An early detection of blue luminescence by neutral PAHs in the direction of the yellow hypergiant HR 5171A?

Here we report our discovery of a band of blue luminescence (BL) in the Red Rectangle (RR) nebula. This enigmatic proto-planetary nebula is also one of the brightest known sources of extended red emission as well as of unidentified infra-red (UIR) band emissions. The spectrum of this newly discovered BL is most likely fluorescence from small neutral polycyclic aromatic hydrocarbon (PAH) molecules. PAH molecules are thought to be widely present in many interstellar and circumstellar environments in our galaxy as well as in other galaxies, and are considered likely carriers of the UIR-band emission. However, no specific PAH molecule has yet been identified in a source outside the solar system, as the set of mid-infra-red emission features attributed to these molecules between the wavelengths of 3.3 micron and 16.4 micron is largely insensitive to molecular sizes. In contrast, near-UV/blue fluorescence of PAHs is more specific as to size, structure, and charge state of a PAH molecule. If the carriers of this near-UV/blue fluorescence are PAHs, they are most likely neutral PAH molecules consisting of 3-4 aromatic rings such as anthracene (C14H10) and pyrene (C16H10). These small PAHs would then be the largest molecules specifically identified in the interstellar medium.

The discovery of a band of blue luminescence in the Red Rectangle proto-planetary nebula has been recently reported by Vijh et al. These authors used the hydrogen Balmer lines and the line-depth technique to extract the intensity of the blue luminescence. This luminescence was attributed to fluorescence from small neutral polycyclic aromatic hydrocarbon molecules in the interstellar medium, consisting of three to four aromatic rings such as anthracene and pyrene. We present in this Letter evidence of another potential carrier responsible for the blue luminescence, namely, ultrasmall silicon nanoparticles of 1 nm in diameter.

Following our initial discovery of blue luminescence in the spectrum of the Red Rectangle (RR) and its identification as fluorescence by small three- to four-ringed polycyclic aromatic hydrocarbon (PAH) molecules, we report on the spatial correlation between the blue luminescence and the 3.3 ?m emission, commonly attributed to small, neutral PAH molecules, and on the newly derived UV/optical attenuation curve for the central source of the RR, HD 44179. Both results provide strong additional evidence for the presence of small PAH molecules with masses of less than 250 amu in the RR, which supports the attribution of the blue luminescence to fluorescence by the same molecules. We contrast the excellent spatial correlation of the two former emissions with the distinctly different spatial distribution of the extended red emission and of the dust-scattered light within the RR. The UV/optical attenuation curve of the central star is unlike any interstellar extinction curve and is interpreted as resulting from circumstellar opacity alone. Major contributions to this opacity are absorptions in broad bands in the mid-UV, contributing to the electronic excitation of the luminescing PAH molecules, and a sharp ionization discontinuity near 7.5 eV in the far-UV, which places a sharp upper limit on the masses of the PAH molecules that are responsible for this absorption. The strength of the far-UV absorption leads to an abundance of PAH molecules of 10-5 relative to hydrogen in the RR. Such small PAHs are perhaps unique to the environment in the RR, where they are shielded from harsh radiation by the dense circumstellar disk.

Blue luminescence (BL) was first discovered in a proto-planetary nebula, the Red Rectangle (RR), surrounding the post-AGB star HD 44179. BL has been attributed to fluorescence by small, 3-4 ringed neutral polycyclic aromatic hydrocarbon (PAH) molecules and was thought to be unique to the RR environment, where such small molecules are actively being produced and shielded from the harsh interstellar radiation by a dense circumstellar disk. In this paper we present the BL spectrum detected in several ordinary reflection nebulae illuminated by stars having temperatures between 10,000 and 23,000 K. All these nebulae are known to also exhibit the infrared emission features called aromatic emission features (AEFs) attributed to large PAHs. We present the spatial distribution of the BL in these nebulae. In the case of Ced 112, the BL is spatially correlated with mid-IR emission structures attributed to AEFs. These observations provide evidence for grain processing and possibly for in situ formation of small grains and large molecules from larger aggregates. Most importantly, the detection of BL in these ordinary reflection nebulae suggests that the BL carrier is an ubiquitous component of the interstellar medium and is not restricted to the particular environment of the RR.

