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We analyzed more than 700 ultraviolet spectra of 45 nova-like stars (NLs) observed with the International Ultraviolet Explorer (IUE) satellite, obtaining reliable data for 42 of them. Combining these with the distances from the Gaia Early Data Release 3 (EDR3) and with results from the literature, for each object we determined the reddening EB − V, the disk spectral energy distribution (SED), the reference (i.e., inclination-corrected) absolute magnitude and disk luminosity (MVref, Ldiskref), and the mass accretion rate (Ṁ), all with propagated errors. The de-reddened UV continuum of NLs in a high state is well approximated by a power-law distribution with index α in the range −2.4 ≤ α ≤ −0.2. The agreement between the power-law extrapolation to the V band and the observed V magnitude is outstanding and implies that for NLs in a high state, the disk continuum dominates not only in the UV but also in the optical, with other possible contributions (white dwarf, M dwarf, and hot spot) being minor. We note that the accretion rate correlates with the period, power-law index, and MVref, making them convenient proxies for Ṁ. The strongest correlation (pH0 < 10−6) is log Ṁ = −0.57 ± 0.06 MVref−5.98 ± 0.29. Nine of the 42 NLs fall within the period gap but all have Ṁ very similar to that of the objects above the gap, contrary to theory expectations but in agreement with other observational work, and indicating that − at least for NLs − the theoretical assumptions of the standard model of the evolution of CVs need substantial revision. Medians and weighted means of log Ṁ (≈ −8.5) are very similar among NL classes, and also to those of old novae, dispelling the prejudice that stars belonging to the SW Sex class of NLs have “exceptionally high” Ṁ compared to other NLs (and old novae). In fact, it is one of the most interesting results of this study that NLs and old novae are indistinguishable in terms of Ṁ and its correlation with MVref. Two NLs (V1315 Aql and BZ Cam) have shells around them, a likely fingerprint of a past nova eruption, but the suggested association with “guest stars” of ancient Chinese chronicles is questionable.
We analyzed more than 700 ultraviolet spectra of 45 nova-like stars (NLs) observed with the International Ultraviolet Explorer (IUE) satellite, obtaining reliable data for 42 of them. Combining these with the distances from the Gaia Early Data Release 3 (EDR3) and with results from the literature, for each object we determined the reddening EB − V, the disk spectral energy distribution (SED), the reference (i.e., inclination-corrected) absolute magnitude and disk luminosity (MVref, Ldiskref), and the mass accretion rate (Ṁ), all with propagated errors. The de-reddened UV continuum of NLs in a high state is well approximated by a power-law distribution with index α in the range −2.4 ≤ α ≤ −0.2. The agreement between the power-law extrapolation to the V band and the observed V magnitude is outstanding and implies that for NLs in a high state, the disk continuum dominates not only in the UV but also in the optical, with other possible contributions (white dwarf, M dwarf, and hot spot) being minor. We note that the accretion rate correlates with the period, power-law index, and MVref, making them convenient proxies for Ṁ. The strongest correlation (pH0 < 10−6) is log Ṁ = −0.57 ± 0.06 MVref−5.98 ± 0.29. Nine of the 42 NLs fall within the period gap but all have Ṁ very similar to that of the objects above the gap, contrary to theory expectations but in agreement with other observational work, and indicating that − at least for NLs − the theoretical assumptions of the standard model of the evolution of CVs need substantial revision. Medians and weighted means of log Ṁ (≈ −8.5) are very similar among NL classes, and also to those of old novae, dispelling the prejudice that stars belonging to the SW Sex class of NLs have “exceptionally high” Ṁ compared to other NLs (and old novae). In fact, it is one of the most interesting results of this study that NLs and old novae are indistinguishable in terms of Ṁ and its correlation with MVref. Two NLs (V1315 Aql and BZ Cam) have shells around them, a likely fingerprint of a past nova eruption, but the suggested association with “guest stars” of ancient Chinese chronicles is questionable.
