Searcharxiv⌕ Search

arXiv subjects

A. Roy

Publications and source records attributed to A. Roy.

At least 73 records · Page 4Linked to original sources
arXiv2017

Probing changes of dust properties along a chain of solar-type prestellar and protostellar cores in Taurus with NIKA

The characterization of dust properties in the interstellar medium (ISM) is key for star formation. Mass estimates are crucial to determine gravitational collapse conditions for the birth of new stellar objects in molecular clouds. However, most of these estimates rely on dust models that need further observational constraints from clouds to prestellar and protostellar cores. We present results of a study of dust emissivity changes based on mm-continuum data obtained with the NIKA camera at the IRAM-30m telescope. Observing dust emission at 1.15 mm and 2 mm allows us to constrain the dust emissivity index ($β$) in the Rayleigh-Jeans tail of the dust spectral energy distribution (SED) far from its peak emission, where the contribution of other parameters (i.e. dust temperature) is important. Focusing on the Taurus molecular cloud, a low-mass star-forming regions in the Gould Belt, we analyze the emission properties of several distinct objects in the B213 filament: three prestellar cores, two Class-0/I protostellar cores and one Class-II object. By means of the ratio of the two NIKA channel-maps, we show that in the Rayleigh-Jeans approximation the dust emissivity index varies among the objects. For one prestellar and two protostellar cores, we produce a robust study using Herschel data to constrain the dust temperature of the sources. By using the Abel transform inversion technique we get accurate radial $β$ profiles. We find systematic spatial variations of $β$ in the protostellar cores that is not observed in the prestellar core. While in the former case $β$ decreases toward the center, in the latter it remains constant. Moreover, $β$ appears anticorrelated with the dust temperature. We discuss the implication of these results in terms of dust grain evolution between pre- and protostellar cores.

astro-ph.GA↗
arXiv2017

Testing the universality of the star formation efficiency in dense molecular gas

Recent studies have suggested that star formation (SF) in dense gas may be governed by essentially the same law in Galactic clouds and external galaxies. This conclusion remains controversial, however, because different tracers have been used to probe the dense gas mass in Galactic and extragalactic studies. We conducted observations of Aquila, Oph., and Orion B in HCN(1-0), HCO$^+$(1-0), and their isotopomers to calibrate the HCN and HCO$^+$ lines used as dense gas tracers in extragalactic studies and to test the possible universality of the star formation efficiency in dense gas, SFE$_{dense}$. H$^{13}$CO$^+$ and H$^{13}$CN were observed to be good tracers of the filaments detected with Herschel. Comparing the luminosities $L_{HCN}$ and $L_{HCO^+}$ with the reference masses $M_{Herschel}^{Av>8}$, the empirical conversion factors $α_{Herschel-HCN}$ and $α_{Herschel-HCO^+}$ were found to be anti-correlated with the local FUV strength. In agreement with Pety et al. (2017), HCN and HCO$^+$ were also found to trace gas down to Av>2. As a result, published extragalactic HCN studies must be tracing all of the moderate density gas. Estimating the contribution of this moderate density gas from the typical column density PDFs in nearby clouds, we obtained the following $G_0$-dependent HCN conversion factor for external galaxies:$α_{Herschel-HCN}^{fit'}=64\times G_0^{-0.34}$. Re-estimating the dense gas masses in external galaxies with $α_{Herschel-HCN}^{fit'}$, we found that SFE$_{dense}$ is remarkably constant over 8 orders of magnitude in dense gas mass. Our results confirm that SFE$_{dense}$ of galaxies is quasi-universal on a wide range of scales from ~1-10pc to >10kpc. Based on the tight link between SF and filamentary structure found in Herschel studies of nearby clouds, we argue that SFE$_{dense}$ is primarily set by the microphysics of SF in filaments.

astro-ph.GA↗
arXiv2017

Observation of the $^1$S$_0$ to $^3$D$_1$ clock transition in $^{175}$Lu$^+$

We report the first laser spectroscopy of the $^1$S$_0$ to $^3$D$_1$ clock transition in $^{175}$Lu$^+$. Clock operation is demonstrated on three pairs of Zeeman transitions, one pair from each hyperfine manifold of the $^3$D$_1$ term. We measure the hyperfine intervals of the $^3$D$_1$ to 10 ppb uncertainty and infer the optical frequency averaged over the three hyperfine transitions to be 353.639 915 952 2 (6) THz. The lifetime of the $^3$D$_1$ state is inferred to be $174^{+23}_{-32}$ hours from the M1 coupling strength.

