Late-type Stars Research Articles

Using ZDI maps to determine magnetic forces and torques at the photospheres of early-type stars

ABSTRACT We use the magnetic field components measured by Zeeman Doppler imaging (ZDI) to calculate the stellar surface force and torque due to magnetic stresses for the fast rotators σ Ori E, 36 Lyn, and CU Vir, and the slow rotator τ Sco. If we assume the stars have spherical photospheres, the estimated torques give spin-down time-scales no larger than 7 × 105 yr. For σ Ori E, the predicted spin-down time-scale, ≃ 6000 yr, is much less than the observationally measured time-scale of ≃ 106 yr. However, for CU Vir, we find that the spin-down time-scale from its ZDI map is 7 × 105 yr in good agreement with its average rate of spin-down from 1960 to 2010. With the exception of τ Sco, the net force due to magnetic stresses at the stellar surface are large compared to the surface-integrated pressure. We discuss possible reasons for the large values of the forces (and torques), and suggest that the likely explanation is that rotation and the magnetic stresses create significant departures from spherical symmetry.

Infrared colours and bolometric corrections of SiO masing stars in the inner Milky Way

We analysed a sample of SiO-masing late-type stars located in the inner Galaxy with the goal of setting better constraints on their obscuration. This reference sample has allowed us to define mathematical relations between their dereddened infrared colours and the observed colours (e.g. The derived equations define a property (the locus) of these late-type stars. Therefore, they enable us to derive the interstellar extinction. With estimated spectral types, it is possible to decompose the total extinction in the two components (interstellar and envelope extinction). These relations are useful for classifying extremely obscured late-type stars located in the inner Galaxy. Estimating the two extinction components can be performed on an individual late-type star, independently of its surroundings and even when few mid-infrared measurements are available.

CARMENES input catalog of M dwarfs

Aims. Knowledge of rotation periods (Prot) is important for understanding the magnetic activity and angular momentum evolution of late-type stars, as well as for evaluating radial velocity signals of potential exoplanets and identifying false positives. We measured photometric and spectroscopic Prot for a large sample of nearby bright M dwarfs with spectral types from M0 to M9, as part of our continual effort to fully characterize the Guaranteed Time Observation programme stars of the CARMENES survey. Methods. We analyse light curves chiefly from the SuperWASP survey and TESS data. We supplemented these with our own follow-up photometric monitoring programme from ground-based facilities, as well as spectroscopic indicator time series derived directly from the CARMENES spectra. Results. From our own analysis, we determined Prot for 129 stars. Combined with the literature, we tabulated Prot for 261 stars, or 75% of our sample. We developed a framework to evaluate the plausibility of all periods available for this sample by comparing them with activity signatures and checking for consistency between multiple measurements. We find that 166 of these stars have independent evidence that confirmed their Prot. There are inconsistencies in 27 periods, which we classify as debated. A further 68 periods are identified as provisional detections that could benefit from independent verification. We provide an empirical relation for the Prot uncertainty as a function of the Prot value, based on the dispersion of the measurements. We show that published formal errors seem to be often underestimated for periods longwards of ∼10 d. We examined rotation–activity relations with emission in X-rays, Hα, Ca II H&K, and surface magnetic field strengths for this sample of M dwarfs. We find overall agreement with previous works, as well as tentative differences in the partially versus fully convective subsamples. We show Prot as a function of stellar mass, age, and galactic kinematics. With the notable exception of three transiting planet systems and TZ Ari, all known planet hosts in this sample have Prot ≳ 15 d. Conclusions. Inherent challenges in determining accurate and precise stellar Prot means independent verification is important, especially for inactive M dwarfs. Evidence of potential mass dependence in activity–rotation relations would suggest physical changes in the magnetic dynamo that warrants further investigation using larger samples of M dwarfs on both sides of the fully convective boundary. Important limitations need to be overcome before the radial velocity technique can be routinely used to detect and study planets around young and active stars.

