Spectral Energy Distribution Slope Research Articles

Can near-to-mid Infrared Spectral Energy Distribution Quantitatively Trace Protoplanetary Disk Evolution?

Infrared (IR) spectral energy distribution (SED) is the major tracer of protoplanetary disks. It was recently proposed to use the near-to-mid IR (or K-24) SED slope α defined between 2 and 24μm as a potential quantitative tracer of disk age. We critically examine the viability of this idea and confront it with additional statistics of IR luminosities and SED shapes. We point out that, because the statistical properties of most of the complicated physical factors involved in disk evolution are still poorly understood in a quantitative sense, the only viable way is to assume them to be random so that an idealized “average disk” can be defined, which allows the α histogram to trace its age. We confirm that the statistics of the zeroth order (luminosity), first order (slope α), and second order characteristics (concavity) of the observed K-24 SEDs indeed carry useful information upon the evolutionary processes of the “average disk”. We also stress that intrinsic diversities in K-24 SED shapes and luminosities are always large at the level of individual stars so that the application of the evolutionary path of the “average disk” to individual stars must be done with care. The data of most curves in plots are provided on GitHub (Disk-age package https://github.com/starage/disk-age/).

PHANGS–JWST First Results: Mapping the 3.3 μm Polycyclic Aromatic Hydrocarbon Vibrational Band in Nearby Galaxies with NIRCam Medium Bands

We present maps of the 3.3 μm polycyclic aromatic hydrocarbon (PAH) emission feature in NGC 628, NGC 1365, and NGC 7496 as observed with the Near-Infrared Camera imager on JWST from the PHANGS–JWST Cycle 1 Treasury project. We create maps that isolate the 3.3 μm PAH feature in the F335M filter (F335MPAH) using combinations of the F300M and F360M filters for removal of starlight continuum. This continuum removal is complicated by contamination of the F360M by PAH emission and variations in the stellar spectral energy distribution slopes between 3.0 and 3.6 μm. We modify the empirical prescription from Lai et al. to remove the starlight continuum in our highly resolved galaxies, which have a range of starlight- and PAH-dominated lines of sight. Analyzing radially binned profiles of the F335MPAH emission, we find that between 5% and 65% of the F335M intensity comes from the 3.3 μm feature within the inner 0.5 r 25 of our targets. This percentage systematically varies from galaxy to galaxy and shows radial trends within the galaxies related to each galaxy’s distribution of stellar mass, interstellar medium, and star formation. The 3.3 μm emission is well correlated with the 11.3 μm PAH feature traced with the MIRI F1130W filter, as is expected, since both features arise from C–H vibrational modes. The average F335MPAH/F1130W ratio agrees with the predictions of recent models by Draine et al. for PAHs with size and charge distributions shifted toward larger grains with normal or higher ionization.

NEOWISE Variability of the New Gaia Herbig Ae/Be and Classical Be Stars—Variability Sample and Statistical Analysis

Light variability is an essential attribute of young stellar objects (YSOs) at the pre-main-sequence (PMS) stage, which can be treated as an indicator to trace star formation. We present a statistical study on the infrared NEOWISE variability of new and large-sized PMS stars as well as Herbig Ae/Be (HAeBe) and classical Be (CBe) stars identified from Gaia Data Release 2. A total of 3792 (including 714 HAeBe), 417, and 473 NEOWISE variable sources were detected from the catalog of 8470 PMS (including 1361 HAeBe), 693 CBe, and 1309 uncertain (possibly belonging to either HAeBe or CBe) stars, respectively. We found that 80% of the PMS variables are dominated by irregular infrared variability, whereas variables in the CBe and uncertain categories exhibit comparable contributions from secular (linear, curved, and periodic) and stochastic (burst, drop, and irregular) variables. This, in combination with infrared W1–W2 colors, W4 luminosities, and the spectral energy distribution slope at near- and mid-infrared wavelengths, suggests different evolutionary stages of the PMS (including HAeBe), uncertain, and CBe stars, from less to more evolved. A statistical analysis of variable source properties (e.g., period and fractional amplitude) further reveals a potential sequence possibly associated with disk evolution for different types of light curves (from linear, to curved, periodic, stochastic, and nonvariable stars). Possible mechanisms for the variable sources are suggested to be related to either extinction via circumstellar matter, or contribution of multiple systems surrounding the YSOs, as opposed to variable accretion (e.g., episodic accretion or accretion burst).

