Dust Emissivity Spectral Index Research Articles

An R V Map of the Milky Way Revealed by LAMOST

The total-to-selective extinction ratio, R V, is a key parameter for tracing the properties of interstellar dust, as it directly determines the variation of the extinction curve with wavelength. By utilizing accurate color excess measurements from the optical to the mid-infrared range, we have derived R V values for approximately three million stars from LAMOST data release 7 using a forward-modeling technique. This extensive data set enables us to construct a comprehensive 2D R V map of the Milky Way within the LAMOST footprint at a spatial resolution of ∼27.′5. Based on reliable sight lines of E(B − V) > 0.1, we find that R V exhibits a Gaussian distribution centered around 3.25 with a standard deviation of 0.25. The spatial variability of R V in the Galactic disk exhibits a wide range, spanning from small scales within individual molecular clouds to large scales up to kiloparsecs. A striking correlation is observed between the distribution of R V and molecular clouds. Notably, we observe lower R V values within the regions of nearby molecular clouds compared to their surrounding areas. Furthermore, we have investigated the relationships between R V and various parameters, including dust temperature, dust emissivity spectral index, column densities and ratios of atomic and molecular hydrogen, as well as the gas-to-dust ratio. We find that these relationships vary with the level of extinction. These analyses provide new insights into the properties and evolution of dust grains in diverse interstellar environments and also hold significant importance for achieving accurate extinction corrections.

ORion Alma New GEneration Survey (ORANGES)

Context. The spectral energy distribution (SED) in the millimetre to centimetre range is an extremely useful tool for characterising the dust in protostellar envelopes as well as free-free emission from the protostar and outflow. Actually, the evolutionary status of solar-type protostars is often based on their SED in the near-infrared to millimetre range. In addition, the presence or absence of free-free emission can be considered an indicator of the source evolutionary stage (Class 0/I versus Class II/III). While many studies have been carried out towards low- and high-mass protostars, little exists so far about solar-type protostars in high-mass star-forming regions, which are likely to be representatives of the conditions where the Solar System was born. Aims. In this work, we focus on the embedded solar-type protostars in the Orion Molecular Cloud (OMC) 2 and 3 filaments, which are bounded by nearby HII regions and which are, therefore, potentially affected by the high-UV illumination of the nearby OB stars. We use various dust parameters to understand whether the small-scale structure (≤1000 au) and the evolutionary status of these solar-type protostars are affected by the nearby HII regions, as is the case for the large-scale (≤104 au) gas chemical composition. Methods. We used the Atacama Large (sub-)Millimeter Array (ALMA) in the 1.3 mm band (246.2 GHz) to image the continuum of 16 young (Class 0/I) OMC-2/3 solar-type protostars, with an angular resolution of 0.25″ (100 au). We completed our data with archival data from the ALMA and VLA VANDAM survey of Orion Protostars at 333 and 32.9 GHz, respectively, to construct the dust SED and extract the dust temperature, the dust emissivity spectral index, the envelope plus disk mass of the sources and to assess whether free-free emission is contaminating their dust SED in the centimetre range. Results. From the millimetre to centimetre range dust SED, we found low dust emissivity spectral indexes (β < 1) for the majority of our source sample and free-free emission towards only 5 of the 16 sample sources. We were also able to confirm or correct the evolutionary status of the source sample reported in the literature. Finally, we did not find any dependence of the source dust parameters on their location in the OMC-2/3 filament. Conclusions. Our results show that the small-scale dust properties of the embedded OMC-2/3 protostars are not affected by the high-UV illumination from the nearby HII regions and that the formation of protostars likely takes place simultaneously throughout the filament.

Gas and star formation from HD and dust emission in a strongly lensed galaxy

ABSTRACTThe molecular gas content of high-redshift galaxies is a highly sought-after property. However, H2 is not directly observable in most environments, so its mass is probed through other emission lines (e.g. CO, [C i], [C ii]), or through a gas-to-dust ratio. Each of these methods depends on several assumptions, and are best used in parallel. In this work, we extend an additional molecular gas tracer to high-redshift studies by observing hydrogen deuteride (HD) emission in the strongly lensed z = 5.656 galaxy SPT0346−52 with ALMA. While no HD(1–0) emission is detected, we are able to place an upper limit on the gas mass of $\rm M_{H_2}\lt 6.4\times 10^{11}\, M_{\odot }$. This is used to find a limit on the $L^{\prime }_{\mathrm{ CO}}$ conversion factor of $\rm \alpha _{CO}\lt 5.8\,M_{\odot}(K\,km\,s^{-1}\,pc^2)^{-1}$. In addition, we construct the most complete spectral energy distribution of this source to date, and fit it with a single-temperature modified blackbody using the nested sampling code multinest, yielding a best-fitting dust mass Mdust = 108.92 ± 0.02 M⊙, dust temperature 78.6 ± 0.5 K, dust emissivity spectral index β = 1.81 ± 0.03, and star formation rate SFR = 3800 ± 100 M⊙ yr−1. Using the continuum flux densities to estimate the total gas mass of the source, we find $\rm M_{H_2}\lt 2.4\times 10^{11}\, M_{\odot }$ , assuming subsolar metallicity. This implies a CO conversion factor of αCO < 2.2, which is between the standard values for MW-like galaxies and starbursts. These properties confirm that SPT0346−52 is a heavily starbursting, gas-rich galaxy.

