Astrometry and infrared observations of the Mira variable stars AP Lyncis, V837 Herculis, and BX Camelopardalis: Implications for the period–luminosity relation of the Milky Way

AP Lyn and V837 Her are long-period Mira variable stars in the Milky Way. We performed VLBI Exploration of Radio Astrometry (VERA) phase-referenced observations towards H2O masers associated with AP Lyn and V837 Her. The annual parallaxes of AP Lyn and V837 Her were obtained to be 2.008 ± 0.038 mas and 1.090 ± 0.014 mas, corresponding to distances of 498 ± 10 pc and 917 ± 12 pc, respectively. From our multi-epoch infrared observations using the Kagoshima University 1 m telescope, we derived the mean J-, H-, and K′-band magnitudes of AP Lyn, V837 Her, and an additional long-period Mira variable BX Cam, whose parallax is known. We derived their pulsation periods to be 433 ± 1 d, 520 ± 1 d, and 458 ± 1 d, respectively, using the K′-band light curves. The MK–log P relation of long-period Mira variables seem to be violated by Mira variable stars with larger-than-expected MK values (like OZ Gem) in the Milky Way because of circumstellar extinction leading to an observed dimming effect. AP Lyn, V837 Her, and BX Cam (like OZ Gem) are dimming from the trend to O-rich stars in the Large Magellanic Cloud. This implies that the high metallicity of the Milky Way galaxy increases the opacity of the Mira-type variable stars and strengthens mass loss.

VLT/X-shooter spectroscopy of massive young stellar objects in the 30 Doradus region of the Large Magellanic Cloud

The process of massive star (M ≥ 8 M⊙) formation is still poorly understood. Observations of massive young stellar objects (MYSOs) are challenging due to their rarity, short formation timescale, large distances, and high circumstellar extinction. Here, we present the results of a spectroscopic analysis of a population of MYSOs in the Large Magellanic Cloud. We took advantage of the spectral resolution and wavelength coverage of X-shooter (300−2500 nm), which is mounted on the European Southern Observatory Very Large Telescope, to detect characteristic spectral features in a dozen MYSO candidates near 30 Doradus, the largest starburst region in the Local Group hosting the most massive stars known. The X-shooter spectra are strongly contaminated by nebular emission. We used a scaling method to subtract the nebular contamination from our objects. We detect Hα, β, [O I] 630.0 nm, Ca II, infrared triplet [Fe II] 1643.5 nm, fluorescent Fe II 1687.8 nm, H2 2121.8 nm, Brγ, and CO bandhead emission in the spectra of multiple candidates. This leads to the spectroscopic confirmation of ten candidates as bona fide MYSOs. We compared our observations with photometric observations from the literature and find all MYSOs to have a strong near-infrared excess. We computed lower limits to the brightness and luminosity of the MYSO candidates, confirming the near-infrared excess and the massive nature of the objects. No clear correlation is seen between the Brγ luminosity and metallicity. Combining our sample with other LMC samples results in a combined detection rate of disk features, such as fluorescent Fe II and CO bandheads, which is consistent with the Galactic rate (40%). Most of our MYSOs show outflow features.

CI Camelopardalis: The first sgB[e]-high mass X-ray binary twenty years on: A supernova imposter in our own Galaxy?

Context. The Galactic supergiant B[e] star CI Camelopardalis (CI Cam) was the first sgB[e] star detected during an X-ray outburst. The star brightened to ∼2 Crab in the X-ray regime (∼5 × 10−8 ergs cm−2 s−1 in the 2–25 keV range) within hours before decaying to a quiescent level in less than two weeks, clearly indicative of binarity. Since the outburst of CI Cam, several sgB[e] stars have been identified as X-ray overluminous for a single star (i.e. LX > 10−7 Lbol). This small population has recently expanded to include two ultra luminous X-ray sources (ULX), Holmberg II X-1 and NGC 300 ULX-1/supernova imposter SN2010da. Aims. Since the discovery of X-ray emission from CI Cam, there have been many developments in the field of massive binary evolution. In light of the recent inclusion of two ULXs in the population of X-ray bright sgB[e] stars, we revisit CI Cam to investigate its behaviour over several timescales and shed further light on the nature of the compact object in the system, its X-ray outburst in 1998 and the binary system parameters. Methods. We analysed archival XMM-Newton EPIC-pn spectra and light curves along with new data from Swift and NuSTAR. We also present high-resolution (R ∼ 85 000) Mercator/HERMES optical spectra, including a spectrum taken 1.02 days after our NuSTAR observation. Results. Despite being in quiescence, CI Cam is highly X-ray variable on timescales of days, both in terms of total integrated flux and spectral shape. We interpret these variations by invoking the presence of an accreting compact companion immersed in a dense, highly structured, aspherical circumstellar envelope. The differences in the accretion flux and circumstellar extinction represent either changes in this environment, triggered by variable mass loss from the star, or the local conditions to the accretor due to its orbital motion. We find no evidence for pulsations in the X-ray light curve. Conclusions. CI Cam has many similarities with SN2010da across mid-IR, optical and X-ray wavelengths suggesting that, subject to distance determination for CI Cam, if CI Cam was located in an external galaxy its 1998 outburst would have led to a classification as a supernova imposter.