The evolution of massive stars surviving the red supergiant (RSG) stage remains unexplored due to the rarity of such objects. The yellow hypergiants (YHGs) appear to be the warm counterparts of post-RSG classes located near the Humphreys-Davidson upper luminosity limit, which are characterized by atmospheric instability and high mass-loss rates. We aim to increase the number of YHGs in M33 and thus to contribute to a better understanding of the pre-supernova evolution of massive stars. Optical spectroscopy of five dust-enshrouded YSGs selected from mid-IR criteria was obtained with the goal of detecting evidence of extensive atmospheres. We also analyzed BVI photometry for 21 of the most luminous YSGs in M33 to identify changes in the spectral type. To explore the properties of circumstellar dust, we performed SED-fitting of multi-band photometry of the 21 YSGs. We find three luminous YSGs in our sample to be YHG candidates, as they are surrounded by hot dust and are enshrouded within extended, cold dusty envelopes. Our spectroscopy of star 2 shows emission of more than one H$\alpha$ component, as well as emission of CaII, implying an extended atmospheric structure. In addition, the long-term monitoring of the star reveals a dimming in the visual light curve of amplitude larger than 0.5 mag that caused an apparent drop in the temperature that exceeded 500 K. We suggest the observed variability to be analogous to that of the Galactic YHG $\rho$ Cas. Five less luminous YSGs are suggested as post-RSG candidates showing evidence of hot or/and cool dust emission. We demonstrate that mid-IR photometry, combined with optical spectroscopy and time-series photometry, provide a robust method for identifying candidate YHGs. Future discovery of YHGs in Local Group galaxies is critical for the study of the late evolution of intermediate-mass massive stars.

Cool debris discs are a relic of the planetesimal formation process around their host star, analogous to the solar system's Edgeworth-Kuiper belt. As such, they can be used as a proxy to probe the origin and formation of planetary systems like our own. The Herschel Open Time Key Programmes "DUst around NEarby Stars" (DUNES) and "Disc Emission via a Bias-free Reconnaissance in the Infrared/Submillimetre" (DEBRIS) observed many nearby, sun-like stars at far-infrared wavelengths seeking to detect and characterize the emission from their circumstellar dust. Excess emission attributable to the presence of dust was identified from around $\sim$ 20% of stars. Herschel's high angular resolution ($\sim$ 7" FWHM at 100 $\mu$m) provided the capacity for resolving debris belts around nearby stars with radial extents comparable to the solar system (50 to 100 au). As part of the DUNES and DEBRIS surveys, we obtained observations of three debris disc stars, HIP 22263 (HD 30495), HIP 62207 (HD 110897), and HIP 72848 (HD 131511), at far-infrared wavelengths with the Herschel PACS instrument. Combining these new images and photometry with ancilliary data from the literature, we undertook simultaneous multi-wavelength modelling of the discs' radial profiles and spectral energy distributions using three different methodologies: single annulus, modified black body, and a radiative transfer code. We present the first far-infrared spatially resolved images of these discs and new single-component debris disc models. We characterize the capacity of the models to reproduce the disc parameters based on marginally resolved emission through analysis of two sets of simulated systems (based on the HIP 22263 and HIP 62207 data) with the noise levels typical of the Herschel images. We find that the input parameter values are recovered well at noise levels attained in the observations presented here.

We present a study of blue straggler stars (BSSs) of open cluster NGC 7142 using AstroSat/UVIT data and other archival data. Using a machine-learning-based algorithm, ML-MOC, on Gaia DR3 data, we find 546 sources as cluster members. Based on the location on the Gaia colour–magnitude diagram, we identify 10 BSS candidates, also detected in UVIT/F148W filter. We study the variable nature of BSSs by constructing their light curves using the Transiting Exoplanet Survey Satellite data. Two BSSs reported as eclipsing binaries in Gaia DR3 are confirmed to be eclipsing binaries based on our analysis and also show the presence of hot companions as per the multiwavelength spectral energy distributions (SEDs). The physical parameters of the hot companions of these two BSSs derived by fitting binary models to their light curves and those derived from the SEDs are found to be in good agreement. Additionally, five more BSSs in the cluster shows UV excess, four of which are likely to have a hot companion based on SEDs. The hot companions with the estimated temperatures ∼14 000–28 000 K, radii ∼0.01–0.05 R⊙, and luminosities ∼0.03–0.1 L⊙ are inferred to be extremely low-mass (<0.2 M⊙), low-mass (0.2–0.4 M⊙), normal-mass (0.4–0.6 M⊙), and high-mass (>0.6 M⊙) white dwarfs (WDs). For the first time in an open cluster, we find the entire range of masses in WDs found as hot companions of BSSs. These masses imply that the Case-A/Case-B mass transfer and merger are responsible for the formation of at least 60 per cent of the BSSs of this cluster.

