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Jane Greaves

Publications and source records attributed to Jane Greaves.

28 records · Page 2Linked to original sources
arXiv2018

Anomalous microwave emission from spinning nanodiamonds around stars

Several interstellar environments produce 'anomalous microwave emission', with brightness-peaks at tens-of-gigahertz frequencies. The emission's origins are uncertain - rapidly-spinning nano-particles could emit electric-dipole radiation, but polycyclic aromatic hydrocarbons proposed as the carrier are now found not to correlate with Galactic signals. The difficulty is to identify co-spatial sources over long lines of sight. Here we identify anomalous microwave emission in three proto-planetary discs. These are the only known systems that host hydrogenated nano-diamonds, in contrast to very common detection of polycyclic aromatic hydrocarbons. Spectroscopy locates the nano-diamonds close to the host-stars, at physically-constrained temperatures. Developing disc models, we reproduce the emission with diamonds 0.75-1.1 nanometres in radius, holding less than or equal to 1-2 per cent of the carbon budget. The microwave-emission:stellar-luminosity ratios are approximately constant, allowing nano-diamonds to be ubiquitous but emitting below detection thresholds in many star-systems. This can unify the findings with similar-sized diamonds found within solar system meteorites. As nano-diamond spectral absorption is seen in interstellar sightlines, these particles are also a candidate for generating galaxy-scale anomalous microwave emission.

astro-ph.SR↗
arXiv2016

The JCMT Gould Belt Survey: First results from SCUBA-2 observations of the Cepheus Flare Region

We present observations of the Cepheus Flare obtained as part of the James Clerk Maxwell Telescope (JCMT) Gould Belt Legacy Survey (GBLS) with the SCUBA-2 instrument. We produce a catalogue of sources found by SCUBA-2, and separate these into starless cores and protostars. We determine masses and densities for each of our sources, using source temperatures determined by the Herschel Gould Belt Survey. We compare the properties of starless cores in four different molecular clouds: L1147/58, L1172/74, L1251 and L1228. We find that the core mass functions for each region typically show shallower-than-Salpeter behaviour. We find that L1147/58 and L1228 have a high ratio of starless cores to Class II protostars, while L1251 and L1174 have a low ratio, consistent with the latter regions being more active sites of current star formation, while the former are forming stars less actively. We determine that, if modelled as thermally-supported Bonnor-Ebert spheres, most of our cores have stable configurations accessible to them. We estimate the external pressures on our cores using archival $^{13}$CO velocity dispersion measurements and find that our cores are typically pressure-confined, rather than gravitationally bound. We perform a virial analysis on our cores, and find that they typically cannot be supported against collapse by internal thermal energy alone, due primarily to the measured external pressures. This suggests that the dominant mode of internal support in starless cores in the Cepheus Flare is either non-thermal motions or internal magnetic fields.

astro-ph.GA↗
arXiv2016

Brown dwarf disks with Herschel: Linking far-infrared and (sub)-mm fluxes

Brown dwarf disks are excellent laboratories to test our understanding of disk physics in an extreme parameter regime. In this paper we investigate a sample of 29 well-characterized brown dwarfs and very low mass stars, for which Herschel far-infrared fluxes as well as (sub)-mm fluxes are available. We have measured new Herschel PACS fluxes for 11 objects and complement these with (sub)-mm data and Herschel fluxes from the literature. We analyze their spectral energy distributions in comparison with results from radiative transfer modeling. Fluxes in the far-infrared are strongly affected by the shape and temperature of the disk (and hence stellar luminosity), whereas the (sub)-mm fluxes mostly depend on disk mass. Nevertheless, there is a clear correlation between far-infrared and (sub)-mm fluxes. We argue that the link results from the combination of the stellar mass-luminosity relation and a scaling between disk mass and stellar mass. We find strong evidence of dust settling to the disk midplane. The spectral slopes between near- and far-infrared are mostly between $-0.5$ and $-1.2$ in our sample, comparable to more massive T Tauri stars, which may imply that the disk shapes are similar as well, though highly-flared disks are rare among brown dwarfs. We find that dust temperatures in the range of 7-15 K, calculated with $T\approx25\,(L/L_\odot)^{0.25}$ K, are appropriate for deriving disk masses from (sub)-mm fluxes for these low luminosity objects. About half of our sample hosts disks with at least one Jupiter mass, confirming that many brown dwarfs harbour sufficient material for the formation of Earth-mass planets in their midst.