Spitzer Mid-infrared Study of Sh 2-208: Evolution of Protoplanetary Disks in Low-metallicity Environments

Abstract This study presents sensitive mid-infrared (MIR) photometry obtained with the Spitzer/IRAC for a young cluster in Sh 2-208 (S208) located in one of the lowest-metallicity H ii regions in the Galaxy, [ O / H ] = − 0.8 dex. Previous studies suggested that the cluster is ∼0.5 Myr old and has a distance of D = 4 kpc, which is consistent with the astrometric distance from Gaia EDR3. In a ∼ 3 .′ 5 × 4 ′ field, 96 sources were detected in at least one MIR band at ≥10σ, covering intermediate-mass stars with a ∼1.0 M ⊙ mass detection limit. A total of 41 probable cluster members were identified based on the spatial distributions of spectral energy distribution slopes derived from the near-infrared (NIR) K S and IRAC bands and extinctions of the sources. The cumulative distribution of the spectral energy distribution slopes for the S208 cluster was not significantly different from those of other clusters in solar-metallicity environments with approximately the same age for intermediate-mass stars, if one also considers nondetected MIR sources identified as S208 cluster members from NIR observations. This suggests that the degree of dust growth/settling does not significantly change with metallicities as different as ∼1 dex. The fraction of stars with MIR disk emissions for the cluster members with ≥1 M ⊙ was 64%–93%, which is comparable to the results in solar-metallicity environments. Although this may suggest that dominant disk dispersal mechanisms for intermediate-mass stars have either no or very weak dependence on metallicity, it can be argued alternatively that this may suggest that the disk dispersal process does not work effectively at this young stage.

The Mouse Pulsar Wind Nebula

Abstract The young energetic pulsar J1747–2958 (τ = 26 kyr, erg s−1) powers the Mouse pulsar wind nebula (PWN), famous for its spectacular tail spanning 45″ in X-rays and 12′ in radio (d ∼ 5 kpc). We present the results of Chandra observations of the PWN and the analysis of archival lower-frequency data. The Chandra HRC image reveals a point-like source at the pulsar position, ≈1′ behind the bow shock apex of the PWN. The flattened appearance of the compact nebula is consistent with an equatorial outflow deformed by the ram pressure, implying that the angle between the pulsar’s spin axis and line of sight is ∼70° (in agreement with the radio and γ-ray pulse profiles). The spatially resolved spectroscopy with Chandra ACIS shows that the power-law (PL) spectrum steepens from Γ = 1.65 ± 0.02 to 3.0 ± 0.1 over the 45′ extent of the X-ray tail. However, the tail’s X-ray spectrum integrated over its 45″ length fits a single absorbed PL with Γ = 2.09 ± 0.03. We also found the Mouse PWN in 150 MHz GMRT data and a possible counterpart in 24 μm Spitzer data. The multiwavelength data suggest that, at low frequencies, the spectrum of the X-ray-emitting tail region can be described by a broken PL with at least one turnover between radio and X-rays. This is consistent with synchrotron cooling of electrons injected at the termination shock (with a spectral energy distribution slope of 2.2) with an equipartition magnetic field B ∼ 200 μG and a bulk flow speed v ∼ 4000 km s−1.

Young star clusters in nearby molecular clouds

The SFiNCs (Star Formation in Nearby Clouds) project is an X-ray/infrared study of the young stellar populations in 22 star forming regions with distances <=1 kpc designed to extend our earlier MYStIX survey of more distant clusters. Our central goal is to give empirical constraints on cluster formation mechanisms. Using parametric mixture models applied homogeneously to the catalog of SFiNCs young stars, we identify 52 SFiNCs clusters and 19 unclustered stellar structures. The procedure gives cluster properties including location, population, morphology, association to molecular clouds, absorption, age (AgeJX), and infrared spectral energy distribution (SED) slope. Absorption, SED slope, and AgeJX are age indicators. SFiNCs clusters are examined individually, and collectively with MYStIX clusters, to give the following results. (1) SFiNCs is dominated by smaller, younger, and more heavily obscured clusters than MYStIX. (2) SFiNCs cloud-associated clusters have the high ellipticities aligned with their host molecular filaments indicating morphology inherited from their parental clouds. (3) The effect of cluster expansion is evident from the radius-age, radius-absorption, and radius-SED correlations. Core radii increase dramatically from ~0.08 to ~0.9 pc over the age range 1--3.5 Myr. Inferred gas removal timescales are longer than 1 Myr. (4) Rich, spatially distributed stellar populations are present in SFiNCs clouds representing early generations of star formation. An Appendix compares the performance of the mixture models and nonparametric Minimum Spanning Tree to identify clusters. This work is a foundation for future SFiNCs/MYStIX studies including disk longevity, age gradients, and dynamical modeling.

