arXiv subjects

S. Casassus

Publications and source records attributed to S. Casassus.

70 records · Page 4Linked to original sources

arXiv2007

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 $100 μ$m data indicate an average dust emissivity of $3.3\pm1.7 μ$K (MJy/sr)$^{-1}$, or a 95 per cent confidence limit of $<6.1 μ$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 ($3.3σ$) 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 $13.6\pm4.2 μ$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.

astro-ph↗

arXiv2006

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, $β=-2.06 \pm 0.03$, is close to the theoretical value, $β=-2.12$ for $T_{e}=9100$ K. We place an upper limit of 24% ~ (2σ) 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.

astro-ph↗

arXiv2005

Morphological analysis of the cm-wave continuum in the dark cloud LDN1622

The spectral energy distribution of the dark cloud LDN1622, as measured by Finkbeiner using WMAP data, drops above 30GHz and is suggestive of a Boltzmann cutoff in grain rotation frequencies, characteristic of spinning dust emission. LDN1622 is conspicuous in the 31 GHz image we obtained with the Cosmic Background Imager, which is the first cm-wave resolved image of a dark cloud. The 31GHz emission follows the emission traced by the four IRAS bands. The normalised cross-correlation of the 31 GHz image with the IRAS images is higher by 6.6sigma for the 12um and 25um bands than for the 60um and 100um bands: C(12+25) = 0.76+/-0.02 and C(60+100) = 0.64+/-0.01. The mid-IR -- cm-wave correlation in LDN 1622 is evidence for very small grain (VSG) or continuum emission at 26-36GHz from a hot molecular phase. In dark clouds and their photon-dominated regions (PDRs) the 12um and 25um emission is attributed to stochastic heating of the VSGs. The mid-IR and cm-wave dust emissions arise in a limb-brightened shell coincident with the PDR of LDN1622, where the incident UV radiation from the Ori OB1b association heats and charges the grains, as required for spinning dust.

astro-ph↗

arXiv2005

Interstellar 12C/13C ratios through CH+ ll 3957,4232 absorption in local clouds: incomplete mixing in the ISM

The 12C/13C isotope ratio is a tracer of stellar yields and the efficiency of mixing in the ISM. 12CH+/13CH+ is not affected by interstellar chemistry, and is the most secure way of measuring 12C/13C in the diffuse ISM. R= 12C/13C is 90 in the solar system. Previous measurements of 12CH+ ll3957.7,4232.3 and 13CH+ ll3958.2,4232.0 absorption toward nearby stars indicate some variations in 12C/13C, with values ranging from 40 to 90 suggesting inefficient mixing. Except for the cloud toward zeta Oph, these R values are strongly affected by noise. With UVES on the VLT we have improved on the previous interstellar 12C/13C measurements. The weighted 12C/13C ratio in the local ISM is 78.27 +- 1.83, while the weighted dispersion of our measurements is 12.7, giving a 6.9 sigma scatter. Thus we report on a 6.9 sigma detection of 16.2% root-mean-square variations in the carbon isotopic ratio on scales of ~100 pc: R= 74.7 +- 2.3 in the zetaOph cloud, while R = 88.6 +- 3.0 toward HD152235 in the Lupus clouds, R = 62.2 +- 5.3 towards HD110432 in the Coalsack, and R = 98.9 +- 10.1 toward HD170740. The observed variations in 13C/12C are the first significant detection of chemical heterogeneity in the local ISM.

astro-ph↗

arXiv2005

Hyperfine splitting of [Al VI]3.66um and the Al isotopic ratio in NGC6302

The core of planetary nebula NGC 6302 is filled with high-excitation photoionised gas at low expansion velocities. It represents a unique astrophysical situation in which to search for hyperfine structure (HFS) in coronal emission lines from highly ionised species. HFS is otherwise blended by thermal or velocity broadening. Spectra containing [AlVI]~3.66um 3P2-3P1, obtained with Phoenix on Gemini-South at resolving powers of up to 75000, resolve the line into five hyperfine components separated by 20 to 60km/s due to the coupling of the I=5/2 nuclear spin of 27Al with the total electronic angular momentum J. 26Al has a different nuclear spin of I=5, and a different HFS, which allows us to place a 3sigma upper limit on the 26Al/27Al abundance ratio of 1/33. We measure the HFS magnetic-dipole coupling constants for [AlVI], and provide the first estimates of the electric-quadrupole HFS coupling constants obtained through astronomical observations of an atomic transition.