CO outflows from young stars in the NGC2023 cluster

Young early-type HAeBe stars are still embedded in the molecular clouds in which they formed. They illuminate reflection nebulae, which shape the surrounding molecular cloud and may trigger star formation. They are therefore ideal places to search for ongoing star formation activity. NGC\,2023 is illuminated by the Herbig Be star HD\,37903. It is the most massive member of a small young cluster with about 30 PMS stars, several of which are Class I objects that still heavily accrete. It might therefore be expected that they might drive molecular outflows. We examined the whole region for outflows. We analyzed previously published APEX data to search for and characterize the outflows in the NGC\,2023 region. This is the first systematic search for molecular outflows in this region. Since the outflows were mapped in several CO transitions, we can determine their properties quite well. We have discovered four molecular outflows in the vicinity of NGC\,2023, three of which are associated with Class I objects. MIR-63, a bright mid-infrared and submillimeter Class I source, is a binary with a separation of 2 and drives two bipolar outflows orthogonal to each other. The large southeast--northwest outflow excites the Herbig-Haro flow HH\,247. MIR-73, a Class I object, which is also a far-infrared source, drives a pole-on outflow. MIR-62 is a Class II object with strong infrared excess and a luminosity of 7 It is not detected in the far-infrared. The Class I sources have bolometric luminosities of about 20 or lower, that is, they are all low-mass stars. One other far-infrared source, MIR-75, may have powered an outflow in the past because it now illuminates an egg-shaped cavity. The four outflows are all powered by young stars and are located in the immediate vicinity of NGC\,2023. They are at a similar evolutionary stage, which suggests that their formation may have been triggered by the expanding region.

Red supergiant candidates for multimessenger monitoring of the next Galactic supernova

ABSTRACT We compile a catalogue of 578 highly probable and 62 likely red supergiants (RSGs) of the Milky Way, which represents the largest list of Galactic RSG candidates designed for continuous follow-up efforts to date. We match distances measured by Gaia DR3, 2MASS photometry, and a 3D Galactic dust map to obtain luminous bright late-type stars. Determining the stars’ bolometric luminosities and effective temperatures, we compare to Geneva stellar evolution tracks to determine likely RSG candidates, and quantify contamination using a catalogue of Galactic AGB in the same luminosity-temperature space. We add details for common or interesting characteristics of RSG, such as multistar system membership, variability, and classification as a runaway. As potential future core-collapse supernova progenitors, we study the ability of the catalogue to inform the Supernova Early Warning System (SNEWS) coincidence network made to automate pointing, and show that for 3D position estimates made possible by neutrinos, the number of progenitor candidates can be significantly reduced, improving our ability to observe the progenitor pre-explosion and the early phases of core-collapse supernovae.

OGLE-2014-BLG-0221Lb: A Jupiter Mass Ratio Companion Orbiting Either a Late-type Star or a Stellar Remnant

We present the analysis of the microlensing event OGLE-2014-BLG-0221, a planetary candidate event discovered in 2014. The photometric light curve is best described by a binary-lens single-source model. Our light-curve modeling finds two degenerate models, with event timescales of t E ∼ 70 days and ∼110 days. These timescales are relatively long, indicating that the discovered system would possess a substantial mass. The two models are similar in their planetary parameters with a Jupiter mass ratio of q ∼ 10−3 and a separation of s ∼ 1.1. Bayesian inference is used to estimate the physical parameters of the lens, revealing that the shorter timescale model predicts 65% and 25% probabilities of a late-type star and white dwarf host, respectively, while the longer timescale model favors a black hole host with a probability ranging from 60% to 95%, under the assumption that stars and stellar remnants have equal probabilities of hosting companions with planetary mass ratios. If the lens is a remnant, this would be the second planet found by microlensing around a stellar remnant. The current separation between the source and lens stars is 41–139 mas depending on the models. This indicates the event is now ready for high-angular-resolution follow-up observations to rule out either of the models. If precise astrometric measurements are conducted in multiple bands, the centroid shift due to the color difference between the source and lens would be detected in the luminous lens scenario.