A Comparative Observational Study of YSO Classification in Four Small Star-forming H ii Regions

Abstract We have developed a new young stellar object (YSO) identification and classification technique using mid-infrared Wide-field Infrared Survey Explorer (WISE) data. We compare this new technique with previous WISE YSO detection and classification methods that used either infrared colors or spectral energy distribution slopes. In this study, we also use the new technique to detect and examine the YSO population associated with four small H ii regions: KR 7, KR 81, KR 120, and KR 140. The relatively simple structure of these regions allows us to effectively use both spatial and temporal constraints to identify YSOs that are potential products of triggered star formation. We are also able to identify regions of active star formation around these H ii regions that are clearly not influenced by the H ii region expansion, and thus demonstrate that star formation is on-going on megayear timescales in some of these molecular clouds.

SPIRAL STRUCTURE AND DIFFERENTIAL DUST SIZE DISTRIBUTION IN THE LkHα 330 DISK

ABSTRACT Dust trapping accelerates the coagulation of dust particles, and, thus, it represents an initial step toward the formation of planetesimals. We report H-band (1.6 μm) linear polarimetric observations and 0.87 mm interferometric continuum observations toward a transitional disk around LkHα 330. As a result, a pair of spiral arms were detected in the H-band emission, and an asymmetric (potentially arm-like) structure was detected in the 0.87 mm continuum emission. We discuss the origin of the spiral arm and the asymmetric structure and suggest that a massive unseen planet is the most plausible explanation. The possibility of dust trapping and grain growth causing the asymmetric structure was also investigated through the opacity index (β) by plotting the observed spectral energy distribution slope between 0.87 mm from our Submillimeter Array observation and 1.3 mm from literature. The results imply that grains are indistinguishable from interstellar medium-like dust in the east side ( ) but are much smaller in the west side , indicating differential dust size distribution between the two sides of the disk. Combining the results of near-infrared and submillimeter observations, we conjecture that the spiral arms exist at the upper surface and an asymmetric structure resides in the disk interior. Future observations at centimeter wavelengths and differential polarization imaging in other bands (Y–K) with extreme AO imagers are required to understand how large dust grains form and to further explore the dust distribution in the disk.

AWISECENSUS OF YOUNG STELLAR OBJECTS IN THE PERSEUS OB2 ASSOCIATION

We have performed a Wide-Field Infrared Survey Explorer based study to identify and characterize young stellar objects (YSOs) in the 12° × 12° Perseus OB2 association. A spectral energy distribution (SED) slope in the range of 3.4–12 μm and 5σ selection criteria were used to select our initial sample. Further manual inspection reduced our final catalog to 156 known and 119 YSO candidates. The spatial distribution of newly found YSOs all over the field shows an older generation of star formation in which most of its massive members have evolved into main-sequence stars. In contrast, the majority of younger members lie within the Perseus molecular cloud and currently active star-forming clusters such as NGC 1333 and IC 348. We also identified an additional 66 point sources that passed YSO selection criteria but are likely AGB stars. However, their spatial distribution suggests that they may contain a fraction of the YSOs. Comparing our results with the commonly used color–color selections, we found that while the color selection method fails in picking up bright but evolved weak disks, our SED fitting method can identify such sources, including transitional disks. In addition, we have less contamination with background sources such as galaxies, but at a price of losing fainter () YSOs. Finally, we employed a Bayesian Monte Carlo SED fitting method to determine the characteristics of each YSO candidate. Distribution of SED slopes and model-driven age and mass confirms separated YSO populations with three suggested age groups of younger than 1 Myr old, 1–5 Myr old, and older than 5 Myr, which agrees with the age of the Per OB2 association and currently star-forming sites within the cloud.