Planck Cold Clumps in the λ Orionis Complex. II. Environmental Effects on Core Formation

Based on the 850 $\mu$m dust continuum data from SCUBA-2 at James Clerk Maxwell Telescope (JCMT), we compare overall properties of Planck Galactic Cold Clumps (PGCCs) in the $\lambda$ Orionis cloud to those of PGCCs in the Orion A and B clouds. The Orion A and B clouds are well known active star-forming regions, while the $\lambda$ Orionis cloud has a different environment as a consequence of the interaction with a prominent OB association and a giant Hii region. PGCCs in the $\lambda$ Orionis cloud have higher dust temperatures ($Td=16.13\pm0.15$ K) and lower values of dust emissivity spectral index ($ \beta=1.65\pm0.02$) than PGCCs in the Orion A (Td=13.79$\pm 0.21$K, $\beta=2.07\pm0.03$) and Orion B ($Td=13.82\pm0.19$K, $\beta=1.96\pm0.02$) clouds. We find 119 sub-structures within the 40 detected PGCCs and identify them as cores. Of total 119 cores, 15 cores are discovered in the $\lambda$ Orionis cloud, while 74 and 30 cores are found in the Orion A and B clouds, respectively. The cores in the $\lambda$ Orionis cloud show much lower mean values of size R=0.08 pc, column density N(H2)=$(9.5\pm1.2) \times 10^{22}$ cm$^{-2}$, number density n(H2)=$(2.9 \pm 0.4)\times10^{5}$ cm$^{-3}$, and mass $M_{core}$=$1.0\pm0.3$ M$_{\odot}$ compared to the cores in the Orion A (R=0.11pc, $N(H2)=(2.3\pm0.3) \times 10^{23}$ cm$^{-2}$, n(H2)=$(3.8\pm0.5) \times 10^{5}$cm$^{-3}$, and $M_{core}$=$2.4 \pm 0.3$ M$_{\odot}$) and Orion B (R=0.16pc, N(H2)=$(3.8 \pm 0.4) \times 10^{23}$cm$^{-2}$, n(H2)=$(15.6\pm1.8)\times10^{5}$ cm$^{-3}$, and $M_{core}$= $2.7\pm0.3$ M$_{\odot}$) clouds. These core properties in the $\lambda$ Orionis cloud can be attributed to the photodissociation and external heating by the nearby Hii region, which may prevent the PGCCs from forming gravitationally bound structures and eventually disperse them. These results support the idea of negative stellar feedback on core formation.

The TOP-SCOPE Survey of Planck Galactic Cold Clumps: Survey Overview and Results of an Exemplar Source, PGCC G26.53+0.17

The low dust temperatures (<14 K) of Planck Galactic Cold Clumps (PGCCs) make them ideal targets to probe the initial conditions and very early phase of star formation. TOP-SCOPE is a joint survey program targeting ~2000 PGCCs in J=1-0 transitions of CO isotopologues and ~1000 PGCCs in 850 micron continuum emisison. The objective of the TOP-SCOPE survey and the joint surveys (SMT 10-m, KVN 21-m and NRO 45-m) is to statistically study the initial conditions occurring during star formation and the evolution of molecular clouds, across a wide range of environments. The observations, data analysis and example science cases for these surveys are introduced with an exemplar source, PGCC G26.53+0.17 (G26), which is a filamentary infrared dark cloud (IRDC). The total mass, the length and the mean line-mass (M/L) of the G26 filament are ~6200 Msun, ~12 pc and ~500 Msun/pc, respectively. Ten massive clumps including eight starless ones are found along the filament. The most massive Clump as a whole may be still in global collapse while its denser part seems to be undergoing expansion due to outflow feedback. The fragmentation in G26 filament from cloud scale to clump scale is in agreement with gravitational fragmentation of an isothermal, non-magnetized, and turbulent supported cylinder. A bimodal behavior in dust emissivity spectral index ($\beta$) distribution is found in G26, suggesting grain growth along the filament. The G26 filament may be formed due to large-scale compression flows evidenced by the temperature and velocity gradients across its natal cloud.

