronal activityor active accretion. This high energy radiation can drive disk dis- sipation via photoevaporation (e.g., Gorti & Hollenbach 2009).Nevertheless, the limited studies that have been performed to ex-plore the disk lifetimes of substellar objects have found disk dis-sipation timescales are at least as long as for solar-mass stars(see, e.g., Luhman & Mamajek 2012, Ercolano et al. 2011,Williams & Cieza 2011, and references therein). Investigationsaimed at establishing the masses of gas and dust around similarlyyoung M stars are key to advance our understanding of disk evo-lution timescales and processes.Among the nearby young moving groups, the TWA is par-ticularly interesting, as it represents an important evolutionarystage in the lives of protoplanetary disks. With an age of ∼ ∼ M (cid:12) , these new mid / late M-dwarfs con-stitute a sample of objects that allows us to probe disks amongthe poorly explored substellar population, and to do so at therelatively advanced age of the TWA. Article number, page 1 of 5 a r X i v : . [ a s t r o - ph . S R ] S e p & A proofs: manuscript no. paper_v6 λ µm -4 -3 -2 -1 F ν [ J y ] T dust = 150 T dust = 620 τ = 8.82e-01 TWA 30B λ µm -4 -3 -2 -1 F ν [ J y ] T dust = 205 T dust = 40 τ = 4.32e-02 TWA 32 λ µm -3 -2 -1 F ν [ J y ] T dust = 240 T dust = 30 τ = 2.50e-02 TWA 33 λ µm -4 -3 -2 -1 F ν [ J y ] T dust = 265 T dust = 50 τ = 2.32e-02 TWA 34
Fig. 1.
Spectral energy distributions for TWA 30B, 32, 33, and 34. Data points are 2MASS (green), ALLWISE (blue), Herschel (purple; see Liu etal. 2015), and ALMA (orange). Two blackbody dust disks are fit in each case. The fractional luminosities, τ = L IR / L bol , are indicated. The poor fitto TWA 30B suggests a more complex model is required.
2. Observations
We carried out an ALMA survey of 15 members or proposedmembers of the TWA as drawn from Looper et al. (2007,2010a,b); Looper (2011); Shkolnik et al. (2011); Rodriguez etal. (2011); Schneider et al. (2012b); Gagné et al. (2014b). Thesestars, listed in Table 1, were chosen as targets as they consti-tute some of the lowest mass members suggested to date for theTWA. None of these had yet been observed with ALMA, thoughmost are known to host dusty circumstellar disks as inferredvia WISE and Herschel infrared excesses (eg, Schneider et al.2012a,b; Liu et al. 2015). For those not in published IR surveys,we noted the WISE color W − W < CO(2–1) and CO(2–1) emission lines witha resolution of 488 kHz, corresponding to a velocity resolutionof 0.6 km / s. We reached a sensitivity of 0.05 mJy / beam in thecontinuum and 5 mJy / beam per 0.6 km / s channel in CO and CO. Calibration and cleaning was performed by the ALMAsta ff with CASA version 4.2.2. Briggs weighting was used withrobust = . × . (cid:48)(cid:48) and corresponds to a scale of 40–80 AU at the mean distance ofthe stars in the TWA.
3. Results
Of the 15 targets observed, four systems were detected in con-tinuum emission. All four sources were unresolved. These first-time detections are listed in Table 1 along with the measuredflux from the primary beam corrected images. Fluxes were mea-sured by fitting a Gaussian to the cleaned image. We also list 3- σ limits, which are three times the RMS error estimated in eachcase. The measured fluxes are consistent with emission fromT ∼
40 K dust grains as determined by modeling the spectral en-ergy distribution (SED). Figure 1 shows SEDs of these 4 systemsalong with blackbody fits to their IR / submm excesses (see, e.g.,Schneider et al. 2012a). In the case of TWA 30B, which displaysstrong variability and may have complex disk structure as a re-sult of the edge-on disk geometry (Looper et al. 2010b; Principeet al., submitted), a more sophisticated model is clearly requiredto accurately describe its SED.These SED models are overly simplistic; unless the emis-sion is coming from narrow rings, the real disks will have arange of temperatures. Nevertheless, the models in Fig. 1 areuseful to demonstrate the presence of cold dust in the system.The fractional luminosity, τ = L IR / L bol , ranges from 2 to 4%for TWA 32, 33, and 34. If we assume the continuum emissionis coming from large (mm-sized) grains which radiate as black- Article number, page 2 of 5.R. Rodriguez et al.: A Molecular Disk Survey of Low-Mass Stars in the TW Hya Association
Name RA Dec. Sp. W1-W3 W1-W4 IR Distance Flux M dust Ref.Type (mag) (mag) Excess (pc) (mJy) (10 − M E )TWA 30B 11:32:18 -30:18:31 M4 4.96 7.23 Y 45 . ± . . ± .