astro-ph↗

arXiv2004

Polarization Observations with the Cosmic Background Imager

Polarization observations of the cosmic microwave background with the Cosmic Background Imager from September 2002 to May 2004 provide a significant detection of the E-mode polarization and reveal an angular power spectrum of polarized emission showing peaks and valleys that are shifted in phase by half a cycle relative to those of the total intensity spectrum. This key agreement between the phase of the observed polarization spectrum and that predicted based on the total intensity spectrum provides support for the standard model of cosmology, in which dark matter and dark energy are the dominant constituents, the geometry is close to flat, and primordial density fluctuations are predominantly adiabatic with a matter power spectrum commensurate with inflationary cosmological models.

astro-ph↗

arXiv2004

Vela X at 31 GHz

We present observations of the Vela X region at 31 GHz using the Cosmic Background Imager (CBI). We find a strong compact radio source (5.9'x4.1', FWHM) about the Vela pulsar, which we associate with the Vela pulsar wind nebula (PWN) recently discovered at lower radio-frequencies. The CBI's 4' resolution for a 45' field of view allows the PWN to be studied in the large-scale context of Vela X. Filamentary structure in Vela X, which stands out in lower frequency maps, is very low-level at 31 GHz. By combining the 10 CBI channels, which cover 26-36 GHz, and 8.4 GHz archive data, we study the spectral energy distribution (SED) of the PWN and the brightest filaments. Our results show that the spectral index alpha (F_{nu} propto nu^alpha) of the PWN is flat, or even marginally positive, with a value of alpha_{8.4}^{31}=0.10+-0.06, while the Vela X filamentary structure has a negative spectral index of alpha_{8.4}^{31}=-0.28+-0.09. The SED inhomogeneity observed in Vela X suggests different excitation processes between the PWN and the filaments. We investigate whether the PWN's flat spectrum is a consequence of variability or truly reflects the SED of the object. The peak of the PWN at 31 GHz is 80''+-20'' south-west of the peak at 8.4 GHz. This shift is confirmed by comparing the 31 GHz CBI image with higher resolution 5 GHz Australia Telescope Compact Array observations, and is likely to be due to SED variations within the PWN. We also investigate the nature of the Vela X filamentary structure.

astro-ph↗

arXiv2004

Extended Mosaic Observations with the Cosmic Background Imager

Two years of microwave background observations with the Cosmic Background Imager (CBI) have been combined to give a sensitive, high resolution angular power spectrum over the range 400 < l < 3500. This power spectrum has been referenced to a more accurate overall calibration derived from WMAP. The data cover 90 deg^2 including three pointings targeted for deep observations. The uncertainty on the l > 2000 power previously seen with the CBI is reduced. Under the assumption that any signal in excess of the primary anisotropy is due to a secondary Sunyaev-Zeldovich anisotropy in distant galaxy clusters we use CBI, ACBAR, and BIMA data to place a constraint on the present-day rms mass fluctuation sigma_8. We present the results of a cosmological parameter analysis on the l < 2000 primary anisotropy data which show significant improvements in the parameters as compared to WMAP alone, and we explore the role of the small-scale cosmic microwave background data in breaking parameter degeneracies.

astro-ph↗

arXiv2003

Anomalous radio emission from dust in the Helix

A byproduct of experiments designed to map the CMB is the recent detection of a new component of foreground Galactic emission. The anomalous foreground at ~ 10--30 GHz, unexplained by traditional emission mechanisms, correlates with 100um dust emission. We report that in the Helix the emission at 31 GHz and 100um are well correlated, and exhibit similar features on sky images, which are absent in Hβ. Upper limits on the 250 GHz continuum emission in the Helix rule out cold grains as candidates for the 31 GHz emission, and provide spectroscopic evidence for an excess at 31 GHz over bremsstrahlung. We estimate that the 100um-correlated radio emission, presumably due to dust, accounts for at least 20% of the 31 GHz emission in the Helix. This result strengthens previous tentative interpretations of diffuse ISM spectra involving a new dust emission mechanism at radio frequencies. Very small grains have not been detected in the Helix, which hampers interpreting the new component in terms of spinning dust. The observed iron depletion in the Helix favors considering the identity of this new component to be magnetic dipole emission from hot ferromagnetic grains. The reduced level of free-free continuum we report also implies an electronic temperature of Te=4600\pm1200K for the free-free emitting material, which is significantly lower than the temperature of 9500\pm500K inferred from collisionally-excited lines (abridged).

