Optical spectroscopy of Galactic field classical Be stars

ABSTRACT In this study, we analyse the emission lines of different species present in 118 Galactic field classical Be stars in the wavelength range of 3800–9000 Å. We re-estimated the extinction parameter (AV) for our sample stars using the newly available data from Gaia DR2 and suggest that it is important to consider AV while measuring the Balmer decrement (i.e. D34 and D54) values in classical Be stars. Subsequently, we estimated the Balmer decrement values for 105 program stars and found that ≈20 per cent of them show D34 ≥ 2.7, implying that their circumstellar disc are generally optically thick in nature. One program star, HD 60855 shows H α in absorption – indicative of disc-less phase. From our analysis, we found that in classical Be stars, H α emission equivalent width values are mostly lower than 40 Å, which agrees with that present in literature. Moreover, we noticed that a threshold value of ∼10 Å of H α emission equivalent width is necessary for Fe ii emission to become visible. We also observed that emission line equivalent widths of H α, P14, Fe ii 5169, and O i 8446 Å for our program stars tend to be more intense in earlier spectral types, peaking mostly near B1-B2. Furthermore, we explored various formation regions of Ca ii emission lines around the circumstellar disc of classical Be stars. We suggest the possibility that Ca ii triplet emission can originate either in the circumbinary disc or from the cooler outer regions of the disc, which might not be isothermal in nature.

The tidal disruption event AT 2018hyz – I. Double-peaked emission lines and a flat Balmer decrement

ABSTRACT We present results from spectroscopic observations of AT 2018hyz, a transient discovered by the All-Sky Automated Survey for Supernova survey at an absolute magnitude of MV ∼ −20.2 mag, in the nucleus of a quiescent galaxy with strong Balmer absorption lines. AT 2018hyz shows a blue spectral continuum and broad emission lines, consistent with previous TDE candidates. High cadence follow-up spectra show broad Balmer lines and He i in early spectra, with He ii making an appearance after ∼70–100 d. The Balmer lines evolve from a smooth broad profile, through a boxy, asymmetric double-peaked phase consistent with accretion disc emission, and back to smooth at late times. The Balmer lines are unlike typical active galactic nucleus in that they show a flat Balmer decrement (Hα/Hβ ∼ 1.5), suggesting the lines are collisionally excited rather than being produced via photoionization. The flat Balmer decrement together with the complex profiles suggests that the emission lines originate in a disc chromosphere, analogous to those seen in cataclysmic variables. The low optical depth of material due to a possible partial disruption may be what allows us to observe these double-peaked, collisionally excited lines. The late appearance of He ii may be due to an expanding photosphere or outflow, or late-time shocks in debris collisions.

MEGARA-IFU detection of extended He ii λ4686 nebular emission in the central region of NGC 1569 and its ionization budget

ABSTRACT We here report the detection of extended He ii λ4686 nebular emission in the central region of NGC 1569 using the integral field spectrograph MEGARA at the 10.4 m Gran Telescopio Canarias. The observations cover a field of view (FoV) of 12.5 arcsec × 11.3 arcsec at a seeing-limited spatial resolution of ∼15 pc and at a spectral resolution of R = 6000 in the wavelength range 4330–5200 Å. The emission extends over a semicircular arc of ∼40 pc width and ∼150 pc diameter around the superstar cluster A (SSC-A). The AV derived using Balmer decrement varies from the Galactic value of 1.6 mag to a maximum of ∼4.5 mag, with a mean value of 2.65 ± 0.60 mag. We infer 124 ± 11 Wolf–Rayet (WR) stars in SSC-A using the He ii λ4686 broad feature and AV = 2.3 mag. The He+ ionizing photon rate from these WR stars is sufficient to explain the luminosity of the He ii nebula. The observationally determined total He+ and H0 ionizing photon rates, their ratio, and the observed number of WR stars in SSC-A are all consistent with the predictions of simple stellar population models at an age of 4.0 ± 0.5 Myr and a mass of (5.5 ± 0.5) × 105 M⊙. Our observations reinforce the absence of WR stars in SSC-B, the second most massive cluster in the FoV. None of the other locations in our FoV where He ii λ4686 emission has been reported from narrow-band imaging observations contain WR stars.

The dust and cold gas content of local star-forming galaxies

ABSTRACT We use dust masses (Mdust) derived from far-infrared data and molecular gas masses (Mmol) based on CO luminosity to calibrate proxies based on a combination of the galaxy Balmer decrement, disc inclination, and gas metallicity. We use such proxies to estimate Mdust and Mmol in the local SDSS sample of star-forming galaxies (SFGs). We study the distribution of Mdust and Mmol along and across the main sequence (MS) of SFGs. We find that Mdust and Mmol increase rapidly along the MS with increasing stellar mass (M*), and more marginally across the MS with increasing SFR (or distance from the relation). The dependence on M* is sub-linear for both Mdust and Mmol. Thus, the fraction of dust (fdust) and molecular gas mass (fmol) decreases monotonically towards large M*. The star formation efficiency (SFE, inverse of the molecular gas depletion time) depends strongly on the distance from the MS and it is constant along the MS. As nearly all galaxies in the sample are central galaxies, we estimate the dependence of fdust and fgas on the host halo mass and find a tight anticorrelation. As the region where the MS is bending is numerically dominated by massive haloes, we conclude that the bending of the MS is due to a lower availability of molecular gas mass in massive haloes rather than a lower efficiency in forming stars.