physics.atom-ph↗
arXiv2016

Search for eta-mesic 4He in the dd->3He n pi0 and dd->3He p pi- reactions with the WASA-at-COSY facility

The search for 4He-eta bound states was performed with the WASA-at-COSY facility via the measurement of the excitation function for the dd->3He n pi0 and dd->3He p pi- processes. The beam momentum was varied continuously between 2.127 GeV/c and 2.422 GeV/c, corresponding to the excess energy for the dd->4He eta reaction ranging from Q=-70 MeV to Q=30 MeV. The luminosity was determined based on the dd->3He n reaction and quasi-free proton-proton scattering via dd->pp n_spectator n_spectator reactions. The excitation functions determined independently for the measured reactions do not reveal a structure which could be interpreted as a narrow mesic nucleus. Therefore, the upper limits of the total cross sections for the bound state production and decay in dd->(4He-eta)_bound->3He n pi0 and dd->(4He-eta)_bound->3He p pi- processes were determined taking into account the isospin relation between both the channels considered. The results of the analysis depend on the assumptions of the N* momentum distribution in the anticipated mesic-4He. Assuming as in the previous works, that this is identical with the distribution of nucleons bound with 20 MeV in 4He, we determined that (for the mesic bound state width in the range from 5 MeV to 50 MeV) the upper limits at 90% confidence level are about 3 nb and about 6 nb for npi0 and ppi- channels, respectively. However, based on the recent theoretical findings of the N*(1535) momentum distribution in the N*-3He nucleus bound by 3.6 MeV, we find that the WASA-at-COSY detector acceptance decreases and hence the corresponding upper limits are 5 nb and 10 nb for npi0 and ppi- channels respectively.

nucl-ex↗
arXiv2016

Measurement of the $ω\to π^+ π^- π^0$ Dalitz plot distribution

Using the production reactions $pd\to {}^3\mbox{He}\,ω$ and $pp\to ppω$, the Dalitz plot distribution for the $ω\to π^+ π^- π^0$ decay is studied with the WASA detector at COSY, based on a combined data sample of $ (4.408\pm 0.042) \times 10^4$ events. The Dalitz plot density is parametrised by a product of the $P$-wave phase space and a polynomial expansion in the normalised polar Dalitz plot variables $Z$ and $ϕ$. For the first time, a deviation from pure $P$-wave phase space is observed with a significance of $4.1σ$. The deviation is parametrised by a linear term $1+2αZ$, with $α$ determined to be $+0.147\pm0.036$, consistent with the expectations of $ρ$-meson-type final-state interactions of the $P$-wave pion pairs.

nucl-ex↗
arXiv2016

Search for an Isospin I =3 Dibaryon

Various theoretical calculations based on QCD or hadronic interactions predict that in addition to the recently observed dibaryon resonance $d^*(2380)$ with $I(J^P) = 0(3^+)$ there should also exist a dibaryon resonance with mirrored quantum numbers $I(J^P) = 3(0^+)$. We report here on a search for such a $NN$-decoupled state in data on the $pp \to ppπ^+π^+π^-π^-$ reaction. Since no clear-cut evidence has been found, we give upper limits for the production cross section of such a resonance in the mass range 2280 - 2500 MeV.

nucl-ex↗
arXiv2016

Electrical detection of spiral spin structures in Pt|Cu_2OSeO_3 heterostructures

The interaction between the itinerant spins in metals and localized spins in magnetic insulators thus far has only been explored in collinear spin systems, such as garnets. Here, we report the spin-Hall magnetoresistance (SMR) sensitive to the surface magnetization of the spin-spiral material, Cu_2OSeO_3. We experimentally demonstrate that the angular dependence of the SMR changes drastically at the transition between the helical spiral and the conical spiral phases. Furthermore, the sign and magnitude of the SMR in the conical spiral state are controlled by the cone angle. We show that this complex behaviour can be qualitatively explained within the SMR theory initially developed for collinear magnets. In addition, we studied the spin Seebeck effect (SSE), which is sensitive to the bulk magnetization. It originates from the conversion of thermally excited low-energy spin waves in the magnet, known as magnons, into the spin current in the adjacent metal contact (Pt). The SSE displays unconventional behavior where not only the magnitude but also the phase of the SSE vary with the applied magnetic field.