07 3.7 1TWA 30A 11:32:18 -30:19:51 M5 1.72 3.64 Y 45 . ± . < < . ± . < < . ± . . ± .
03 1.5 4TWA 34 10:28:45 -28:30:37 M5 1.35 2.75 Y 47 . ± . . ± .
06 2.5 4TWA 32 12:26:51 -33:16:12 M6 1.77 3.77 Y 59 . ± . . ± .
05 15.8 3, 5J1045-2819 10:45:52 -28:19:30 M6 0.32 < . ± . < < . ± . < < < . ± . < < / . ± . < < < < . ± . < < < . ± . < < . ± . < < / < < . ± . < < / < . ± . < < Table 1.
Targets for our ALMA observations. The third column indicates if the source had prior indications of IR excess from WISE ( W − W > W W
4, or both. Distances listed here have been calculated via the BANYAN II kinematic tool (Gagnéet al. 2014a). Also listed are the continuum detections and 3- σ upper limits. Dust masses are estimated as described in Section 3 and using T dust ∼
40 K. All sources are unresolved with ALMA.References: (1) Looper et al. 2010b, (2) Looper et al. 2010a, (3) Shkolnik et al. 2011, (4) Schneider et al. 2012b, (5) Rodriguez et al. 2011, (6)Looper 2011, (7) Gagné et al. 2014b, (8) Looper et al. 2007. bodies at ∼
40 K, then the dust grains need to be located a fewAU from these low-mass stars.In addition to the four Table 1 stars that are coincident withsubmm continuum sources, we detected continuum sources thatare well o ff set (typically by ∼ (cid:48)(cid:48) ) from three systems (TWA31, J1247-3816, and J1207-3900). Given the typical positionalaccuracy of ALMA observations is better than 0.1 (cid:48)(cid:48) , we concludethese are background sources, likely of extragalactic origin.Assuming optically thin dust, we can estimate the dust massfrom: M dust = F ν D κ ν B ν ( T dust ) , where we adopt κ ν = .
15 cm / g (see Rodriguez et al. 2010 andreferences therein) and T dust =
40 K. We tabulate the resultingestimates of M d , as well as 3- σ upper limits, in Table 1. Theinferred dust masses range from ∼ T d ≈ L ∗ / L (cid:12) ) / .While consistent with results for disks orbiting young, earliertype stars, the Andrews et al. (2013) relationship does not nec-essarily hold for very low-mass objects such as those in Table 1(van der Plas et al., in prep). Young M5 stars have log L ∗ / L (cid:12) ≈−
2, which suggests a dust temperature of ∼ σ upper limit of 0.05 mJy / beam.Among our sample only TWA 34 displays detectable COemission, which we discuss in Section 3.1. For those systemsin our sample with no CO detections, we can infer a 3- σ upper Fig. 2.
Velocity map for the CO(2–1) emission in TWA 34. limit of ∼ M E in molecular gas following the prescriptionin Section 3.1 and assuming optically thin CO, CO:H of 10 − ,and a distance of ∼
60 pc.
Among the Table 1 systems, only TWA 34 shows evidence of CO emission. Although the CO emission is unresolved foreach individual velocity channel, we find the centroid changeswith velocity, allowing us to generate the first moment mapin Figure 2. Figure 2 suggests that TWA 34 is orbited by amolecule-rich disk viewed at intermediate to high inclination,with North-South rotation.
Article number, page 3 of 5 & A proofs: manuscript no. paper_v6
We show in Figure 3 the integrated line profile of the COemission, Hanning smoothed with a kernel size of 3 channels.This double-peaked CO line profile is indicative of Keplerian ro-tation. Hence, to characterize this emission, we fit a parametrizedKeplerian model as described in Kastner et al. (2008). That is, wefit a parametric line profile function described by: F = (cid:40) F (( v − v ) / v d ) q − | v − v | > v d F (( v − v ) / v d ) p d | v − v | < v d where F is the peak line intensity, ν is the rest frequency inthe star / disk system frame, v d is the projected rotational veloc-ity near the outer edge of the disk, and p d and q are quasi-physical disk parameters (see Kastner et al. 2008 for details).We fix q = p d = . ± .