astro-ph.SR↗
arXiv2015

The AU Mic Debris Disk: far-infrared and submillimeter resolved imaging

We present far-infrared and submillimeter maps from the Herschel Space Observatory and the James Clerk Maxwell Telescope of the debris disk host star AU Microscopii. Disk emission is detected at 70, 160, 250, 350, 450, 500 and 850 micron. The disk is resolved at 70, 160 and 450 micron. In addition to the planetesimal belt, we detect thermal emission from AU Mic's halo for the first time. In contrast to the scattered light images, no asymmetries are evident in the disk. The fractional luminosity of the disk is $3.9 \times 10^{-4}$ and its mm-grain dust mass is 0.01 MEarth (+/- 20%). We create a simple spatial model that reconciles the disk SED as a blackbody of 53 +/- 2 K (a composite of 39 and 50 K components) and the presence of small (non-blackbody) grains which populate the extended halo. The best fit model is consistent with the "birth ring" model explored in earlier works, i.e., an edge-on dust belt extending from 8.8-40 AU, but with an additional halo component with an $r^{-1.5}$ surface density profile extending to the limits of sensitivity (140 AU). We confirm that AU Mic does not exert enough radiation force to blow out grains. For stellar mass loss rates of 10-100x solar, compact (zero porosity) grains can only be removed if they are very small, consistently with previous work, if the porosity is 0.9, then grains approaching 0.1 micron can be removed via corpuscular forces (i.e., the stellar wind).

astro-ph.SR↗
arXiv2015

Stellar Multiplicity and Debris Disks: An Unbiased Sample

Circumstellar dust disks have been observed around many nearby stars. However, many stars are part of binary or multiple stellar systems. A natural question arises regarding the presence and properties of such disks in systems with more than one star. To address this, we consider a sample of 449 systems (spectral types A-M) observed with the Herschel Space Observatory as part of the DEBRIS program. We have examined the stellar multiplicity of this sample by gathering information from the literature and performing an adaptive optics imaging survey at Lick Observatory. Five new companions were revealed with our program. In total, we identify 188 (42%) binary or multiple star systems. The multiplicity of the sample is examined with regards to the detection of circumstellar disks for stars of spectral types AFGK. In general, disks are less commonly detected around binaries than single stars, though the disk frequency is comparable among A stars regardless of multiplicity. However, this sample reveals the period distribution of disk-bearing binaries is consistent with that of non-disk binaries and with comparison field samples. We find that the properties of disks in binary systems are not statistically different from those around single stars. Although the frequency of disk-bearing FGK binaries may be lower than in single star systems, the processes behind disk formation and the characteristics of these disks are comparable among both populations.