Radio and infrared study of the star-forming region IRAS 20286+4105

A multi-wavelength investigation of the star forming complex IRAS 20286+4105, located in the Cygnus-X region, is presented here. Near-infrared K-band data is used to revisit the cluster / stellar group identified in previous studies. The radio continuum observations, at 610 and 1280 MHz show the presence of a HII region possibly powered by a star of spectral type B0 - B0.5. The cometary morphology of the ionized region is explained by invoking the bow-shock model where the likely association with a nearby supernova remnant is also explored. A compact radio knot with non-thermal spectral index is detected towards the centre of the cloud. Mid-infrared data from the Spitzer Legacy Survey of the Cygnus-X region show the presence of six Class I YSOs inside the cloud. Thermal dust emission in this complex is modelled using Herschel far-infrared data to generate dust temperature and column density maps. Herschel images also show the presence of two clumps in this region, the masses of which are estimated to be {\sim} 175 M{\sun} and 30 M{\sun}. The mass-radius relation and the surface density of the clumps do not qualify them as massive star forming sites. An overall picture of a runaway star ionizing the cloud and a triggered population of intermediate-mass, Class I sources located toward the cloud centre emerges from this multiwavelength study. Variation in the dust emissivity spectral index is shown to exist in this region and is seen to have an inverse relation with the dust temperature.

THE JCMT GOULD BELT SURVEY: EVIDENCE FOR DUST GRAIN EVOLUTION IN PERSEUS STAR-FORMING CLUMPS

ABSTRACT The dust emissivity spectral index, β, is a critical parameter for deriving the mass and temperature of star-forming structures and, consequently, their gravitational stability. The β value is dependent on various dust grain properties, such as size, porosity, and surface composition, and is expected to vary as dust grains evolve. Here we present β, dust temperature, and optical depth maps of the star-forming clumps in the Perseus Molecular Cloud determined from fitting spectral energy distributions to combined Herschel and JCMT observations in the 160, 250, 350, 500, and 850 μm bands. Most of the derived β and dust temperature values fall within the ranges of 1.0–2.7 and 8–20 K, respectively. In Perseus, we find the β distribution differs significantly from clump to clump, indicative of grain growth. Furthermore, we also see significant localized β variations within individual clumps and find low-β regions correlate with local temperature peaks, hinting at the possible origins of low-β grains. Throughout Perseus, we also see indications of heating from B stars and embedded protostars, as well evidence of outflows shaping the local landscape.

ALMA observations of anisotropic dust mass loss in the inner circumstellar environment of the red supergiant VY Canis Majoris

The processes leading to dust formation and the subsequent role it plays in driving mass loss in cool evolved stars is an area of intense study. Here we present high resolution ALMA Science Verification data of the continuum emission around the highly evolved oxygen-rich red supergiant VY CMa. These data enable us to study the dust in its inner circumstellar environment at a spatial resolution of 129 mas at 321 GHz and 59 mas at 658 GHz, thus allowing us to trace dust on spatial scales down to 11 R$_{\star}$ (71 AU). Two prominent dust components are detected and resolved. The brightest dust component, C, is located 334 mas (61 R$_{\star}$) South East of the star and has a dust mass of at least $2.5\times 10^{-4}$ M$_{\odot}$. It has a dust emissivity spectral index of $\beta =-0.1$ at its peak, implying that it is optically thick at these frequencies with a cool core of $T_{d}\lesssim 100$ K. Interestingly, not a single molecule in the ALMA data has emission close to the peak of this massive dust clump. The other main dust component, VY, is located at the position of the star and contains a total dust mass of $4.0 \times 10^{-5} $M$_{\odot}$. It also contains a weaker dust feature extending over $60$ R$_{\star}$ to the North with the total component having a typical dust emissivity spectral index of $\beta =0.7$. We find that at least $17\%$ of the dust mass around VY CMa is located in clumps ejected within a more quiescent roughly spherical stellar wind, with a quiescent dust mass loss rate of $5 \times 10^{-6}$ M$_{\odot} $yr$^{-1}$. The anisotropic morphology of the dust indicates a continuous, directed mass loss over a few decades, suggesting that this mass loss cannot be driven by large convection cells alone.

