arXiv subjects

L. Bronfman

Publications and source records attributed to L. Bronfman.

At least 127 records · Page 7Linked to original sources

arXiv2007

Clumpy photon-dominated regions in Carina. I. [CI] and mid-J CO lines in two 4'x4' fields

The Carina region is an excellent astrophysical laboratory for studying the feedback mechanisms of newly born, very massive stars within their natal giant molecular clouds (GMCs) at only 2.35 kpc distance. We use a clumpy PDR model to analyse the observed intensities of atomic carbon and CO and to derive the excitation conditions of the gas. The NANTEN2-4m submillimeter telescope was used to map the [CI] 3P1-3P0, 3P2-3P1 and CO 4-3, 7-6 lines in two 4'x4' regions of Carina where molecular material interfaces with radiation from the massive star clusters. One region is the northern molecular cloud near the compact OB cluster Tr14, and the second region is in the molecular cloud south of etaCar and Tr16. These data were combined with 13CO SEST spectra, HIRES/IRAS 60um and 100um maps of the FIR continuum, and maps of 8um IRAC/Spitzer and MSX emission. We used the HIRES far-infrared dust data to create a map of the FUV field heating the gas. The northern region shows an FUV field of a few 1000 in Draine units while the field of the southern region is about a factor 10 weaker. We constructed models consisting of an ensemble of small spherically symmetric PDR clumps within the 38" beam (0.43pc), which follow canonical power-law mass and mass-size distributions. We find that an average local clump density of 2x10**5 cm-3 is needed to reproduce the observed line emission at two selected interface positions. Stationary, clumpy PDR models reproduce the observed cooling lines of atomic carbon and CO at two positions in the Carina Nebula.

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arXiv2007

Molecular Outflows and a Mid-Infrared Census of the Massive Star-Formation Region Associated with IRAS 18507+0121

We have observed the central region of the IR-dark cloud filament associated with IRAS 18507+0121 at millimeter wavelengths in CO(1-0), 13CO(1-0), and C18O(1-0) line emission and with the Spitzer Space Telescope at mid-IR wavelengths. Five massive outflows from two cloud cores were discovered. Three outflows are centered on or near an Ultracompact HII region (G34.4+0.23) while the remaining two outflows originate from the millimeter core G34.4+0.23 MM. Modeling of the SEDs of the mid-IR sources identified 31 young stellar objects in the filament with a combined stellar mass of ~127 +/- 27 Msun. An additional 22 sources were identified as probable cluster members based on the presence of strong 24 micron emission. The total star formation efficiency in the G34.4 cloud filament is estimated to be ~7% while the massive and intermediate mass star formation efficiency in the entire cloud filament is estimated to be roughly 2%. A comparison of the gravitational binding energy with the outflow kinetic energy suggests that the compact core containing G34.4+0.23 MM is being destroyed by its molecular outflows whereas the outflows associated with more massive core surrounding the G34.4 UC HII region are not likely to totally disrupt the cloud. Additionally, a qualitative evaluation of the region appears to suggest that stars in this region may have formed in two stages: first lower mass stars formed and then, a few Myrs later, the more massive stars began to form.

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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.

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arXiv2007

New evidence on the origin of the microquasar GRO J1655-40

Aims. Motivated by the new determination of the distance to the microquasar GRO J1655-40 by Foellmi et al. (2006), we conduct a detailed study of the distribution of the atomic and molecular gas, and dust around the open cluster NGC 6242, the possible birth place of the microquasar. The proximity and relative height of the cluster on the galactic disk provides a unique opportunity to study SNR evolution and its possible physical link with microquasar formation. Methods. We search in the interstellar atomic and molecular gas around NGC 6242 for traces that may have been left from a supernova explosion associated to the formation of the black hole in GRO J1655-40. Furthermore, the 60/100 mu IR color is used as a tracer of shocked-heated dust. Results. At the kinematical distance of the cluster the observations have revealed the existence of a HI hole of 1.5*1.5 degrees in diameter and compressed CO material acumulated along the south-eastern internal border of the HI cavity. In this same area, we found extended infrared emission with characteristics of shocked-heated dust. Based on the HI, CO and FIR emissions, we suggest that the cavity in the ISM was produced by a supernova explosion occured within NGC 6242. The lower limit to the kinematic energy transferred by the supernova shock to the surrounding interstellar medium is ~ 10^{49} erg and the atomic and molecular mass displaced to form the cavity of ~ 16.500 solar masses. The lower limit to the time elapsed since the SN explosion is ~ 2.2*10^{5} yr, which is consistent with the time required by GRO J1655-40 to move from the cluster up to its present position. The observations suggest that GRO J1655-40 could have been born inside NGC 6242, being one of the nearest microquasars known so far.