cond-mat.mtrl-sci↗
arXiv2016

Characterizing filaments in regions of high-mass star formation: High-resolution submilimeter imaging of the massive star-forming complex NGC 6334 with ArTéMiS

Herschel observations of nearby molecular clouds suggest that interstellar filaments and prestellar cores represent two fundamental steps in the star formation process. The observations support a picture of low-mass star formation according to which ~ 0.1 pc-wide filaments form first in the cold interstellar medium, probably as a result of large-scale compression of interstellar matter by supersonic turbulent flows, and then prestellar cores arise from gravitational fragmentation of the densest filaments. Whether this scenario also applies to regions of high-mass star formation is an open question, in part because Herschel data cannot resolve the inner width of filaments in the nearest regions of massive star formation. We used the bolometer camera ArTeMiS on the APEX telescope to map the central part of the NGC6334 complex at a factor of > 3 higher resolution than Herschel at 350 microns. Combining ArTeMiS data with Herschel data allowed us to study the structure of the main filament of the complex with a resolution of 8" or < 0.07 pc at d ~ 1.7 kpc. Our study confirms that this filament is a very dense, massive linear structure with a line mass ranging from ~ 500 Msun/pc to ~ 2000 Msun/pc over nearly 10 pc. It also demonstrates that its inner width remains as narrow as W ~ 0.15 +- 0.05 pc all along the filament length, within a factor of < 2 of the characteristic 0.1 pc value found with Herschel for lower-mass filaments in the Gould Belt. While it is not completely clear whether the NGC 6334 filament will form massive stars or not in the future, it is two to three orders of magnitude denser than the majority of filaments observed in Gould Belt clouds, and yet has a very similar inner width. This points to a common physical mechanism for setting the filament width and suggests that some important structural properties of nearby clouds also hold in high-mass star forming regions.

astro-ph.GA↗
arXiv2016

Globules and Pillars in Cygnus X I. Herschel Far-infrared imaging of the Cyg OB2 environment

The radiative feedback of massive stars on molecular clouds creates pillars, globules and other features at the interface between the HII region and molecular cloud. We present here Herschel observations between 70 and 500 micron of the immediate environment of the Cygnus OB2 association, performed within the HOBYS program. All structures were detected based on their appearance at 70 micron, and have been classified as pillars, globules, evaporating gasous globules (EGGs), proplyd-like objects, and condensations. From the 70 and 160 micron flux maps, we derive the local FUV field on the PDR surfaces. In parallel, we use a census of the O-stars to estimate the overall FUV-field, that is 10^3-10^4 G_0 close to the central OB cluster (within 10 pc) and decreases down to a few tens G_0, in a distance of 50 pc. From a SED fit to the four longest Herschel wavelengths, we determine column density and temperature maps and derive masses, volume densities and surface densities for these structures. We find that the morphological classification corresponds to distinct physical properties. Pillars and globules have the longest estimated photoevaporation lifetimes, a few 10^6 yr, while all other features should survive less than that. These lifetimes are consistent with that found in simulations of turbulent, UV-illuminated clouds. We propose a tentative evolutionary scheme in which pillars can evolve into globules, which in turn then evolve into EGGs, condensations and proplyd-like objects.

astro-ph.GA↗
arXiv2016

Study of Performance of Bakelite Resistive Plate Chamber (RPC)

Resistive Plate Chamber (RPC) is a type of gaseous detector having excellent time and position resolutions. VECC is involved in the R\&D of indigenously developed bakelite RPCs. The largest size of bakelite RPC developed in India is 100 cm $\times$ 100 cm. We present here the test results of a bakelite sample along with the cosmic ray test results of a bakelite RPC (30 cm $\times$ 30 cm $\times$ 0.2cm) fabricated at VECC. The steps taken towards the development of a large size (240 cm $\times$ 120 cm $\times$ 0.2 cm) bakelite RPC have also been discussed.