We present a statistical, photometric, and spectral energy distribution (SED) analysis to characterize the blue straggler stars (BSSs) populations in the Galactic old open cluster Berkeley 39. Berkeley 39 is a 6.16 Gyr old open cluster located at a distance of 3.99 Kpc.Gaia DR3 astrometry data have been used to estimate the membership probabilities using ensemble-based unsupervised machine learning techniques. We identified 21 BSS candidates on the colour–magnitude diagram, with 19 of them being detected in the Swift/UVOT UVW2 filter. We analysed the radial surface density profile and examined the cluster dynamical states and mass segregation effect. The SEDs of 19 BSSs are constructed using multiwavelength data covering UV to IR wavelengths. A single-component SED is fitted successfully for 14 BSS candidates. We discovered hot companions in five BSS candidates. These hot companions have temperatures of approximately 14 000 to 23 000 K, radii ranging from 0.04 to 0.13 R$_{\odot }$, and luminosities ranging from 0.16 to 2.91 L$_{\odot }$. Among these, three are most likely extremely low-mass white dwarfs (WDs) with masses around 0.17 to 0.18 M$_{\odot }$, and two are low-mass WDs with masses around 0.18 to 0.39 M$_{\odot }$. This confirms that they are post-mass transfer (Case A or Case B) systems. We also investigated the variable characteristics of BSSs by analysing their light curves using data from TESS. Our analysis confirms that two BSSs identified as eclipsing binaries in Gaia DR3 are indeed eclipsing binaries. Additionally, one of the two eclipsing binary BSSs shows evidence of having hot companions, as indicated by the multiwavelength SEDs.

Context. About a year ago, a super-Jovian planet was directly imaged around the nearby young solar-type star AF Lep. The 2.8 MJup planet orbiting at a semimajor axis of 8.2 au matches the predicted location based on the HIPPARCOS-Gaia astrometric acceleration. Aims. Our aim is to expand the atmospheric exploration of AF Lep b by modeling all available observations obtained with SPHERE at VLT (in the range 0.95–1.65, at 2.105, and at 2.253 µm), and NIRC2 at Keck (at 3.8 µm) with self-consistent atmospheric models. Methods. To understand the physical properties of this exoplanet, we used ForMoSA. This forward-modeling code compares observations with grids of pre-computed synthetic atmospheric spectra using Bayesian inference methods. We used Exo-REM, an atmospheric radiative-convective equilibrium model, including the effects of nonequilibrium processes and clouds. Results. From the atmospheric modeling we derived solutions at the low Teff of ~750 K. Our analysis also favors a metal-rich atmosphere (>0.4) and solar to super-solar carbon-to-oxygen ratio (~0.6). We tested the robustness of the estimated values for each parameter by cross-validating our models using the leave-one-out strategy, where all points are used iteratively as validation points. Our results indicate that the photometry point at 3.8 µm strongly drives the metal-rich and super-solar carbon-to-oxygen solutions. Conclusions. Our atmospheric forward-modeling analysis strongly supports the planetary nature of AF Lep b. Its spectral energy distribution is consistent with that of a young, cold, early-T super-Jovian planet. We recover physically consistent solutions for the surface gravity and radius, which allows us to reconcile atmospheric forward modeling with evolutionary models, in agreement with the previously published complementary analysis done by retrievals. Finally, we identified that future data at longer wavelengths are necessary before we can conclude about the metal-rich nature of AF Lep b.

Among the about one dozen luminous X-ray bursters in the cores of globular clusters, only three have optical/UV counterparts unambiguously identified. We have observed two of these counterparts spectroscopically with the low resolution optical and UV gratings of the Faint Object Spectrograph (FOS) aboard the Hubble Space Telescope (HST), yielding broad (1150-2500A and 3600-7000A) wavelength coverage in each case. For the counterpart AC211 to the X-ray source in NGC 7078/M15, our small aperture FOS data reveal both strong absorption and emission features, as well as a broad spectral energy distribution (SED), that are akin to those determined from earlier ground-based and IUE spectra, but with some important differences. These differences suggest that only the HST spectra of AC211 are free of significant contamination from nearby cluster stars. Our FOS data for the counterpart "Star S" in NGC 6712 permit an investigation of the SED for this second object as well; in this case a composite (and/or variable) SED is indicated. However, intriguingly in the case of "Star S", we find little evidence for any identifiable features--emission or absorption--attributable to the system, although features as strong as those seen for AC211 would be readily detected. These HST optical and UV spectra of AC211 and "Star S" may presage a spectral diversity among the counterparts to X-ray bursters in globular cluster cores.