Silicate Extinction Profile Based on the Stellar Spectrum by Spitzer/IRS

The 9.7 and 18 μm interstellar spectral features, arising from the Si–O stretching and O–Si–O bending modes of amorphous silicate dust, are the strongest extinction features in the infrared. Here we use the “pair method” to determine the silicate extinction profile by comparing the Spitzer/IRS spectra of 49 target stars with obvious extinction with those of unreddened stars of the same spectral type. The 9.7 μm extinction profile is determined from all the 49 stars and the 18 μm profile is determined from six stars. It is found that the profile has a peak wavelength at ∼9.2–9.8 and ∼18–22 μm, respectively. The peak wavelength of the 9.7 μm feature seems to become shorter for stars of late spectral types; meanwhile the FWHM seems irrelevant to the spectral type, which may be related to circumstellar silicate emission. The silicate optical depth at 9.7 μm, Δτ 9.7, mostly increases with the color excess in J − K S ( EJKS ). The mean ratio of the visual extinction to the 9.7 μm silicate absorption optical depth is A V /Δτ 9.7 ≈ 17.8, in close agreement with that of the solar-neighborhood diffuse interstellar medium. When EJKS>4 , this proportionality changes. The correlation coefficient between the peak wavelength and the FWHM of the 9.7 μm feature is 0.4, which indicates a positive correlation considering the uncertainties of the parameters. The method is compared with replacing the reference star by an atmospheric model spectral energy distribution and no significant difference is observed.

Derivative Spectroscopy and its Application at Detecting the Weak Emission/Absorption Lines

The development of spectroscopic survey telescopes like Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST), Apache Point Observatory Galactic Evolution Experiment and Sloan Digital Sky Survey has opened up unprecedented opportunities for stellar classification. Specific types of stars, such as early-type emission-line stars and those with stellar winds, can be distinguished by the profiles of their spectral lines. In this paper, we introduce a method based on derivative spectroscopy (DS) designed to detect signals within complex backgrounds and provide a preliminary estimation of curve profiles. This method exhibits a unique advantage in identifying weak signals and unusual spectral line profiles when compared to other popular line detection methods. We validated our approach using synthesis spectra, demonstrating that DS can detect emission signals three times fainter than Gaussian fitting. Furthermore, we applied our method to 579,680 co-added spectra from LAMOST Medium-Resolution Spectroscopic Survey, identifying 16,629 spectra with emission peaks around the Hα line from 10,963 stars. These spectra were classified into three distinct morphological groups, resulting in nine subclasses as follows. (1) Emission peak above the pseudo-continuum line (single peak, double peaks, emission peak situated within an absorption line, P Cygni profile, Inverse P Cygni profile); (2) Emission peak below the pseudo-continuum line (sharp emission peak, double absorption peaks, emission peak shifted to one side of the absorption line); (3) Emission peak between the pseudo-continuum line.

Elusive hot stripped helium stars in the Galaxy

Context. Stellar evolution theory predicts the existence of helium-core remnants of the primary components of intermediate-mass close binaries that lost most of their hydrogen-helium envelopes due to the mass exchange. These remnants are expected to be observed as hot helium-rich stars with masses of (1−7) M⊙, located in the area of the Hertzsprung-Russell diagram between OB subdwarfs and Wolf-Rayet stars. While several thousands of such stripped helium stars are expected to exist in the Galaxy, none of them have been identified so far. Aims. We aim to provide comprehensive predictions of the numbers and fundamental properties of stripped helium stars and their binary companions in the Galaxy. This is a necessary first step to guide observations, to enable a comparison between binary evolution models and realistic stellar populations, and to determine the feedback parameters of stripped helium stars in the Galaxy. Methods. We expanded the previously considered space of parameters describing close binary systems producing stripped helium stars and applied a population synthesis based on a grid of evolutionary models computed by the code MESA, using a spin-dependent algorithm for the treatment of mass and angular momentum loss from the system. Results. We show that the number of Galactic binaries hosting (1−7) M⊙ He-stars is ≃20 000 and that it steeply declines with an increase in the He-star mass (≃3000 with mass ≳2 M⊙). The decisive factor that defines the low number of stripped He-stars is runaway mass-loss after Roche lobe overflow (RLOF) by primary components of the binaries, which leads to the formation of common envelopes and the further merger of components. This effect strongly restricts the initial ranges of masses of components of the progenitors of stripped stars and orbital periods. In addition, stripped helium stars are much less numerous than expected, since a significant fraction of binaries in which the primaries have masses less than (5−7) M⊙ produce subdwarfs with masses ≲1 M⊙. Our calculations show that the overwhelming majority of helium stars reside in binaries with an early-type companion star and can be identified neither by the UV excess nor by emission features. The large periods of a significant fraction of binaries hosting stripped stars (≳several hundred days) also hamper their discovery.

