The Galactic Disk Distribution of Planetary Nebulae with Warm Dust Emission Features: II

Can the distribution of warm-dust compositions in IR-bright galactic disk PNe be linked to the underlying stellar population? The PNe with warm dust emission represent a homogeneous population, which is presumably young and minimally affected by a possible dependence of PN lifetime on progenitor mass. The sample in paper I thus allows testing the predictions of single star evolution, through a comparison with synthetic distributions and under the assumption that tip-of-the-AGB and PN statistics are similar. We construct a schematic model for AGB evolution (adapted from Groenewegen & de Jong 1993), whose free-parameters are calibrated with the luminosity function (LF) of C stars in the LMC, the initial-final mass relation, and the range of PN compositions. The observed metallicity gradient and distribution of star forming regions with galactocentric radius (Bronfman et al. 2000) allow us to synthesise the galactic disk PN progenitor population. We find the fraction of O-rich PNe, f(O), is a tight constraint on AGB parameters. For our best model, a minimum PN progenitor mass Mmin=1Msun predicts that about 50% of all young PNe should be O-rich, compared to an observed fraction of 22%; thus Mmin=1.2Msun, at a 2sigma confidence level. By contrast, current AGB models for single stars can account neither for the continuous range of N enrichment (Leisy & Dennefeld 1996), nor for the observation that the majority of very C-rich PNe have Peimbert type I (paper I). f(O) is thus an observable much easier to model. The decrease in f(O) with galactocentric radius, as reported in paper I, is a strong property of the synthetic distribution, independent of Mmin. This trend reflects the sensitivity of the surface temperature of AGB stars and of the core mass at the first thermal pulse to the galactic metallicity gradient.Comment: accepted by MNRA

Multi-GPU maximum entropy image synthesis for radio astronomy

The maximum entropy method (MEM) is a well known deconvolution technique in radio-interferometry. This method solves a non-linear optimization problem with an entropy regularization term. Other heuristics such as CLEAN are faster but highly user dependent. Nevertheless, MEM has the following advantages: it is unsupervised, it has a statistical basis, it has a better resolution and better image quality under certain conditions. This work presents a high performance GPU version of non-gridding MEM, which is tested using real and simulated data. We propose a single-GPU and a multi-GPU implementation for single and multi-spectral data, respectively. We also make use of the Peer-to-Peer and Unified Virtual Addressing features of newer GPUs which allows to exploit transparently and efficiently multiple GPUs. Several ALMA data sets are used to demonstrate the effectiveness in imaging and to evaluate GPU performance. The results show that a speedup from 1000 to 5000 times faster than a sequential version can be achieved, depending on data and image size. This allows to reconstruct the HD142527 CO(6-5) short baseline data set in 2.1 minutes, instead of 2.5 days that takes a sequential version on CPU.Comment: 11 pages, 13 figure

CBI limits on 31 GHz excess emission in southern HII regions

We have mapped four regions of the southern Galactic plane at 31 GHz with the Cosmic Background Imager. From the maps, we have extracted the flux densities for six of the brightest \hii regions in the southern sky and compared them with multi-frequency data from the literature. The fitted spectral index for each source was found to be close to the theoretical value expected for optically thin free-free emission, thus confirming that the majority of flux at 31 GHz is due to free-free emission from ionised gas with an electron temperature of $\approx 7000-8000$ K. We also found that, for all six sources, the 31 GHz flux density was slightly higher than the predicted value from data in the literature. This excess emission could be due to spinning dust or another emission mechanism. Comparisons with $100 \mu$m data indicate an average dust emissivity of $3.3\pm1.7 \mu$K (MJy/sr)$^{-1}$, or a 95 per cent confidence limit of $<6.1 \mu$K (MJy/sr)$^{-1}$. This is lower than that found in diffuse clouds at high Galactic latitudes by a factor of $\sim 3-4$. The most significant detection ($3.3\sigma$) was found in $G284.3-0.3$ (RCW49) and may account for up to $\approx 30$ per cent of the total flux density observed at 31 GHz. Here, the dust emissivity of the excess emission is $13.6\pm4.2 \mu$K (MJy/sr)$^{-1}$ and is within the range observed at high Galactic latitudes. Low level polarised emission was observed in all six sources with polarisation fractions in the range $0.3-0.6$ per cent. This is likely to be mainly due to instrumental leakage and is therefore upper an upper limit to the free-free polarisation. It corresponds to an upper limit of $\sim1$ per cent for the polarisation of anomalous emission.Comment: Accepted in MNRAS. 12 pages, 10 figures, 5 table

Dust masses of disks around 8 Brown Dwarfs and Very Low-Mass Stars in Upper Sco OB1 and Ophiuchus