002 Jy, a systemic velocity of 2 . ± . / sin the LSR frame, v d of 2 . ± . / s, and an integrated in-tensity of 0 . ± .
03 Jy km / s. The parameter v d can be used toestimate the outer radius of the disk as detected in CO emission, R d , from v d = GM ∗ / R d where M ∗ is the mass of the star, 0.08 M (cid:12) (Bara ff e et al. 2015).We thereby estimate the CO disk orbiting TWA 34 is ∼
11 AUin radius. This suggests that the disk around TWA 34 is morecompact than those seen around younger brown dwarfs and low-mass stars in Taurus (Ricci et al. 2014). We note that the COemission in Figure 2 is marginally resolved and suggests a largerCO radius of ∼ ∼ M E following the pre-scriptions of Zuckerman et al. (2008), Kastner et al. (2008),Rodriguez et al. (2010), and references therein. We assumeoptically thin CO, a C: C ratio of 89, a CO:H ratio of10 − , temperature of ∼
40 K, and a 3- σ CO upper limit of ∼ / s. The CO upper limit is determined assuminga linewidth identical to that of CO. This estimate for the gasmass is ∼ ∼ ratio; due, forexample, to freezing out of CO on cold dust grains. These var-ious processes result in large uncertainties when estimating thegas mass of disks based on CO measurements. From our best-fit Keplerian profile (see prior section), we haveobtained an estimate for the systemic velocity of TWA 34. Inthe barycentric frame of reference, this velocity corresponds to13 . ± . / s. This is the first accurate radial velocity mea-surements available for TWA 34 (see also Murphy et al. 2015).At a distance of 47 . ± . F l u x D e n s i t y ( J y ) Fig. 3. CO(2–1) emission line profile of TWA 34, the only target inour sample with detected CO gas. The red, thin line represents the best-fit Keplerian profile. in PPMXL (Roeser et al. 2010), we can estimate UVW veloci-ties of − . ± . − . ± . − . ± . / s. These agreevery well with the average velocity of the TWA ( − . − . − .
52 km / s; Malo et al. 2013). This new velocity measurementthereby further supports the conclusion that TWA 34 is a mem-ber of the TWA.
4. Discussion
Among the 15 low-mass TWA members and candidate mem-bers listed in Table 1, only 4 yielded ALMA continuum detec-tion at 1.3 mm, despite the fact that many show some evidenceof warm circumstellar dust. In the absence of cold dust grains,the warm dust grains detected by WISE would have 1.3 mmemission < − M E or less. This is similar to what has been observed forother ∼
10 Myr-old debris disks around M stars (see Wyatt 2008and references therein). We were more sensitive to moleculargas masses and achieved a limit of a few times 10 − M E for gasin H , assuming CO / H of 10 − . As the case for TWA 34 andother disks shows, the gas-to-dust ratio is unlikely to be ∼ ffi ciently removed, even in cases where a signifi-cant mass of primordial dust has survived. However, studies haveidentified signatures of on-going gas accretion in some of thesesystems, for example around TWA 30A, 30B, and 31 (Looperet al. 2010a,b; Shkolnik et al. 2011). Hence, at least in certain Article number, page 4 of 5.R. Rodriguez et al.: A Molecular Disk Survey of Low-Mass Stars in the TW Hya Association cases, it is likely that disk CO gas has frozen out onto dust grains,suppressing the gas-phase CO abundance.
5. Conclusions
We have carried out an ALMA survey of 15 low-mass TWAmembers and candidates to search for molecular gas in the formof CO and CO as well as provide constraints on continuumdust emission. Among systems targeted, four (TWA 30B, 32, 33,and 34) have detected dust emission consistent with the existenceof cold dust grains in the disk. Circumstellar dust grain temper-atures of ∼
40 K are consistent with the mid-infrared to submmSEDs for these systems. All continuum sources are unresolved.While most of our sample shows indications of warm dust basedon WISE measurements, the ALMA non-detections suggest anycold grains present in the outer disk may have already grown tocm size or larger.Only one system, TWA 34, shows signatures of moleculargas in its disk in the form of CO (2–1) emission. The COemission has velocity structure indicative of Keplerian rotation.The systemic velocity for the system, as determined from the COdetection, is consistent with membership in the TWA. Amongthe sample of known ∼ / disk systems, TWA 34,at just ∼
50 pc from Earth, is the lowest mass star thus far identi-fied as harboring cold molecular gas in an orbiting disk.
Acknowledgements.
This paper makes use of the following ALMA data:ADS / JAO.ALMA / NRAO and NAOJ. We thank our referee, Greg Herczeg, for the de-tailed and useful review of our manuscript. D.R.R. acknowledges support fromFONDECYT grant 3130520. G.v.d.P. acknowledges support from FONDECYTgrant 3140393. D.P. acknowledges support from FONDECYT grant 3150550.G.v.d.P. acknowledges support from FONDECYT grant 3140393 and by the Mil-lennium Nucleus RC130007 (Chilean Ministry of Economy). J.K.’s research onyoung stars near Earth is supported by National Science Foundation grant AST-1108950 and NASA Astrophysics Data Analysis Program grant NNX12H37G,both to RIT. S.M. acknowledges the support of the STFC grant ST / K001051 / References
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