We investigate the fundamental properties of core-collapse supernova (SN) progenitors from single stars at solar metallicity. For this purpose, we combine Geneva stellar evolutionary models with initial masses of Mini = 20−120 M⊙ with atmospheric and wind models using the radiative transfer code CMFGEN. We provide synthetic photometry and high-resolution spectra of hot stars at the pre-SN stage. For models with Mini = 9−20 M⊙, we supplement our analysis using publicly available MARCS model atmospheres of RSGs to estimate their synthetic photometry. We employ well-established observational criteria of spectroscopic classification and find that, depending on their initial mass and rotation, massive stars end their lives as red supergiants (RSG), yellow hypergiants (YHG), luminous blue variables (LBV), and Wolf-Rayet (WR) stars of the WN and WO spectral types. For rotating models, we obtained the following types of SN progenitors: WO1–3 (Mini ≥ 32 M⊙), WN10–11 (25 < Mini < 32 M⊙), LBV (20 ≤ Mini ≤ 25 M⊙), G1 Ia+ (18 < Mini < 20 M⊙), and RSGs (9 ≤ Mini ≤ 18 M⊙). For non-rotating models, we found spectral types WO1–3 (Mini > 40 M⊙), WN7–8 (25 < Mini ≤ 40 M⊙), WN11h/LBV (20 < Mini ≤ 25 M⊙), and RSGs (9 ≤ Mini ≤ 20 M⊙). Our rotating models indicate that SN IIP progenitors are all RSG, SN IIL/b progenitors are 56% LBVs and 44% YHGs, SN Ib progenitors are 96% WN10-11 and 4% WOs, and SN Ic progenitors are all WO stars. We find that the most massive and luminous SN progenitors are not necessarily the brightest ones in a given filter, since this depends on their luminosity, temperature, wind density, and the way the spectral energy distribution compares to a filter bandpass. We find that SN IIP progenitors (RSGs) are bright in the RIJHKS filters and faint in the UB filters. SN IIL/b progenitors (LBVs and YHGs), and SN Ib progenitors (WNs) are relatively bright in optical/infrared filters, while SN Ic progenitors (WOs) are faint in all optical filters. We argue that SN Ib and Ic progenitors from single stars should be undetectable in the available pre-explosion images with the current magnitude limits, in agreement with observational results.

We present an initial analysis of AKARI 18$\mu $m-selected galaxies using all 9 photometric bands at 2–24$\mu $m available in the Infrared Camera (IRC), in order to demonstrate new capabilities of AKARI cosmological surveys with this unprecedented wavelength coverage at mid-infrared (MIR) bands. We detected 72 sources at 18$\mu $m in an area of 50.2 arcmin$^2$ in the AKARI/IRC monitor field towards the North Ecliptic Pole (NEP). From this sample, 25 galaxies with probable redshifts of $z \gtrsim$ 0.5 were selected with a single colour cut (N2$-$N3$\gt$ 0.1) for a detailed spectral energy distribution (SED) analysis with ground-based $BVRi'z'JK$ data. Using an SED radiative transfer model of starbursts covering the wavelength range UV–submm, we derived photometric redshifts from the optical-MIR SEDs of 18$\mu $m-selected galaxies. From the best-fit SED models, we showed that the IRC all-band photometry is capable of tracing a steep rise in the flux at the blue side of the polycyclic aromatic hydrocarbon (PAH) 6.2$\mu $m emission feature. This indicates that the IRC all-band photometry is useful to constrain the redshift of infrared galaxies, specifically for dusty galaxies with a less prominent 4000Å break. Also, we found that the flux dip between the PAH 7.7 and 11.2$\mu $m emission feature is recognizable in the observed SEDs of galaxies at $z \sim 1$. This pilot study suggests the possibility of detecting many interesting galaxy properties in the NEP-Deep and Wide surveys, such as a systematic difference in SEDs between high–and low-$z$ Ultra-Luminous InfraRed Galaxies, and a large variation of the PAH inter-band strength ratio in galaxies at high redshifts.