Optical and X-ray studies of three polars: RX J0859.1+0537, RX J0749.1–0549, and RX J0649.8–0737

ABSTRACT We present optical photometric and spectroscopic observations, and an analysis of archival X-ray data of three polars: RX J0859.1+0537, RX J0749.1–0549, and RX J0649.8–0737. Optical light curves of these three polars reveal eclipse-like features that are deep, total, and variable in shape. The optical and X-ray modulations of RX J0859.1+0537, RX J0749.1–0549, and RX J0649.8–0737 are both found to occur at the orbital periods of 2.393 ± 0.003 h, 3.672 ± 0.001 h, and 4.347 ± 0.001 h, respectively. RX J0859.1+0537 is found to be a polar that lies in the region of the period gap, whereas RX J0749.1–0549 and RX J0649.8–0737 are found to be long-period polars above the period gap. The eclipse length is found to be 61 min for RX J0749.1–0549 in the Rband, which is the highest among the long-period eclipsing polars. The radius of the eclipsed light source is found to be more than the actual size of the white dwarf for these three systems, indicating that the eclipsed component is not only the white dwarf but also appears to include the presence of an extended accretion region. Optical spectra of these systems show the presence of high-ionization emission lines along with the strong Balmer emission lines with an inverted Balmer decrement. Cyclotron harmonics are also detected in the optical spectra from which we infer magnetic field strength of the surface of the white dwarf to be 49 ± 2 MG, 43.5 ± 1.4 MG, and 44 ± 1 MG for RX J0859.1+0537, RX J0749.1–0549, and RX J0649.8–0737, respectively.

The Correlation Between Fluxes of Balmer Emission Lines of H? and H? and Colors of a Galaxy in VNIR Region

ABSTRACT We have studied the correlation between the flux and colors of a galaxy sample from SDSS, Data Release 13 as measured by the standard Balmer decrement technique in both visible and near-infrared region, covering the spectral region from (4500-8500) Ao. We analyzed the spectral properties of the sample in the three (g, r, i) optical bands and found that the main sample tends to be redder which gives an indication that the sample has higher metallicity or older age. We carried out the standard procedures IRAF software package to get a deeper insight into the flux in Hβ and Hα wavelengths range. We have concluded that the distribution of H- Beta and H- alpha line profiles is Gaussian in the two regions. We found also a linear correlation between Balmer decrement (Hα /Hβ ) in lognormal units and the rest frame colors of the sample with the best fit of 3.4764. Mean and root means square (rms) profiles of Hβ and Hα shows a maximum peak in the red region at the radial velocity of 1000km/s and the profiles were high symmetry in this region.

Magnetic fields and thermodynamic conditions in the pre-peak phase of M6.4 / 3N solar flare

We present a study of the pre-peak phase of the solar flare of M6.4 / 3N class which arose on July 19, 2000 in the NOAA 9087 active region. The effective magnetic field Beff was measured using the FeI 6301.5 Ǻ, FeI 6302.5 Ǻ, Hα and Hβ spectral lines. It was found that at the brightest place of the flare, which was projected onto a small sunspot of N polarity, Beff was close to each other on all four lines and corresponded to 1.0-1.2 kG. At the same time, the modulus of the magnetic field at the level of FeI 6302.5 formation, determined by the splitting of peaks V of the Stokes parameter and the localization of the σ-components in the I ± V profiles, was in the range 1.6–2.6 kG. The bisectors of the I + V and I – V profiles of the FeI 6301.5 line are parallel to each other, indicating a simple one-component structure of the magnetic field at the level of the middle photosphere under the flare. The Balmer decrement of Imax (Hα) / Imax (Hβ) by Hα and Hβ lines was 1.16. The semi-empirical model of the photospheric layers of the flare was constructed using Stokes I observations of non-magneticsensitive FeI 5123.7 and 5434.5 lines by solving the inverse equilibrium transfer problem using Tikhonov stabilizers. For the distribution of temperature with height, the effects of deviation from the LTE were found to be significant for the layers of the lower photosphere corresponding to the heights h ≥ 0 (i.e. τ 5 ≤ 1). In the entire thickness of the photosphere (h = 0–500 km), the flare temperature is lower compared to the non-perturbed atmosphere, while it is slightly higher for h> 500 km. The micro-turbulent velocity is increased at altitudes h> 200–500 km, while at altitudes h <200 km it is reduced. The obtained results indicate that the upper photosphere and the lower chromosphere are perturbed during solar flares, even when the magnetic field is quasi-homogeneous in the lower layers (middle photosphere).