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arXiv2006

A comparison of density structures of a star forming and a non-star-forming globule. DCld303.8-14.2 and Thumbprint Nebula

We present a study of radial density structure of the star forming globule, DCld303.8-14.2 (DC303), and a non-star forming globule, Thumbprint Nebula (TPN), using near-infrared data taken with the ISAAC instrument on the Very Large Telescope. We derive the extinction through the globules using the color excess technique and examine the radial density distribution using Bonnor-Ebert and power-law models. The two globules have significantly different density structures. The extinction profile of DC303 is best fitted with a single power-law with an exponent p = 2.29 \pm 0.08. An unstable Bonnor-Ebert model with a dimensionless parameter ξ_max = 23 \pm 3 provides equally good fit to data. The extinction profile of TPN flattens at small radii, making the profile significantly different from the profile of DC303. We are unable to fit the Bonnor-Ebert model for TPN in a robust manner, but derive the lower limit ξ_max > 8 for the dimensionless outer edge. The density profile derived for TPN is typical compared to recently observed pre-protostellar globules, with high ξ_max value which could be interpreted as the presence of significant additional support or very slow contraction.

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arXiv2006

Implications of the Cosmic Background Imager Polarization Data

We present new measurements of the power spectra of the E-mode of CMB polarization, the temperature T, the cross-correlation of E and T, and upper limits on the B-mode from 2.5 years of dedicated Cosmic Background Imager (CBI) observations. Both raw maps and optimal signal images in the uv-plane and real space show strong detections of the E-mode (11.7 sigma for the EE power spectrum overall) and no detection of the B-mode. The power spectra are used to constrain parameters of the flat tilted adiabatic Lambda-CDM models: those determined from EE and TE bandpowers agree with those from TT, a powerful consistency check. There is little tolerance for shifting polarization peaks from the TT-forecast locations, as measured by the angular sound crossing scale theta = 100 ell_s = 1.03 +/- 0.02 from EE and TE cf. 1.044 +/- 0.005 with the TT data included. The scope for extra out-of-phase peaks from subdominant isocurvature modes is also curtailed. The EE and TE measurements of CBI, DASI and BOOMERANG are mutually consistent, and, taken together rather than singly, give enhanced leverage for these tests.

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arXiv2005

Molecular Gas, Kinematics, and OB Star Formation in the Spiral Arms of the Southern Milky Way

The rotation curve for the IV galactic quadrant, within the solar circle, is derived from the Columbia University - U. de Chile CO(J=1-0) survey of molecular gas. A new sampling, four times denser in longitude than in our previous analysis, is used to compute kinematical parameters that require derivatives w/r to galactocentric radius; the angular velocity Omega(R), the epicyclic frequency kappa(R), and the parameters A(R) and B(R) describing, respectively, gas shear and vorticity. The face-on surface density of molecular gas is computed from the CO data in galactocentric radial bins for the subcentral vicinity, the same spectral region used to derive the rotation curve, where the two-fold ambiguity in kinematical distances is minimum. The rate of massive star formation per unit area is derived, for the same radial bins, from the luminosity of IRAS point-like sources with FIR colors of UC HII regions detected in the CS(J=2-1) line. Massive star formation occurs preferentially in three regions of high molecular gas density, coincident with lines of sight tangent to spiral arms. The molecular gas motion in these arms resembles that of a solid body, characterized by constant angular velocity and by low shear and vorticity. The formation of massive stars in the arms follows the Schmidt law, Sigma_{MSFR} is proportional to [Sigma_{gas}]^n, with an index of n = 1.2 +- 0.2 Our results suggest that the large scale kinematics, through shear, regulate global star formation in the Galactic disk.