Evidence for large grains in the star-forming filament OMC 2/3

We present a new 3.3 mm continuum map of the Orion Molecular Cloud (OMC) 2/3 region. When paired with previously published maps of 1.2 mm continuum and NH3-derived temperature, we derive the emissivity spectral index of dust emission in this region, tracking its changes across the filament and cores. We find that the median value of the emissivity spectral index is 0.9, much shallower than previous estimates in other nearby molecular clouds. We find no significant difference between the emissivity spectral index of dust in the OMC 2/3 filament and the starless or protostellar cores. Furthermore, the temperature and emissivity spectral index, β, are anticorrelated at the 4σ level. The low values of the emissivity spectral index found in OMC 2/3 can be explained by the presence of millimetre-sized dust grains in the dense regions of the filaments to which these maps are most sensitive. Alternatively, a shallow dust emissivity spectral index may indicate non-power-law spectral energy distributions, significant free–free emission, or anomalous microwave emission. We discuss the possible implications of millimetre-sized dust grains compared to the alternatives.

A resolved analysis of cold dust and gas in the nearby edge-on spiral NGC 891

We investigate the connection between dust and gas in the nearby edge-on spiral galaxy NGC 891. High resolution Herschel PACS and SPIRE 70, 100, 160, 250, 350, and 500 $\mu$m images are combined with JCMT SCUBA 850 $\mu$m observations to trace the far-infrared/submillimetre spectral energy distribution (SED). Maps of the HI 21 cm line and CO(J=3-2) emission trace the atomic and molecular hydrogen gas, respectively. We fit one-component modified blackbody models to the integrated SED, finding a global dust mass of 8.5$\times$10$^{7}$ M$_{\odot}$ and an average temperature of 23$\pm$2 K. We also fit the pixel-by-pixel SEDs to produce maps of the dust mass and temperature. The dust mass distribution correlates with the total stellar population as traced by the 3.6 $\mu$m emission. The derived dust temperature, which ranges from approximately 17 to 24 K, is found to correlate with the 24 $\mu$m emission. Allowing the dust emissivity index to vary, we find an average value of $\beta$ = 1.9$\pm$0.3. We confirm an inverse relation between the dust emissivity spectral index and dust temperature, but do not observe any variation of this relationship with vertical height from the mid-plane of the disk. A comparison of the dust properties with the gaseous components of the ISM reveals strong spatial correlations between the surface mass densities of dust and the molecular hydrogen and total gas surface densities. Observed asymmetries in the dust temperature, and the H$_{2}$-to-dust and total gas-to-dust ratios hint that an enhancement in the star formation rate may be the result of larger quantities of molecular gas available to fuel star formation in the NE compared to the SW. Whilst the asymmetry likely arises from dust obscuration due to the geometry of the line-of-sight projection of the spiral arms, we cannot exclude an enhancement in the star formation rate in the NE side of the disk.

GASEOUS CO ABUNDANCE—AN EVOLUTIONARY TRACER FOR MOLECULAR CLOUDS

Planck cold clumps are among the most promising objects to investigate the initial conditions of the evolution of molecular clouds. In this work, by combing the dust emission data from the survey of Planck satellite with the molecular data of $^{12}$CO/$^{13}$CO (1-0) lines from observations with the Purple Mountain Observatory (PMO) 13.7 m telescope, we investigate the CO abundance, CO depletion and CO-to-H$_{2}$ conversion factor of 674 clumps in the early cold cores (ECC) sample. The median and mean values of the CO abundance are 0.89$\times10^{-4}$ and 1.28$\times10^{-4}$, respectively. The mean and median of CO depletion factor are 1.7 and 0.9, respectively. The median value of $X_{CO-to-H_{2}}$ for the whole sample is $2.8\times10^{20}$ cm$^{-2}$K$^{-1}$km$^{-1}$ s. The CO abundance, CO depletion factor and CO-to-H$_{2}$ conversion factor are strongly (anti-)correlated to other physical parameters (e.g. dust temperature, dust emissivity spectral index, column density, volume density and luminosity-to-mass ratio). To conclude, the gaseous CO abundance can be used as an evolutionary tracer for molecular clouds.