astro-ph↗

arXiv2003

The Anisotropy of the Microwave Background to l = 3500: Deep Field Observations with the Cosmic Background Imager

We report measurements of anisotropy in the cosmic microwave background radiation over the multipole range l ~ 200 - 3500 with the Cosmic Background Imager based on deep observations of three fields. These results confirm the drop in power with increasing l first reported in earlier measurements with this instrument, and extend the observations of this decline in power out to l \~ 2000. The decline in power is consistent with the predicted damping of primary anisotropies. At larger multipoles, l = 2000 - 3500, the power is 3.1 sigma greater than standard models for intrinsic microwave background anisotropy in this multipole range, and 3.5 sigma greater than zero. This excess power is not consistent with expected levels of residual radio source contamination but, for sigma_8 >~ 1, is consistent with predicted levels due to a secondary Sunyaev-Zeldovich anisotropy. Further observations are necessary to confirm the level of this excess and, if confirmed, determine its origin.

astro-ph↗

arXiv2003

The Anisotropy of the Microwave Background to l = 3500: Mosaic Observations with the Cosmic Background Imager

Using the Cosmic Background Imager, a 13-element interferometer array operating in the 26-36 GHz frequency band, we have observed 40 sq deg of sky in three pairs of fields, each ~ 145 x 165 arcmin, using overlapping pointings (mosaicing). We present images and power spectra of the cosmic microwave background radiation in these mosaic fields. We remove ground radiation and other low-level contaminating signals by differencing matched observations of the fields in each pair. The primary foreground contamination is due to point sources (radio galaxies and quasars). We have subtracted the strongest sources from the data using higher-resolution measurements, and we have projected out the response to other sources of known position in the power-spectrum analysis. The images show features on scales ~ 6 - 15 arcmin, corresponding to masses ~ (5 - 80)*10^{14} Msun at the surface of last scattering, which are likely to be the seeds of clusters of galaxies. The power spectrum estimates have a resolution Delta-l = 200 and are consistent with earlier results in the multipole range l <~ 1000. The power spectrum is detected with high signal-to-noise ratio in the range 300 <~ l <~ 1700. For 1700 <~ l <~ 3000 the observations are consistent with the results from more sensitive CBI deep-field observations. The results agree with the extrapolation of cosmological models fitted to observations at lower l, and show the predicted drop at high l (the "damping tail").

astro-ph↗

arXiv2001

8-13um Dust Emission Features in Galactic Bulge Planetary Nebulae

A sample of 25 IR-bright planetary nebulae (PNe) towards the Galactic bulge is analysed through 8-13um spectroscopy. The classification of the warm-dust emission features provides a measure of the C/O chemical balance, and represents the first C/O estimates for bulge PNe. Out of 13 PNe with identified dust types, 4 PNe have emission features associated with C-based grains, while the remaining 9 have O-rich dust signatures. The low fraction of C-rich PNe, <30%, contrasts with that for local PNe, around 80%, although it follows the trend for a decreasing frequency of C-rich PNe with galactocentric radius (paper~I). We investigate whether the PNe discussed here are linked to the bulge stellar population (similar to type~IV, or halo, PNe) or the inner Galactic disk (a young and super-metal-rich population). Although 60% of the PNe with warm dust are convincing bulge members, none of the C-rich PNe satisfy our criteria, and they are probably linked to the inner Galactic disk. In the framework of single star evolution, the available information on bulge PNe points towards a progenitor population similar in age to that of local PNe (type I PNe are found in similar proportions), but super-metal-rich (to account for the scarcity of C-rich objects). Yet the metallicities of bulge PNe, as inferred from [O/H], fail to reach the required values - except for the C-rich objects. It is likely that the sample discussed here is derived from a mixed disk/bulge progenitor population and dominated by type IV PNe, as suggested by Peimbert (1992). The much higher fraction of O-rich PNe in this sample than in the solar neighbourhood should result in a proportionally greater injection of silicate grains into the inner Galactic medium.

astro-ph↗

arXiv2000

The Galactic Disk Distribution of Planetary Nebulae With Warm Dust Emission Features: I