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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.

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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.

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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.

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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).

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arXiv2003

Discovery of a Massive Protostar near IRAS 18507+0121

We have observed the massive star forming region, IRAS 18507+0121, at millimeter wavelengths in 3 mm continuum emission and H13CO+(J=1-0) and SiO(v=0, J=2-1) line emission, and at near-infrared wavelengths between 1.2 and 2.1 microns. Two compact molecular cores are detected: one north and one south separated by about 40". The northern molecular core contains a newly discovered, deeply embedded, B2 protostar surrounded by several hundred solar masses of warm gas and dust, G34.4+0.23 MM. Based on the presence of warm dust emission and the lack of detection at near-infrared wavelengths, we suggest that G34.4+0.23 MM may represent the relatively rare discovery of a massive protostar (e.g. analogous to a low-mass "Class 0" protostar). The southern molecular core is associated with a near-infrared cluster of young stars and an ultracompact (UC) HII region, G34.4+0.23, with a central B0.5 star. The fraction of near-infrared stars with excess infrared emission indicative of circumstellar material is greater than 50% which suggests an upper limit on the age of the IRAS 18507+0121 star forming region of 3 Myrs.

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arXiv2003

A parsec-scale flow associated with the IRAS 16547-4247 radio jet

IRAS 16547-4247 is the most luminous (6.2 x 10^4 Lsun) embedded young stellar object known to harbor a thermal radio jet. We report the discovery using VLT-ISAAC of a chain of H_2 2.12 um emission knots that trace a collimated flow extending over 1.5 pc. The alignment of the H_2 flow and the central location of the radio jet implies that these phenomena are intimately linked. We have also detected using TIMMI2 an isolated, unresolved 12 um infrared source towards the radio jet . Our findings affirm that IRAS 16547-4247 is excited by a single O-type star that is driving a collimated jet. We argue that the accretion mechanism which produces jets in low-mass star formation also operates in the higher mass regime.

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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.

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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").

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arXiv2003

Cosmological Parameters from Cosmic Background Imager Observations and Comparisons with BOOMERANG, DASI, and MAXIMA

We report on the cosmological parameters derived from observations with the Cosmic Background Imager (CBI), covering 40 square degrees and the multipole range 300 < l < 3500. The angular scales probed by the CBI correspond to structures which cover the mass range from 10^14 to 10^17 M_sun, and the observations reveal, for the first time, the seeds that gave rise to clusters of galaxies. These unique, high-resolution observations also show damping in the power spectrum to l ~ 2000, which we interpret as due to the finite width of the photon-baryon decoupling region and the viscosity operating at decoupling. Because the observations extend to much higher l the CBI results provide information complementary to that probed by the Boomerang, DASI, Maxima, and VSA experiments. As the observations are pushed to higher multipoles no anomalies relative to standard models appear, and extremely good consistency is found between the cosmological parameters derived for the CBI observations over the range 610 < l < 2000 and observations at lower l [abridged].

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arXiv2001

First Intrinsic Anisotropy Observations with the Cosmic Background Imager

We present the first results of observations of the intrinsic anisotropy of the cosmic microwave background radiation with the Cosmic Background Imager from a site at 5080 m altitude in northern Chile. Our observations show a sharp decrease in C_l in the range l=400 - 1500. The broadband amplitudes we have measured are deltaT(band) = 58.7 (-6.3, +7.7) microK for l = 603 (-166, +180) and 29.7 (-4.2, +4.8) microK for l = 1190 (-224, +261), where these are half-power widths in l. Such a decrease in power at high l is one of the fundamental predictions of the standard cosmological model, and these are the first observations which cover a broad enough l range to show this decrease in a single experiment. The C_l we have measured enable us to place limits on the density parameter, Omega(tot) <= 0.4 or $Omega(tot) >= 0.7 (90% confidence).

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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.

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