Diffuse interstellar bands in Gaia DR3 RVS spectra

Diffuse interstellar bands (DIBs) are weak and broad interstellar absorption features in astronomical spectra that originate from unknown molecules. To measure DIBs in spectra of late-type stars more accurately and more efficiently, we developed a random forest model to isolate the DIB features from the stellar components. We applied this method to 780 thousand spectra collected by the Gaia Radial Velocity Spectrometer (RVS) that were published in the third data release (DR3). After subtracting the stellar components, we modeled the DIB at 8621 Å (λ8621) with a Gaussian function and the DIB around 8648 Å (λ8648) with a Lorentzian function. After quality control, we selected 7619 reliable measurements for DIB λ8621. The equivalent width (EW) of DIB λ8621 presented a moderate linear correlation with dust reddening, which was consistent with our previous measurements in Gaia DR3 and the newly focused product release. The rest-frame wavelength of DIB λ8621 was updated as λ0 = 8623.141 ± 0.030 Å in vacuum, corresponding to 8620.766 Å in air, which was determined by 77 DIB measurements toward the Galactic anticenter. The mean uncertainty of the fit central wave-length of these 77 measurements is 0.256 Å. With the peak-finding method and a coarse analysis, DIB λ8621 was found to correlate better with the neutral hydrogen than with the molecular hydrogen (represented by 12CO J = (1−0) emission). We also obtained 179 reliable measurements of DIB λ8648 in the RVS spectra of individual stars for the first time, further confirming this very broad DIB feature. Its EW and central wavelength presented a linear relation with those of DIB λ8621. A rough estimation of λ0 for DIB λ8648 was 8646.31 Å in vacuum, corresponding to 8643.93 Å in air, assuming that the carriers of λ8621 and λ8648 are comoving. Finally, we confirmed the impact of stellar residuals on the DIB measurements in Gaia DR3, which led to a distortion of the DIB profile and a shift of the center (≲0.5 Å), but the EW was consistent with our new measurements. With our measurements and analyses, we propose that the approach based on machine learning can be widely applied to measure DIBs in numerous spectra from spectroscopic surveys.

Maser polarization through anisotropic pumping

Context. Polarized emission from masers is an excellent tool to study magnetic fields in maser sources. The linear polarization of the majority of masers is understood as an interplay of maser saturation and anisotropic pumping. However, for the latter mechanism, no quantitative modeling has been presented yet. Aims. We aim to construct a comprehensive model of maser polarization, including quantitative modeling of both anisotropic pumping and the effects of maser saturation on the polarization of masers. Methods. We extended regular (isotropic) maser excitation modeling with a dimension that describes the molecular population alignments, as well as including the linear polarization dimension to the radiative transfer. The results of the excitation analysis yielded the anisotropic pumping and decay parameters, which were subsequently used in one-dimensional proper maser polarization radiative transfer modeling. Results. We present the anisotropic pumping parameters for a variety of transitions from class I CH3OH masers, H2O masers, and SiO masers. SiO masers are highly anisotropically pumped due to them occurring in the vicinity of a late-type star, which irradiates the maser region with a strong directional radiation field. Class I CH3OH masers and H2O masers occur in association with shocks, and they are modestly anisotropically pumped due to the anisotropy of the excitation region. Conclusions. Our modeling constitutes the first quantitative constraints on the anisotropic pumping of masers. We find that anisotropic pumping can explain the high polarization yields of SiO masers, as well as the modest polarization of unsaturated class I CH3OH masers. The common 22 GHz H2O maser has a relatively weak anisotropic pumping; in contrast, we predict that the 183 GHz H2O maser is strongly anisotropically pumped. Finally, we outline a mechanism through which non-Zeeman circular polarization is produced, when the magnetic field changes direction along the propagation through an anisotropically pumped maser.