We investigate the galactic disk distribution of a sample of planetary nebulae characterised in terms of their mid-infrared spectral features. The total number of galactic disk PNe with 8-13mum spectra is brought up to 74 with the inclusion of 24 new objects, whose spectra we present for the first time. 54 PNe have clearly identified warm dust emission features, and form a sample which we use to construct the distribution of the C/O chemical balance in galactic disk PNe. The dust emission features complement the information on the progenitor masses brought by the gas-phase N/O ratios: PNe with unidentified infrared emission bands have the highest N/O ratios, while PNe with the silicate signature have either very high N enrichment or close to none, and SiC emission features coincide with a range of moderate N-enrichments. We find a trend for a decreasing proportion of O-rich PNe towards the third and fourth galactic quadrants. Two independent distance scales confirmed that the proportion of O-rich PNe decreases from 30+-9% inside the solar circle, to 14+-7% outside. PNe with warm dust are also the youngest. PNe with no warm dust are uniformly distributed in C/O and N/O ratios, and do not appear to be confined to C/O~1. They also have higher 6cm fluxes, as expected from more evolved PNe. We show that the IRAS fluxes are a good representation of the bolometric flux for warm-dust PNe. The requirement F(12um)>0.5Jy should probe a good portion of the galactic disk, and the dominant selection effects are rooted in the PN catalogues.

astro-ph↗

arXiv2000

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.

astro-ph↗

arXiv2000

The radial distribution of OB star formation in the Galaxy

We present the azimuthally averaged radial distribution of 748 regions of OB star formation in the whole galactic disk, based on our previous CS(2-1) survey of UC H II regions. Embedded massive stars produce a total FIR luminosity of 1.39E8 Lsun within the range 0.2<R/Ro<2 in galactocentric radius. We find 492 massive star forming regions within the solar circle, producing 81% of the total FIR luminosity. Separate analyses of the 349 sources in the I and II quadrant (north), and of the 399 sources in the III and IV quadrant (south), yield FIR luminosities (extrapolated to the complete galactic disk) of 1.17E8 Lsun and of 1.60E8 Lsun, respectively. Massive star formation is distributed in a layer with its centroid Zo(R) following that of molecular gas for all galactocentric radii, both north and south. Its thickness for R<Ro is ~73 pc (FWHM), 62% the thickness of the molecular gas disk. The FIR luminosity produced by massive stars has a well defined maximum at R=0.55Ro, with a gaussian FWHM of 0.28Ro - compared with 0.51Ro for the H2 surface density distribution. Toward the outer Galaxy, down from the maximum, the face-on FIR surface luminosity decays exponentially with a scale length of 0.21Ro, compared with 0.34Ro for the H2 surface density. Massive star formation per unit H2 mass is maximum for R~0.55Ro in the southern Galaxy, with a FIR surface luminosity to H2 surface density ratio of 0.41Lsun/Msun, compared with 0.21Lsun/Msun at the same radius in the north, and with an average of ~0.18Lsun/Msun for the whole galactic disk within the solar circle.

astro-ph↗

arXiv1999

The luminosity function of galactic ultra-compact HII regions and the IMF for massive stars

The population of newly formed massive stars, while still embedded in their parent molecular clouds, is studied on the galactic disk scale. We analyse the luminosity function of IRAS point-like sources, with far-infrared (FIR) colours of ultra-compact HII regions, that have been detected in the CS(2-1) line - a tracer of high density molecular gas. The FIR luminosities of 555 massive star forming regions (MSFRs), 413 of which lie within the solar circle, are inferred from their fluxes in the four IRAS bands and from their kinematic distances, derived using the CS(2-1) velocity profiles. The luminosity function (LF) for the UCHII region candidates shows a peak well above the completeness limit, and is different within and outside the solar circle (96% confidence level). While within the solar circle the LF has a maximum for 2E5 Lo, outside the solar circle the maximum is at 5E4 Lo. We model the LF using three free parameters: -alpha, the exponent for the initial mass function (IMF) expressed in log(M/Mo); -beta, the exponent for a power law distribution in N*, the number of stars per MSFR; and N*max, an upper limit for N*. While alpha has a value of \~ 2.0 throughout the Galaxy, beta changes from ~ 0.5 inside the solar circle to \~ 0.7 outside, with a maximum for the number of stars per MSFR of ~650 and \~450 (with 1 <M/Mo< 120). While the IMF appears not to vary, the average number of stars per MSFR within the solar circle is higher than for the outer Galaxy.

astro-ph↗