astro-ph.SR↗
arXiv2013

Protoplanetary Disk Masses from Stars to Brown Dwarfs

We present SCUBA-2 850um observations for 7 very low mass stars (VLMS) and brown dwarfs (BDs): 3 in Taurus, 4 in the TWA, and all classical T Tauri (cTT) analogs. We detect 2 of the 3 Taurus disks, but none of the TWA ones. Our 3sigma limits correspond to a dust mass of 1.2 MEarth in Taurus and a mere 0.2 MEarth in the TWA (3--10x deeper than previous work). We combine our data with other sub-mm/mm surveys of Taurus, rho Oph and the TWA to investigate trends in disk mass and grain growth during the cTT phase. We find : (1) The minimum disk outer radius required to explain the upper envelope of sub-mm/mm fluxes is 100 AU for intermediate-mass stars, solar-types and VLMS, and 20 AU for BDs. (2) While the upper envelope of disk masses increases with Mstar from BDs to VLMS to solar-types, no increase is seen from solar-type to intermediate-mass stars. We propose this is due to enhanced photoevaporation around intermediate masses. (3) Many disks around Taurus and rho Oph intermediate-mass and solar-type stars evince an opacity index beta of 0--1, indicating large grains. Of the only four VLMS/BDs in these regions with multi-wavelength data, three are consistent with large grains, though optically thick disks are not ruled out. (4) For the TWA VLMS (TWA 30A, B), combining our fluxes with Mdot and ages suggests substantial grain growth by 10 Myr. The degree of grain growth in the TWA BDs (2M1207A, SSPM1102) remains largely unknown. (5) A Bayesian analysis shows that mean(log[Mdisk/Mstar]) = -2.4, roughly constant all the way from intermediate-mass stars to VLMS/BDs, and (6) the disk mass in close solar-type Taurus binaries is significantly lower than in singles (by a factor of 10), while that in wide solar-type Taurus binaries is closer to that in singles (lower by a factor of 3). (7) We discuss the implications for planet formation, and for the dependence of Mdot on Mstar.

astro-ph.SR↗
arXiv2012

Interferometric Identification of a Pre-Brown Dwarf

It is not known whether brown dwarfs (stellar-like objects with masses less than the hydrogen-burning limit, 0.075 Msun) are formed in the same way as solar-type stars or by some other process. Here we report the clear-cut identification of a self-gravitating condensation of gas and dust with a mass in the brown-dwarf regime, made through millimeter interferometric observations. The level of thermal millimeter continuum emission detected from this object indicates a mass ~ 0.02-0.03 Msun, while the small radius < 460 AU and narrow spectral lines imply a dynamical mass of 0.015-0.02 Msun. The identification of such a pre-brown dwarf core supports models according to which brown dwarfs are formed in the same manner as hydrogen-burning stars.

astro-ph.GA↗
arXiv2010

A Pilot Study for the SCUBA-2 'All-Sky' Survey

We have carried out a pilot study for the SCUBA-2 'All-Sky' Survey, SASSy, a wide and shallow mapping project at 850 microns, designed to find rare objects, both Galactic and extragalactic. Two distinct sets of exploratory observations were undertaken, and used to test the SASSy approach and data reduction pipeline. The first was a 0.5 by 0.5 degrees map around the nearby galaxy NGC 2559. The galaxy was easily detected at 156 mJy, but no other convincing sources are present in the map. Comparison with other galaxies with similar wavelength coverage indicates that NGC 2559 has relatively warm dust. The second observations cover 1 square degree around the W5-E HII region. As well as diffuse structure in the map, a filtering approach was able to extract 27 compact sources with signal-to-noise greater than 6. By matching with data at other wavelengths we can see that the SCUBA-2 data can be used to discriminate the colder cores. Together these observations show that the SASSy project will be able to meet its original goals of detecting new bright sources which will be ideal for follow-up observations with other facilities.

astro-ph.CO↗
arXiv2010

Resolving debris discs in the far-infrared: early highlights from the DEBRIS survey

We present results from the earliest observations of DEBRIS, a Herschel Key Programme to conduct a volume- and flux-limited survey for debris discs in A-type through M-type stars. PACS images (from chop/nod or scan-mode observations) at 100 and 160 micron are presented toward two A-type stars and one F-type star: beta Leo, beta UMa and eta Corvi. All three stars are known disc hosts. Herschel spatially resolves the dust emission around all three stars (marginally, in the case of beta UMa), providing new information about discs as close as 11 pc with sizes comparable to that of the Solar System. We have combined these data with existing flux density measurements of the discs to refine the SEDs and derive estimates of the fractional luminosities, temperatures and radii of the discs.

astro-ph.SR↗