AHerschelview of the far-infrared properties of submillimetre galaxies

We study a sample of 61 submillimetre galaxies (SMGs) selected from ground-based surveys, with known spectroscopic redshifts and observed with Herschel as part of the PACS Evolutionary Probe (PEP) and the Herschel Multi-tiered Extragalactic Survey (HerMES) key programmes. We use the broad far-infrared wavelength coverage (100-600um) provided by the combination of PACS and SPIRE observations. Using a power-law temperature distribution model to derive infrared luminosities and dust temperatures, we measure a dust emissivity spectral index for SMGs of beta=2.0+/-0.2. Our results unveil the diversity of the SMG population. Some SMGs exhibit extreme infrared luminosities of ~10^13 Lsun and relatively warm dust components, while others are fainter (~10^12 Lsun) and are biased towards cold dust temperatures. The extreme infrared luminosities of some SMGs (LIR>10^12.7 Lsun, 26/61 systems) imply SFRs of >500Msun yr^-1. Such high SFRs are difficult to reconcile with a secular mode of star formation, and may instead correspond to a merger-driven stage in the evolution of these galaxies. Another observational argument in favour of this scenario is the presence of dust temperatures warmer than that of SMGs of lower luminosities (~40K as opposed to ~25K), consistent with observations of local ULIRGs triggered by major mergers and with results from hydrodynamic simulations of major mergers combined with radiative transfer calculations. Luminous SMGs are also offset from normal star-forming galaxies in the stellar mass-SFR plane, suggesting that they are undergoing starburst events with short duty cycles, compatible with the major merger scenario. On the other hand, a significant fraction of the low infrared luminosity SMGs have cold dust temperatures, are located close to the main sequence of star formation, and thus might be evolving through a secular mode of star formation. [abridged]

Variations of the spectral index of dust emissivity from Hi-GAL observations of the Galactic plane

Variations in the dust emissivity are critical for gas mass determinations derived from far-infrared observations, but also for separating dust foreground emission from the Cosmic Microwave Background (CMB). Hi-GAL observations allow us for the first time to study the dust emissivity variations in the inner regions of the Galactic plane at resolution below 1 degree. We present maps of the emissivity spectral index derived from the combined Herschel PACS 160 \mu m, SPIRE 250 \mu m, 350 \mu m, and 500 \mu m data, and the IRIS 100 \mu m data, and we analyze the spatial variations of the spectral index as a function of dust temperature and wavelength in the two Science Demonstration Phase Hi-GAL fields, centered at l=30{\deg} and l=59{\deg}. Applying two different methods, we determine both dust temperature and emissivity spectral index between 100 and 500 \mu m, at an angular resolution of 4'. Combining both fields, the results show variations of the emissivity spectral index in the range 1.8-2.6 for temperatures between 14 and 23 K. The median values of the spectral index are similar in both fields, i.e. 2.3 in the range 100-500 \mu m, while the median dust temperatures are equal to 19.1 K and 16.0 K in the l=30{\deg} and l=59{\deg} field, respectively. Statistically, we do not see any significant deviations in the spectra from a power law emissivity between 100 and 500 \mu m. We confirm the existence of an inverse correlation between the emissivity spectral index and dust temperature, found in previous analyses.

THE DUST EMISSIVITY SPECTRAL INDEX IN THE STARLESS CORE TMC-1C

In this paper, we present a dust emission map of the starless core TMC-1C taken at 2100 μm. Along with maps at 160, 450, 850, and 1200 μm, we study the dust emissivity spectral index from the (sub)millimeter spectral energy distribution, and find that it is close to the typically assumed value of β = 2. We also map the dust temperature and column density in TMC-1C, and find that at the position of the dust peak (AV ∼ 50) the line-of-sight-averaged temperature is ∼7 K. Employing simple Monte Carlo modeling, we show that the data are consistent with a constant value for the emissivity spectral index over the whole map of TMC-1C.

Discovery of a Cold Extended Condensation in the Orion A Complex

The central region of the Orion A cloud complex has been observed in the submillimeter range, using the French balloon-borne 2 m telescope PRONAOS-SPM during its first flight, in 1994 September. An area covering 50' × 10' and including the M42 Nebula has been mapped in four submillimeter photometric bands: λ180-240, 240-340, 340-560, and 560-1050 μm, with high sensitivity (≤2 MJy sr-1) and an angular resolution from 2' to 35. Four brightness enhancements are visible and have been identified with the following sources: (1) the brightest peak corresponding to the central core of the nebula, in the BN/KL source direction; (2) an extended emission region around 5' × 8', to the southeast of BN/KL, correlated with 100 μm IRAS and 13CO emission (this has been identified as a gas-density enhancement associated with a shock region); (3) the edge of an ionized part of the cloud, correlated with the 100 μm emission; and (4) a very cold and extended condensation, discovered at 16' northwest of BN/KL. The emission spectra obtained for these sources show a variation of the dust emissivity spectral index within a large range, from 1 ± 0.1 to 2.2 ± 0.2. The cold condensation discovered (T = 12.5 ± 3 K) has a very low brightness emission, undetected on the 100 μm IRAS map. It extends over approximately 0.7 pc (FWHM), with a total mass of 11 M☉ and a total luminosity of 2.4 L☉.