physics.ins-det↗
arXiv2016

A census of dense cores in the Taurus L1495 cloud from the Herschel Gould Belt Survey

We present a catalogue of dense cores in a $\sim 4^\circ\times2^\circ$ field of the Taurus star-forming region, inclusive of the L1495 cloud, derived from Herschel SPIRE and PACS observations in the 70 $μ$m, 160 $μ$m, 250 $μ$m, 350 $μ$m, and 500 $μ$m continuum bands. Estimates of mean dust temperature and total mass are derived using modified blackbody fits to the spectral energy distributions. We detect 525 starless cores of which $\sim10$-20% are gravitationally bound and therefore presumably prestellar. Our census of unbound objects is $\sim85$% complete for $M>0.015\,M_\odot$ in low density regions ($A_V\stackrel{<}{_\sim}5$ mag), while the bound (prestellar) subset is $\sim85$% complete for $M>0.1\,M_\odot$ overall. The prestellar core mass function (CMF) is consistent with lognormal form, resembling the stellar system initial mass function, as has been reported previously. All of the inferred prestellar cores lie on filamentary structures whose column densities exceed the expected threshold for filamentary collapse, in agreement with previous reports. Unlike the prestellar CMF, the unbound starless CMF is not lognormal, but instead is consistent with a power-law form below $0.3\,M_\odot$ and shows no evidence for a low-mass turnover. It resembles previously reported mass distributions for CO clumps at low masses ($M\stackrel{<}{_\sim}0.3\,M_\odot$). The volume density PDF, however, is accurately lognormal except at high densities. It is consistent with the effects of self-gravity on magnetized supersonic turbulence. The only significant deviation from lognormality is a high-density tail which can be attributed unambiguously to prestellar cores.

astro-ph.GA↗
arXiv2016

Evidence of Formation of Superdense Nonmagnetic Cobalt

Magnetism of 3d transition metals at high density has always received wide interest due to existence of magnetism at the core of the Earth. For ferromagnetic cobalt, although there is a theoretical prediction that its magnetic moment would vanish when the density increases to about 1.4 times the normal value, so far there is no experimental evidence. We have discovered the existence of ultrathin superdense nonmagnetic cobalt layers in a polycrystalline cobalt thin film. The densities of these layers are about 1.3-1.4 times the normal density of Co. This has been revealed by X-ray reflectometry experiments, which provide a depth profile of the electron scattering length density, and corroborated by polarized neutron reflectometry (PNR) experiments. The superdense Co layer has an fcc structure, unlike hcp structure for bulk Co, as revealed by transmission electron microscopy. The magnetic depth profile, obtained by PNR, shows that the superdense Co layers at the film-substrate interface and near the top of the film are nonmagnetic. The major part of the Co film has the usual density and magnetic moment.

cond-mat.mtrl-sci↗
arXiv2016

Measurement of the $\vec{n}p \to dπ^0π^0$ Reaction with Polarized Beam in the Region of the $d^*(2380)$ Resonance

We report on a high-statistics measurement of the most basic double pionic fusion reaction $\vec{n}p \to dπ^0π^0$ over the energy region of the $d^*(2380)$ resonance by use of a polarized deuteron beam and observing the double fusion reaction in the quasifree scattering mode. The measurements were performed with the WASA detector setup at COSY. The data reveal substantial analyzing powers and confirm conclusions about the $d^*$ resonance obtained from unpolarized measurements. We also confirm the previous unpolarized data obtained under complementary kinematic conditions.

nucl-ex↗
arXiv2015

High statistics study of the reaction $γp\to p\;2π^0$

The photoproduction of 2$π^0$ mesons off protons was studied with the Crystal Barrel/TAPS experiment at the electron accelerator ELSA in Bonn. The energy of photons produced in a radiator was tagged in the energy range from 600\,MeV to 2.5\,GeV. Differential and total cross sections and $pπ^0π^0$ Dalitz plots are presented. Part of the data was taken with a diamond radiator producing linearly polarized photons, and beam asymmetries were derived. Properties of nucleon and $Δ$ resonances contributing to the $pπ^0π^0$ final state were determined within the BnGa partial wave analysis. The data presented here allow us to determine branching ratios of nucleon and $Δ$ resonances for their decays into $pπ^0π^0$ via several intermediate states. Most prominent are decays proceeding via $Δ(1232)π$, $N(1440)1/2^+π$, $N(1520)3/2^-π$, $N(1680)5/2^+π$, but also $pf_0(500)$, $pf_0(980)$, and $pf_2(1270)$ contribute to the reaction.