Dynamics of wind and the dusty environments in the accreting T Tauri stars RY Tauri and SU Aurigae

Classical T Tauri stars with ages of less than 10 Myr possess accretion discs. Magnetohydrodynamic processes at the boundary between the disc and the stellar magnetosphere control the accretion and ejections gas flows. We carried out a long series of simultaneous spectroscopic and photometric observations of the classical T Tauri stars, RY Tauri and SU Aurigae, with the aim to quantify the accretion and outflow dynamics at time-scales from days to years. It is shown that dust in the disc wind is the main source of photometric variability of these stars. In RY Tau, we observed a new effect: during events of enhanced outflow, the circumstellar extinction becomes lower. The characteristic time of changes in outflow velocity and stellar brightness indicates that the obscuring dust is near the star. The outflow activity in both stars is changing on a time-scale of years. Periods of quiescence in the variability of the Hα profile were observed during the 2015–2016 period in RY Tau and during the 2016–2017 period in SU Aur. We interpret these findings in the framework of the magnetospheric accretion model, and we discuss how the global stellar magnetic field can influence the long-term variations of the outflow activity.

Polyaromatic disordered carbon grains as carriers of the UV bump: Far-UV to mid-IR spectroscopy of laboratory analogs

Context. A multiwavelength study of laboratory carbons with varying degrees of hydrogenation and sp2 hybridization is required to characterize the structure of the carbonaceous carriers of interstellar and circumstellar extinction. Aims. We study the spectral properties of carbonaceous dust analogs from the far-ultraviolet to the mid-infrared and correlate features in both spectral ranges to the aromatic/aliphatic degree. Methods. Analogs to carbonaceous interstellar dust encountered in various phases of the interstellar medium have been prepared in the laboratory. These are amorphous hydrogenated carbons (a-C:H), analogs to the diffuse interstellar medium component, and soot particles, analogs to the polyaromatic component. Thin films (d < 100 nm) have been measured in transmission in the vacuum-ultraviolet (VUV; 120–210 nm) within the atmospheric pressure experiment (APEX) chamber of the DISCO beam line at the SOLEIL synchrotron radiation facility. Spectra of these films were further measured through the UV-Vis (210 nm–1 μm) and in the mid-infrared (3–15 μm). Results. Tauc optical gaps, Eg, are derived from the visible spectra. The major spectral features are fitted through the VUV to the mid-infrared to obtain positions, full-widths at half maximum (FWHM), and integrated intensities. These are plotted against the position of the π-π∗ electronic transitions peak. Unidentified or overlapping features in the UV are identified by correlations with complementary infrared data. A correlation between the optical gap and position of the π-π∗ electronic transitions peak is found. The latter is also correlated to the position of the sp3 carbon defect band at ~8 μm, the aromatic C=C stretching mode position at ~6 μm, and the H/C ratio. Conclusions. Ultraviolet and infrared spectroscopy of structurally diverse carbon samples are used to constrain the nanostructural properties of carbon carriers of both circumstellar and interstellar extinction, such as the associated coherent lengths and the size of polyaromatic units. Our study suggests that carriers of the interstellar UV bump should exhibit infrared bands akin to the A/B classes of the aromatic infrared bands, while the circumstellar bump carriers should exhibit bands corresponding to the B/C classes.

The evolution of red supergiants to supernovae

With red supergiants (RSGs) predicted to end their lives as Type IIP core collapse supernova (CCSN), their behaviour before explosion needs to be fully understood. Mass loss rates govern RSG evolution towards SN and have strong implications on the appearance of the resulting explosion. To study how the mass-loss rates change with the evolution of the star, we have measured the amount of circumstellar material around 19 RSGs in a coeval cluster. Our study has shown that mass loss rates ramp up throughout the lifetime of an RSG, with more evolved stars having mass loss rates a factor of 40 higher than early stage RSGs. Interestingly, we have also found evidence for an increase in circumstellar extinction throughout the RSG lifetime, meaning the most evolved stars are most severely affected. We find that, were the most evolved RSGs in NGC2100 to go SN, this extra extinction would cause the progenitor’s initial mass to be underestimated by up to 9M⊙.