Chemical abundances and ionizing mechanisms in the star-forming double-ring of AM 0644-741 using MUSE data

ABSTRACT We present the analysis of archival Very Large Telescope Multi-Unit Spectroscopic Explorer observations of 179 H ii regions in the star-forming double-ring collisional galaxy AM 0644-741 at 98.6 Mpc. We determined ionic abundances of He, N, O, and Fe using the direct method for the brightest H ii region (ID 39); we report $\log \rm {(\frac{N}{O})}=-1.3\pm 0.2$ and $12+\log \rm {(\frac{O}{H})}=8.9\pm 0.2$. We also find the so-called ‘blue-bump’, broad He ii λ4686, in the spectrum of this knot of massive star formation; its luminosity being consistent with the presence of ∼430 Wolf–Rayet (WR) stars of the nitrogen late-type. We determined the O abundances for 137 H ii regions using the strong-line method; we report a median value of $12+\log \rm {(\frac{O}{H})}=8.5\pm 0.8$. The location of three objects, including the WR complex, coincide with that of an Ultra Luminous X-ray source. Nebular He ii is not detected in any H ii region. We investigate the physical mechanisms responsible for the observed spectral lines using appropriate diagnostic diagrams and ionization models. We find that the H ii regions are being photoionized by star clusters with ages ∼2.5–20 Myr and ionization potential −3.5 <log 〈U〉<−3.0. In these diagrams, a binary population is needed to reproduce the observables considered in this work.

Exploring Be phenomena in OBA stars: A mid-infrared search

As early-type stars with a rotation speed close to their critical velocity, Be stars experience an event called the Be phenomenon. The material in their equator is ejected into outside space during the Be phenomenon and forms a circumstellar disk. The mechanism triggering these events remains poorly understood, and observations of these events are limited because the duration of these events ranges from months to years. Long-term epoch photometry in the infrared bands is expected to be ideal for detecting Be phenomena because the brightness variation is larger than that in the optical, and the effect of interstellar extinction is weaker as well. We conducted a systematic search for Be phenomena among Milky Way OBA stars in the mid-infrared. We examined the brightness and colour variations of known classical Be stars using the WISE W1 and W2 photometry bands to quantify their characteristics. Subsequently, we established a set of criteria to identify similar photometric variations in a large sample of OBA stars. We found 916 OBA stars that show Be phenomena in the past 13 yr, 736 of which are newly discovered. The peak-to-peak variations in magnitude and colour were found to be correlated, indicating that a decretion disk is common. The increase in colour was observed to be strongly correlated with the emission of the Hα line, providing further evidence of the association with circumstellar disks. The brightness variation of a star with Be phenomena can be up to 1.5 mag, and the colour variations can be up to 0.4 mag. The median durations for the disk build-up and decay phases are 474 and 524 days, respectively (durations shorter than 180 days are not sampled). The search for Be phenomena in the WISE bands greatly enlarges the number of stars showing disk variation, and it enables multi-band photometry analysis of these events with the help of current and future optical photometry surveys.

Stellar activity and differential rotation of HD 111395

Aims. Stellar activity cycles and rotation periods are important parameters for characterising the stellar dynamo, which operates in late-type main-sequence stars. However, the number of stars with well-known cycle and rotation periods is rather low, so new detections are still important. Methods. To find activity cycles and rotation periods, we utilised the TIGRE telescope to monitor stars for periodic variations in chromospheric activity indicators. We employed the widely used CaII H&K lines and the CaII infrared triplet lines as stellar activity indicators. To verify a periodic variation and to determine the corresponding period, we performed a frequency analysis via the generalised Lomb-Scargle method of the taken time series. Results. We studied CaII data of the G5V star HD 111395 and derive an activity cycle period of 949 ± 5 d (≈2.6 yr). This cycle is coincident with coronal measurements from the X-ray telescope eROSITA on board SRG. Furthermore, the TIGRE CaII time series show a long-term trend that indicates an additional long-term cycle. Using the few available literature S-index data points, we estimate a probable cycle length of 12–15 yr for this potential long-term cycle. Finally, we determined rotation periods from each observation season. We computed a mean rotation period of 16.76 ± 0.36 d averaged over all observation seasons and chromospheric indicators. However, we also find a strong variation in the mean seasonal rotation periods, which follows the derived cycle period; therefore, we interpret this behaviour as a sign of surface differential rotation.