Absorption of crystalline water ice in the far infrared at different temperatures

The optical properties of ice in the far infrared are important for models of protoplanetary and debris disks. In this report we derive a new set of data for the absorption (represented by the imaginary part of the refractive index $\kappa$) of crystalline water ice in this spectral range, including a detailed inspection of the temperature dependence, which had not been done in such detail before. We measured the transmission of three ice layers with different thicknesses at temperatures $\vartheta = 10...250$K and present data at wavelengths $\lambda=80...625$ microns. We found a change in the spectral dependence of $\kappa$ at a wavelength of $175 \pm 6$ microns. At shorter wavelengths, $\kappa$ exhibits a constant flat slope and no significant temperature dependence. Long-ward of that wavelength, the slope gets steeper and has a clear, approximately linear temperature dependence. This change in the behaviour is probably caused by a characteristic absorption band of water ice. The measured data were fitted by a power-law model that analytically describes the absorption behaviour at an arbitrary temperature. This model can readily be applied to any object of interest, for instance a protoplanetary or a debris disk. To illustrate how the model works, we simulated the spectral energy distribution (SED) of the resolved, large debris disk around the nearby solar-type star HD 207129. Replacing our ice model by another, commonly used data set for water ice results in a different SED slope at longer wavelengths. This leads to changes in the characteristic model parameters of the disk, such as the inferred particle size distribution, and affects the interpretation of the underlying collisional physics of the disk.

ASPITZERVIEW OF THE YOUNG OPEN CLUSTER NGC 2264

We have performed mid-IR photometry of the young open cluster NGC 2264 using the images obtained with the Spitzer Space Telescope IRAC and MIPS instruments and present a normalized classification scheme of young stellar objects in various color-color diagrams to make full use of the information from multicolor photometry. These results are compared with the classification scheme based on the slope of the spectral energy distribution (SED). From the spatial distributions of Class I and II stars, we have identified two subclusterings of Class I objects in the CONE region of Sung et al. The disked stars in the other star forming region S MON are mostly Class II objects. These three regions show a distinct difference in the fractional distribution of SED slopes as well as the mean value of SED slopes. The fraction of stars with primordial disks is nearly flat between log m = 0.2 -- -0.5, and that of transition disks is very high for solar mass stars. In addition, we have derived a somewhat higher value of the primordial disk fraction for NGC 2264 members located below the main pre-main sequence locus (so-called BMS stars). This result supports the idea that BMS stars are young stars with nearly edge-on disks. We have also found that the fraction of primordial disks is very low near the most massive star S Mon and increases with distance from S Mon.

The use of genetic algorithms to model protoplanetary discs

The protoplanetary discs of T Tauri and Herbig Ae/Be stars have previously been studied using geometric disc models to fit their spectral energy distribution (SED). The simulations provide a means to reproduce the signatures of various circumstellar structures, which are related to different levels of infrared excess. With the aim of improving our previous model, which assumed a simple flat-disc configuration, we adopt here a reprocessing flared-disc model that assumes hydrostatic, radiative equilibrium. We have developed a method to optimize the parameter estimation based on genetic algorithms (GAs). This paper describes the implementation of the new code, which has been applied to Herbig stars from the Pico dos Dias Survey catalogue, in order to illustrate the quality of the fitting for a variety of SED shapes. The star AB Aur was used as a test of the GA parameter estimation, and demonstrates that the new code reproduces successfully a canonical example of the flared-disc model. The GA method gives a good quality of fit, but the range of input parameters must be chosen with caution, as unrealistic disc parameters can be derived. It is confirmed that the flared-disc model fits the flattened SEDs typical of Herbig stars; however, embedded objects (increasing SED slope) and debris discs (steeply decreasing SED slope) are not well fitted with this configuration. Even considering the limitation of the derived parameters, the automatic process of SED fitting provides an interesting tool for the statistical analysis of the circumstellar luminosity of large samples of young stars.