Nova eruptions occur in cataclysmic variables when enough material has been accreted onto the surface of the white dwarf primary. As a consequence, the material that has been accumulated until then is expelled into the interstellar medium, forming an expanding nova shell around the system. Understanding the physical process that shapes the morphology of nova shells is essential to fully comprehend how the ejection mechanism operates during nova eruptions. Because of its closeness and age, the nova shell around the classical nova RR Pic (Nova Pic 1925) is an ideal target for studying the evolving morphology of nova shells. The use of integral field spectroscopy (IFS) is a technique that has received little attention in the study of nova shells, despite the advantages in using it when studying the morphology and kinematics of nova shells. In this work, we present an IFS study of the RR Pic nova shell, with a particular emphasis on the extraction of the 3D morphology of the shell. The nova shell was observed by the Multi-Unit Spectroscopic Explorer (MUSE) instrument placed at the ESO-VLT. By measuring the extension of the nova shell in these new observations, and comparing it against previous measurements, we were able to determine the expansion history of the ejected material. We used this information, together with the distance to the system based on Gaia EDR3 parallaxes, and the systemic velocity of the system reported in the literature to obtain the physical 3D view of the shell. The MUSE datacube confirms the presence of the nova shell in H$ H$ and O iii and very faintly in N ii . A comparison with previous observations suggests that the shell continues in its free-expansion phase but with the different parts of the shell apparently expanding at different rates. The data analysis corroborates the previous vision that the shell is composed of an equatorial ring and polar filaments traced by H$ At the same time, the new data also reveal that O iii is confined in gaps located in the tropical regions of the shell where no Hydrogen is observed. The flux measurements indicate that sim 99<!PCT!> of the shell flux is confined to the equatorial ring, while the polar filaments show a flux asymmetry between the NE and SW filaments, with the latter being sim 2.5 times brighter. We have estimated the mass of the shell to be sim 5times 10$^ $M$_ From the analysis of the 3D-extracted data, we determine that the ring structure extends sim 8\,000 au from the central binary, and has a position angle of sim 155 deg and an inclination of sim 74 deg. The analysis of the equatorial ring reveals it is composed of a main ring and several small clouds, extending up to a height of sim 4\,000 au above and below the main plane of the equatorial ring. The radial profile of the whole ring structure is reminiscent of a bow shock. Our data have proven the capabilities of observing nova shells using IFS, and how the nova shell around RR Pic is an interesting object of study. Further and continuous observations of the shell across the electromagnetic spectrum are required to confirm the results and ideas presented in this work.
Vela X-1 is a runaway X-ray binary system hosting a massive donor star, whose strong stellar wind creates a bow shock as it interacts with the interstellar medium. This bow shock has previously been detected in Hα and IR, but, similar to all but one bow shock from a massive runaway star (BD+43o3654), has escaped detection in other wavebands. We report on the discovery of 1.3 GHz radio emission from the Vela X-1 bow shock with the MeerKAT telescope. The MeerKAT observations reveal how the radio emission closely traces the Hα line emission, both in the bow shock and in the larger-scale diffuse structures known from existing Hα surveys. The Vela X-1 bow shock is the first stellar-wind-driven radio bow shock detected around an X-ray binary. In the absence of a radio spectral index measurement, we explore other avenues to constrain the radio emission mechanism. We find that thermal/free-free emission can account for the radio and Hα properties, for a combination of electron temperature and density consistent with earlier estimates of ISM density and the shock enhancement. In this explanation, the presence of a local ISM over-density is essential for the detection of radio emission. Alternatively, we consider a non-thermal/synchrotron scenario, evaluating the magnetic field and broad-band spectrum of the shock. However, we find that exceptionally high fractions (≳ 13%) of the kinetic wind power would need to be injected into the relativistic electron population to explain the radio emission. Assuming lower fractions implies a hybrid scenario, dominated by free-free radio emission. Finally, we speculate about the detectability of radio bow shocks and whether it requires exceptional ISM or stellar wind properties.
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