From convective stellar dynamo simulations to Zeeman-Doppler images

Context. Zeeman-Doppler imaging (ZDI) is used to reconstruct the surface magnetic field of late-type stars from high-resolution spectropolarimetric observations. The results are usually described in terms of characteristics of the field topology, such as poloidality versus toroidality and axisymmetry versus non-axisymmetry, in addition to the field strength. Aims. In this study, we want to test how well these characteristics are preserved when applying the ZDI method to simulated data. We are particularly interested in how accurately the field topology is preserved and to what extent stellar parameters, such as projected rotation velocity and rotation axis inclination, influence the reconstruction. Methods. For these tests, we used published magnetic field vector data from direct numerical magnetohydrodynamic simulations taken near the surface of the simulation domain. These simulations have variable rotation rates and therefore represent different levels of activity of an otherwise Sun-like setup with a convective envelope of solar thickness. Our ZDI reconstruction is based on spherical harmonics expansion. By comparing the original values to those of the reconstructed images, we study the ability to reconstruct the surface magnetic field in terms of various characteristics of the field. Results. In general, the ZDI method works as expected. The main large-scale features are reasonably well recovered, but the strength of the recovered magnetic field is just a fraction of the original input. The quality of the reconstruction shows clear correlations with the data quality. Furthermore, there are some spurious dependencies between stellar parameters and the characteristics of the field. Conclusions. Our study uncovers some limits of ZDI. Firstly, the recovered field strength will generally be lower than the ‘real’ value, as smaller structures with opposite polarities will be blurred in the inversion. This is also seen in the relative distribution of magnetic energy in terms of the angular degree ℓ. Secondly, the axisymmetry is overestimated. The poloidality versus toroidality is better recovered. The reconstruction works better for a stronger field and faster rotation velocity. Still, the ZDI method works surprisingly well even for a weaker field and slow rotation provided the data have a high signal-to-noise ratio and good rotation phase coverage.

Thirty Bright High Velocity B, A, and F Stars that may have Late-type Companions

I found thirty high velocity stars that have large V − K values compared to standard stars of similar spectral types indicating the possible presence of late-type companion stars. All these thirty stars are relatively bright nearby stars with accurate Gaia DR3 parallaxes and radial velocities. There is hardly any relevant literature on these stars. There are 18 high galactic latitude stars in this sample. In order to further understand their chemical composition, evolutionary status, binary companions and how these stars acquired high radial velocities high resolution spectroscopy, radial velocity and photometric monitoring studies are very much needed.

The 3D geometry of reflection nebulae IC 59 and IC 63 with their illuminating star gamma Cas

ABSTRACT The early-type star gamma Cas illuminates the reflection nebulae IC 59 and IC 63, creating two photodissociation regions (PDRs). Uncertainties about the distances to the nebulae and the resulting uncertainty about the density of the radiation fields incident on their surfaces have hampered the study of these PDRs during the past three decades. We employed far-ultraviolet (UV) – optical nebula – star colour differences of dust-scattered light to infer the locations of the nebulae relative to the plane of the sky containing gamma Cas, finding IC 63 to be positioned behind the star and IC 59 in front of the star. To obtain the linear distances of the nebulae relative to gamma Cas, we fit far-infrared archival Herschel flux data for IC 59 and IC 63 with modified blackbody curves and relate the resulting dust temperatures with the luminosity of gamma Cas, yielding approximate distances of 4.15 pc for IC 59 and 2.3 pc for IC 63. With these distances, using updated far-UV flux data in the 6–13.6 eV range for gamma Cas with two recent determinations of the interstellar extinction for gamma Cas, we estimate that the far-UV radiation density at the surface of IC 63 takes on values of G0 = 58 or G0 = 38 with respective values for E(B − V) for gamma Cas of 0.08 and 0.04 mag. This is a substantial reduction from the range 150 ≤ G0 ≤ 650 used for IC 63 during the past three decades. The corresponding, even lower new values for IC 59 are G0 = 18 and G0 = 12.

Origin and Composition of the Galactic Diffuse X-Ray Emission Spectra by Unresolved X-Ray Sources