ASpitzer Space TelescopeStudy of Disks in the Young σ Orionis Cluster

We report new Spitzer Space Telescope observations, using the IRAC and MIPS instruments, of the young (~3 Myr) σ Orionis cluster. We identify 336 stars as members of the cluster, using optical and near-infrared color-magnitude diagrams. Using the spectral energy distribution slopes in the IRAC spectral range, we place objects into several classes: non-excess stars, stars with optically thick disks (such as classical T Tauri stars), class I (protostellar) candidates, and stars with "evolved disks"; the last exhibit smaller IRAC excesses than optically thick disk systems. In general, this classification agrees with the location expected in IRAC-MIPS color-color diagrams for these objects. We find that the evolved disk systems are mostly a combination of objects with optically thick but nonflared disks, suggesting grain growth and/or settling, and transition disks, systems in which the inner disk is partially or fully cleared of small dust. In all, we identify seven transition disk candidates and three possible debris disk systems. As in other young stellar populations, the fraction of disks depends on the stellar mass, ranging from ~10% for stars in the Herbig Ae/Be mass range (>2 M☉) to ~35% for those in the T Tauri mass range (1-0.1 M☉). The IRAC infrared excesses found in stellar clusters and associations with and without central high-mass stars are similar, suggesting that external photoevaporation is not very important in many clusters. Finally, we find no correlation between the X-ray luminosity and the disk infrared excess, suggesting that the X-rays are not strongly affected by disk accretion.

Models for achromatic light-curve breaks in gamma-ray burst afterglows: jets, structured outflows and energy injection

The steepening (break) of the power-law fall-off observed in the optical emission of some GRB afterglows at epoch ~1 day is often attributed to a collimated outflow (jet), undergoing lateral spreading. Wider opening GRB ejecta with a non-uniform energy angular distribution (structured outflows) or the cessation of energy injection in the afterglow can also yield light-curve breaks. We determine the optical and X-ray light-curve decay indices and spectral energy distribution slopes for 10 GRB afterglows with optical light-curve breaks (980519, 990123, 990510, 991216, 000301, 000926, 010222, 011211, 020813, 030226), and use these properties to test the above models for light-curve steepening. It is found that the optical breaks of six of these afterglows can be accommodated by either energy injection or by structured outflows. In the refreshed shock model, a wind-like stratification of the circumburst medium (as expected for massive stars as GRB progenitors) is slightly favoured. A spreading jet interacting with a homogeneous circumburst medium is required by the afterglows 990510, 000301, 011211, and 030226. The optical pre- and post-break decays of these four afterglows are incompatible with a wind-like medium. The current sample of 10 afterglows with breaks suggests that the distribution of the break magnitude (defined as the increase of the afterglow decay exponent) is bimodal, with a gap at 1. If true, this bimodality favours the structured outflow model, while the gap location indicates a homogeneous circumburst environment.

Probing the embedded YSOs of the R CrA region through VLT-ISAAC spectroscopy

Near IR spectra at low (R ∼ 800) and medium (R ∼ 9000) resolution, obtained with ISAAC at VLT, have been used to pose constraints on the evolutionary state and accretion properties of a sample of five embedded YSOs located in the R CrA core. This sample includes three Class I sources (HH100 IR, IRS2 and IRS5), and two sources with NIR excesses (IRS6 and IRS3). IRS5 and IRS6 have been discovered to be binaries with a separation between the two components of 78 and 97 AU, respectively. Absorption lines, typical of late-type photospheres, have been detected in the medium resolution spectra of all the observed targets, including HH 100 IR and IRS2 which have high values of infrared continuum veiling (r K = 6 and 3, respectively). These two sources also present low resolution spectra rich in emission lines (HI, CO and plenty of other permitted lines from neutral atoms) likely originating in the disk-star-wind connected regions. Among the features observed in HH100 IR and IRS2, Na I at 2.205 μm and CO at 2.3 μm, which are more commonly used for stellar classification, are detected in emission instead of absorption. Several strong photospheric lines which lie around 2.12 and 2.23 μm and whose ratio is sensitive to both effective temperature and gravity are proposed as independent diagnostic tools for this type of sources. We derived spectral types, veiling and stellar luminosity for the five observed sources, which in turn have been used to infer their mass (ranging between 0.3-1.2 M ○. ) and age (between 10 5 and 10 6 yr) adopting pre-main sequence evolutionary tracks. We find that in HH100 IR and IRS2 most of the bolometric luminosity is due to accretion (L acc /L bol ∼ 0.8 and 0.6 respectively), while the other three investigated sources, including the Class I object IRS5a, present low accretion activity (L acc /L bol < 0.2). Mass accretion rates of the order of 2 x 10 -6 and 3 x 10 -7 M ○. yr -1 are derived for HH100 IR and IRS2, respectively, i.e. higher by an order of magnitude than the average values derived for T Tauri stars. We observe a general correlation between the accretion luminosity, the IR veiling and the emission line activity of the sources. In particular, we find that the correlation between L acc and L Brγ , previously reported for optical T Tauri stars, can be extended to the embedded sources, up to at least one order of magnitude larger line luminosity. A correlation between the accretion activity and the spectral energy distribution slope is recognizable with the notable exception of IRS5a. Our analysis therefore shows how the definition of the evolutionary stage of deeply embedded YSOs by means of IR colors needs to be refined.