Context. V838 Monocerotis is a peculiar binary that underwent an immense stellar explosion in 2002, leaving behind an expanding cool supergiant and a hot B3V companion. Five years after the outburst, the B3V companion disappeared from view, and has not returned to its original state. Aims. We investigate the changes in the light curve and spectral features to explain the behaviour of V838 Mon during the current long-lasting minimum. Methods. A monitoring campaign has been performed over the past 13 years with the Nordic Optical Telescope to obtain optical photometric and spectroscopic data. The datasets are used to analyse the temporal evolution of the spectral features and the spectral energy distribution, and to characterise the object. Results. Our photometric data show a steady brightening in all bands over the past 13 years, which is particularly prominent in the blue. This rise is also reflected in the spectra, showing a gradual relative increase in the continuum flux at shorter wavelengths. In addition, a slow brightening of the Hα emission line starting in 2015 was detected. These changes might imply that the B3V companion is slowly reappearing. During the same time interval, our analysis reveals a considerable change in the observed colours of the object along with a steady decrease in the strength and width of molecular absorption bands in our low-resolution spectra. These changes suggest a rising temperature of the cool supergiant along with a weakening of its wind, most likely combined with a slow recovery of the secondary due to the evaporation of the dust and accretion of the material from the shell in which the hot companion is embedded. From our medium-resolution spectra, we find that the heliocentric radial velocity of the atomic absorption line of Ti I 6556.06 Å has been stable for more than a decade. We propose that Ti I lines are tracing the velocity of the red supergiant in V838 Mon, and do not represent the infalling matter as previously stated.

The 10-micron silicate feature observed with Spitzer in active galactic nuclei (AGN) reveals some puzzling behavior. It (1) has been detected in emission in type 2 sources, (2) shows broad, flat-topped emission peaks shifted toward long wavelengths in several type 1 sources, and (3) is not seen in deep absorption in any source observed so far. We solve all three puzzles with our clumpy dust radiative transfer formalism. (1) We present the spectral energy distribution (SED) of SST1721+6012, the first type 2 quasar observed to show a clear 10-mic silicate feature in emission. We constructed a large database of clumpy torus models and performed extensive fitting of the observed SED, constraining several of the torus parameters. We find that the source bolometric luminosity is ~3*10^12 L_sun. Our modeling suggests that <35% of objects with tori sharing characteristics and geometry similar to the best fit would have their central engines obscured. This relatively low obscuration probability can explain the clear appearance of the 10-mic emission feature in SST1721+6012 together with its rarity among other QSO2. (2) We also fitted the SED of PG1211+143, one of the first type 1 QSOs with a 10-mic silicate feature in emission. Among similar sources, this QSO appears to display an unusually broadened feature whose peak is shifted toward longer wavelengths. Although this led to suggestions of non-standard dust chemistry in these sources, our analysis fits such SEDs with standard galactic dust; the apparent peak shifts arise from radiative transfer effects. (3) We find that the distribution of silicate feature strengths among clumpy torus models closely resembles the observed distribution, and the feature never occurs deeply absorbed. (abridged)

We report mid- to far-infrared imaging and photometry from 7 to 37 μm with SOFIA/FORCAST and 2 μm adaptive optics imaging with LBTI/LMIRCam of a large sample of red supergiants (RSGs) in four Galactic clusters: RSGC1, RSGC2 = Stephenson 2, RSGC3, and NGC 7419. The RSGs in these clusters cover their expected range in luminosity and initial mass from ≈9 to more than 25 M ⊙. The population includes examples of very late-type RSGs such as MY Cep, which may be near the end of the RSG stage, high-mass-losing maser sources, yellow hypergiants, and post-RSG candidates. Many of the stars and almost all of the most luminous have spectral energy distributions (SEDs) with extended infrared excess radiation at the longest wavelengths. To best model their SEDs, we use the DUSTY code with a variable radial density distribution function to estimate their mass-loss rates. Our –luminosity relation for 42 RSGs basically follows the classical de Jager curve, but at luminosities below 105 , we find a significant population of RSGs with below the de Jager relation. At luminosities above 105 L ⊙, there is a rapid transition to higher mass-loss rates that approximates and overlaps the de Jager curve. We recommend that instead of using a linear relation or single curve, the empirical –luminosity relation is better represented by a broad band. Interestingly, the transition to much higher at about 105 L ⊙ corresponds approximately to an initial mass of 18–20 M ⊙, which is close to the upper limit for RSGs becoming Type II supernovae.