We present the results of ALMA band 7 observations of dust and CO gas in the disks around 7 objects with spectral types ranging between M5.5 and M7.5 in Upper Scorpius OB1, and one M3 star in Ophiuchus. We detect unresolved continuum emission in all but one source, and the $^{12}$CO J=3-2 line in two sources. We constrain the dust and gas content of these systems using a grid of models calculated with the radiative transfer code MCFOST, and find disk dust masses between 0.1 and 1 M$_\oplus$, suggesting that the stellar mass / disk mass correlation can be extrapolated for brown dwarfs with masses as low as 0.05 M$_\odot$. The one disk in Upper Sco in which we detect CO emission, 2MASS J15555600, is also the disk with warmest inner disk as traced by its H - [4.5] photometric color. Using our radiative transfer grid, we extend the correlation between stellar luminosity and mass-averaged disk dust temperature originally derived for stellar mass objects to the brown dwarf regime to $\langle T_{dust} \rangle \approx 22 (L_{*} /L_{\odot})^{0.16} K$, applicable to spectral types of M5 and later. This is slightly shallower than the relation for earlier spectral type objects and yields warmer low-mass disks. The two prescriptions cross at 0.27 L$_\odot$, corresponding to masses between 0.1 and 0.2 M$_\odot$ depending on age.Comment: 9 pages,6 figures, accepted to ApJ on 26/01/201

An upper limit on anomalous dust emission at 31 GHz in the diffuse cloud [LPH96]201.663+1.643

[LPH96]201.663+1.643, a diffuse H{\sc ii} region, has been reported to be a candidate for emission from rapidly spinning dust grains. Here we present Cosmic Background Imager (CBI) observations at 26-36 GHz that show no evidence for significant anomalous emission. The spectral index within the CBI band, and between CBI and Effelsberg data at 1.4/2.7 GHz, is consistent with optically thin free-free emission. The best-fitting temperature spectral index from 2.7 to 31 GHz, $\beta=-2.06 \pm 0.03$, is close to the theoretical value, $\beta=-2.12$ for $T_{e}=9100$ K. We place an upper limit of 24% ~ (2\sigma) for excess emission at 31 GHz as seen in a 6\arcmin FWHM beam. Current spinning dust models are not a good fit to the spectrum of LPH96. No polarized emission is detected in the CBI data with an upper limit of 2% on the polarization fraction.Comment: 5 pages, 3 figures, submitted to ApJ

Survey of Planetary Nebulae at 30 GHz with OCRA-p

We report the results of a survey of 442 planetary nebulae at 30 GHz. The purpose of the survey is to develop a list of planetary nebulae as calibration sources which could be used for high frequency calibration in future. For 41 PNe with sufficient data, we test the emission mechanisms in order to evaluate whether or not spinning dust plays an important role in their spectra at 30 GHz. The 30-GHz data were obtained with a twin-beam differencing radiometer, OCRA-p, which is in operation on the Torun 32-m telescope. Sources were scanned both in right ascension and declination. We estimated flux densities at 30 GHz using a free-free emission model and compared it with our data. The primary result is a catalogue containing the flux densities of 93 planetary nebulae at 30 GHz. Sources with sufficient data were compared with a spectral model of free-free emission. The model shows that free-free emission can generally explain the observed flux densities at 30 GHz thus no other emission mechanism is needed to account for the high frequency spectra.Comment: 10 pages, 7 Postscript figures, to be published in A&

An excess of emission in the dark cloud LDN 1111 with the Arcminute Microkelvin Imager

We present observations of the Lynds' dark nebula LDN 1111 made at microwave frequencies between 14.6 and 17.2 GHz with the Arcminute Microkelvin Imager (AMI). We find emission in this frequency band in excess of a thermal free--free spectrum extrapolated from data at 1.4 GHz with matched uv-coverage. This excess is > 15 sigma above the predicted emission. We fit the measured spectrum using the spinning dust model of Drain & Lazarian (1998a) and find the best fitting model parameters agree well with those derived from Scuba data for this object by Visser et al. (2001).Comment: accepted MNRA

The ALMA Early Science View of FUor/EXor Objects - V. Continuum Disc Masses and Sizes

Low-mass stars build a significant fraction of their total mass during short outbursts of enhanced accretion known as FUor and EXor outbursts. FUor objects are characterized by a sudden brightening of ∼5 mag at visible wavelengths within 1 yr and remain bright for decades. EXor objects have lower amplitude outbursts on shorter time-scales. Here we discuss a 1.3 mm Atacama Large Millimeter/submillimeter Array (ALMA) mini-survey of eight outbursting sources (three FUors, four EXors, and the borderline object V1647 Ori) in the Orion Molecular Cloud. While previous papers in this series discuss the remarkable molecular outflows observed in the three FUor objects and V1647 Ori, here we focus on the continuum data and the differences and similarities between the FUor and EXor populations. We find that FUor discs are significantly more massive (∼80–600 MJup) than the EXor objects (∼0.5–40 MJup). We also report that the EXor sources lack the prominent outflows seen in the FUor population. Even though our sample is small, the large differences in disc masses and outflow activity suggest that the two types of objects represent different evolutionary stages. The FUor sources seem to be rather compact (Rc \u3c 20–40 au) and to have a smaller characteristic radius for a given disc mass when compared to T Tauri stars. V1118 Ori, the only known close binary system in our sample, is shown to host a disc around each one of the stellar components. The disc around HBC 494 is asymmetric, hinting at a structure in the outer disc or the presence of a second disc