Galactic diffuse X-ray emission (GDXE) can be spatially segmented into Galactic center X-ray emission (GCXE), Galactic ridge X-ray emission (GRXE), and Galactic bulge X-ray emission (GBXE). The X-ray spectra of GDXE are expressed by the assembly of compact X-ray sources, which are either white dwarfs (WDs) or X-ray active stars consisting of binaries with late-type stars. WDs have either a strong magnetic field or a weak magnetic field. WDs and X-ray active stars are collectively called compact X-ray stars. However, spectral fittings by the assembly of all compact X-ray stars for GCXE, GRXE, and GBXE are rejected, leaving significant excess near the energies of the Kα, Heα, and Lyα lines. These excesses are found in the collisional ionization equilibrium (CIE) plasma. Thus, the spectra of GRXE and GBXE are improved by adding CIE supernova remnants (SNRs). However, the GCXE spectrum is still unacceptable, with significant data excess due to radiative recombination emission (recombining plasma (RP)). The GCXE fit is then significantly improved by adding aged RP-SNRs. Aged RP-SNRs are made by a past big flare of Sgr A* emitting either hard X-rays or low-energy cosmic rays. The big flares may excite Fe and Ni atoms in cold diffuse gas (cold matter (CM)) and emit fluorescent X-ray lines. The CIE-SNRs, RP-SNRs, and CM are called diffuse X-ray sources. This paper presents the spectral fits by the assembly of all the compact and diffuse X-ray sources together with high-quality spectra and a combined fit among all the GDXE of GCXE, GRXE, and GBXE. This provides the first scenario to quantitatively and comprehensively predict the origin of the GDXE spectra.

The most variable VVV sources: eruptive protostars, dipping giants in the nuclear disc and others

ABSTRACT We have performed a comprehensive search of a VISTA Variables in the Via Lactea (VVV) data base of 9.5 yr light curves for variable sources with ΔKs ≥ 4 mag, aiming to provide a large sample of high amplitude eruptive young stellar objects (YSOs) and detect unusual or new types of infrared variable source. We find 222 variable or transient sources in the Galactic bulge and disc, most of which are new discoveries. The sample mainly comprises novae, YSOs, microlensing events, Long Period Variable stars (LPVs), and a few rare or unclassified sources. Additionally, we report the discovery of a significant population of aperiodic late-type giant stars suffering deep extinction events, strongly clustered in the Nuclear Disc of the Milky Way. We suggest that these are metal-rich stars in which radiatively driven mass loss has been enhanced by super-solar metallicity. Among the YSOs, 32/40 appear to be undergoing episodic accretion. Long-lasting YSO eruptions have a typical rise time of ∼2 yr, somewhat slower than the 6–12 month time-scale seen in the few historical events observed on the rise. The outburst durations are usually at least 5 yr, somewhat longer than many lower amplitude VVV events detected previously. The light curves are diverse in nature, suggesting that multiple types of disc instability may occur. Eight long-duration extinction events are seen wherein the YSO dims for a year or more, attributable to inner disc structure. One binary YSO in NGC 6530 displays periodic extinction events (P=59 d) similar to KH 15D.

VLTI/GRAVITY Provides Evidence the Young, Substellar Companion HD 136164 Ab Formed Like a “Failed Star”

Young, low-mass brown dwarfs orbiting early-type stars, with low mass ratios (q ≲ 0.01), appear to be intrinsically rare and present a formation dilemma: could a handful of these objects be the highest-mass outcomes of “planetary” formation channels (bottom up within a protoplanetary disk), or are they more representative of the lowest-mass “failed binaries” (formed via disk fragmentation or core fragmentation)? Additionally, their orbits can yield model-independent dynamical masses, and when paired with wide wavelength coverage and accurate system age estimates, can constrain evolutionary models in a regime where the models have a wide dispersion depending on the initial conditions. We present new interferometric observations of the 16 Myr substellar companion HD 136164 Ab (HIP 75056 Ab) made with the Very Large Telescope Interferometer (VLTI)/GRAVITY and an updated orbit fit including proper motion measurements from the Hipparcos–Gaia Catalog of Accelerations. We estimate a dynamical mass of 35 ± 10 M J (q ∼ 0.02), making HD 136164 Ab the youngest substellar companion with a dynamical mass estimate. The new mass and newly constrained orbital eccentricity (e = 0.44 ± 0.03) and separation (22.5 ± 1 au) could indicate that the companion formed via the low-mass tail of the initial mass function. Our atmospheric fit to a SPHINX M-dwarf model grid suggests a subsolar C/O ratio of 0.45 and 3 × solar metallicity, which could indicate formation in a circumstellar disk via disk fragmentation. Either way, the revised mass estimate likely excludes bottom-up formation via core accretion in a circumstellar disk. HD 136164 Ab joins a select group of young substellar objects with dynamical mass estimates; epoch astrometry from future Gaia data releases will constrain the dynamical mass of this crucial object further.