nucl-ex↗
arXiv2015

Measurements of Branching Ratios for $η$ Decays into Charged Particles

The WASA-at-COSY experiment has collected $3\times10^{7}$ events with $η$-mesons produced via the reaction $pd\rightarrow{^{3}\textrm{He}}η$ at $\textrm{T} = 1.0 \textrm{GeV}$. Using this data set, we evaluate the branching ratios of the decays $η\toπ^{+}π^{-}γ$, $η\to e^{+}e^{-}γ$, $η\toπ^{+}π^{-}e^{+}e^{-}$ and $η\to e^{+}e^{-}e^{+}e^{-}$. The branching ratios are normalized to the $η\rightarrowπ^{+}π^{-}π^{0}$ decay. In addition an upper limit on a $CP$-violating asymmetry in $η\rightarrow π^{+}π^{-}e^{+}e^{-}$ is extracted.

nucl-ex↗
arXiv2015

A possible link between the power spectrum of interstellar filaments and the origin of the prestellar core mass function

Two major features of the prestellar CMF are: 1) a broad peak below 1 Msun, presumably corresponding to a mean gravitational fragmentation scale, and 2) a characteristic power-law slope, very similar to the Salpeter slope of the stellar initial mass function (IMF) at the high-mass end. While recent Herschel observations have shown that the peak of the prestellar CMF is close to the thermal Jeans mass in marginally supercritical filaments, the origin of the power-law tail of the CMF/IMF at the high-mass end is less clear. Inutsuka (2001) proposed a theoretical scenario in which the origin of the power-law tail can be understood as resulting from the growth of an initial spectrum of density perturbations seeded along the long axis of filaments by interstellar turbulence. Here, we report the statistical properties of the line-mass fluctuations of filaments in nearby molecular clouds observed with Herschel using a 1-D power spectrum analysis. The observed filament power spectra were fitted by a power-law function $(P_{true}(s) \propto s^α)$ after removing the effect of beam convolution at small scales. A Gaussian-like distribution of power-spectrum slopes was found centered at -1.6, close to that of the one-dimensional velocity power spectrum generated by subsonic Kolomogorov turbulence (-1.67). An empirical correlation, $P^{0.5}(s_0) \propto ^{1.4 \pm 0.1} $, was also found between the amplitude of each filament power spectrum $P(s_0)$ and the mean column density along the filament $ $. Finally, the dispersion of line-mass fluctuations along each filament $σ_{\rm M_{line}}$ was found to scale with the physical length $L$ of the filament, roughly as $σ_{M_{line}} \propto L^{0.7}$. Overall, our results are consistent with the suggestion that the bulk of the CMF/IMF results from the gravitational fragmentation of filaments.

astro-ph.GA↗
arXiv2015

Photoproduction of ω Mesons off the Proton

The differential cross sections and unpolarized spin-density matrix elements for the reaction $γp\to pω$ were measured using the CBELSA/TAPS experiment for initial photon energies ranging from the reaction threshold to 2.5 GeV. These observables were measured from the radiative decay of the $ω$ meson, $ω\toπ^0γ$. The cross sections cover the full angular range and show the full extent of the $t$-channel forward rise. The overall shape of the angular distributions in the differential cross sections and unpolarized spin-density matrix elements are in fair agreement with previous data. In addition, for the first time, a beam of linearly-polarized tagged photons in the energy range from 1150 MeV to 1650 MeV was used to extract polarized spin-density matrix elements. These data were included in the Bonn-Gatchina partial wave analysis (PWA). The dominant contribution to $ω$ photoproduction near threshold was found to be the $3/2^+$ partial wave, which is primarily due to the sub-threshold $N(1720)\,3/2^+$ resonance. At higher energies, pomeron-exchange was found to dominate whereas $π$-exchange remained small. These $t$-channel contributions as well as further contributions from nucleon resonances were necessary to describe the entire dataset: the $1/2^-$, $3/2^-$, and $5/2^+$ partial waves were also found to contribute significantly.

nucl-ex↗