The Variability of QSOs in the Optical Band

Constraints on the emission mechanism of AGNs can be provided by the variability of their spectral energy distribution (SED). Recently Di Clemente et al. (1996) have shown that the positive correlation of QSO variability with redshift can be due to a hardening of the spectrum in bright phases, coupled with the increase of the rest-frame frequency of the (fixed) observing band, for increasing redshift. Direct evidences of slope changes in the SEDs of a limited number of individual AGNs have been provided by Cutri et al. (1985), Edelson et al. (1990), and Kinney et al. (1991). In the following we present some preliminary results of a direct measure of variations of the SED slope in the complete, magnitude limited, sample of QSOs of the Selected Area 57 (Koo, Kron &amp; Cudworth 1986; Trèvese et al. 1989 (T89)). The data are derived from two sets of prime focus plates of the SA 57, in the U, BJ, F and N bands, obtained with the 4-m telescope at the Kitt Peak National Observatory at two epochs separated by one year. Photometric methods and signal-to-noise optimization are described in T89 and Trèvese et al. (1994). The quasar sample, of &lt; z &gt; ⋍ 1.4, consists of 33 objects from T89, plus the brightest 3 members of the the Bershady, Trèvese and Kron (1998) sample of extended objects selected on the basis of variability. In Figure 1a Δα is reported versus the changes Δlog fv in the BJ band and shows positive correlation, indicating a hardening of the spectrum in the bright phase. A special care is needed to avoid spurious Δα – Δlog fv correlations (see Massaro &amp; Trèvese 1996). The correlation coefficient is r=0.46 with a probability P(&gt;r)=0.995, after the exclusion of one deviant point, whose inclusion would produce a higher correlation. The slope of the linear regression of ΔαΔlogv is b=1.9. Assuming that the spectra are, dominated by the big blue bump (BBB) in the sampled spectral region, around λ ≍ 2000 Å, we can use the simple approximation of a single black-body spectrum. To check the hypothesis that both the slope and brightness changes are caused by a temperature variations only, we derive the black-body temperatures from the SED slope as deduced from a linear fit of the log fv v.s. logv relation, based on the U, BJ and F data. The average slope (excluding the two highest and lowest z objects) is 〈α〉=-0.4±0.6 and the average temperature is T ≍ 25000 K. For a black-body of fixed emitting surface and varying temperature T, the changes of the local SED slope and the relevant luminosity variations are related by (dα/dT)/(dlogBv/dT) ≡ f(x), where x ≡ hv/kT. The slope b of the linear regression of Δα v.s. log fv of Figure la is compared with the function f(x) values of the sample determined by the dispersion of α. We can conclude that dα/dlogBv and α are consistent for x ≍ 3. For an average sampling wavelength &lt; λ &gt; ≍ 2000 Å of the sample, this corresponds to an average temperature T ≍ 2.5 × 104 of a black-body of fluctuating temperature.

Emission Lines and the Spectral Energy Distributions of Quasars

AbstractMany years of study have failed to establish conclusively relationships between a quasar’s spectral energy distribution (SED) and the emission lines it is thought to produce. This is at least partially due to the lack of well-observed SEDs. We present initial results from a line–SED study for a sample of 43 quasars and active galaxies for which we have optical and ultraviolet spectra and far-infrared–X-ray SEDs. We present the results of tests for correlations between line equivalent widths and SED luminosity and slope parameters and compare these results to those from earlier studies. We find that the Baldwin effect is weaker when the luminosity is defined close to the ionizing continuum of that line and conclude that the detailed SED is likely to be important in making further progress.