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The VLA Nascent Disk and Multiplicity Survey of Perseus Protostars (VANDAM). IV. Free–Free Emission from Protostars: Links to Infrared Properties, Outflow Tracers, and Protostellar Disk Masses
摘要： Emission from protostars at centimeter radio wavelengths has been shown to trace the free–free emission arising from ionizing shocks as a result of jets and outflows driven by protostars. Therefore, measuring properties of protostars at radio frequencies can provide valuable insights into the nature of their outflows and jets. We present a C-band (4.1 and 6.4 cm) survey of all known protostars (Class 0 and Class I) in Perseus as part of the VLA Nascent Disk and Multiplicity (VANDAM) Survey. We examine the known correlations between radio flux density and protostellar parameters, such as bolometric luminosity and outflow force, for our sample. We also investigate the relationship between radio flux density and far-infrared line luminosities from Herschel. We show that free–free emission most likely originates from J-type shocks; however, the large scatter indicates that those two types of emission probe different time and spatial scales. Using C-band fluxes, we removed an estimation of free–free contamination from the corresponding Ka-band (9 mm) flux densities that primarily probe dust emission from embedded disks. We find that the compact (<1″) dust emission is lower for Class I sources (median dust mass 96 M⊕) relative to Class 0 (248 M⊕), but several times higher than in Class II (5–15 M⊕). If this compact dust emission is tracing primarily the embedded disk, as is likely for many sources, this result provides evidence of decreasing disk masses with protostellar evolution, with sufficient mass for forming giant planet cores primarily at early times.
关键词： techniques: interferometric,radio continuum: stars,stars: protostars,protoplanetary disks,stars: winds, outflows,stars: formation
更新于2025-09-19 17:15:36
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Distinguishing circumstellar from stellar photometric variability in Eta Carinae
摘要： The interacting binary Eta Carinae remains one of the most enigmatic massive stars in our Galaxy despite over four centuries of observations. In this work, its light curve from the ultraviolet to the near-infrared is analysed using spatially resolved HST observations and intense monitoring at the La Plata Observatory, combined with previously published photometry. We have developed a method to separate the central stellar object in the ground-based images using HST photometry and applying it to the more numerous ground-based data, which supports the hypothesis that the central source is brightening faster than the almost-constant Homunculus. After detrending from long-term brightening, the light curve shows periodic orbital modulation (?V ～ 0.6 mag) attributed to the wind-wind collision cavity as it sweeps around the primary star and it shows variable projected area to our line-of-sight. Two quasi-periodic components with time scales of 2-3 and 8-10 yr and low amplitude, ?V < 0.2 mag, are superimposed on the brightening light-curve, being the only stellar component of variability found, which indicates minimal stellar instability. Moreover, the light curve analysis shows no evidence of 'shell ejections' at periastron. We propose that the long-term brightening of the stellar core is due to the dissipation of a dusty clump in front of the central star, which works like a natural coronagraph. Thus, the central stars appear to be more stable than previously thought since the dominant variability originates from a changing circumstellar medium. We predict that the brightening phase, due mainly to dust dissipation, will be completed around 2032 ± 4 yr, when the star will be brighter than in the 1600's by up to ?V ～ 1 mag.
关键词： stars: individual (η Carinae),stars: winds, outflows,(stars:) binaries: general,stars: evolution,(ISM:) dust, extinction
更新于2025-09-19 17:15:36
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Near-infrared evidence for a sudden temperature increase in Eta Carinae
摘要： Aims. Eta Car’s ultra-violet, optical, and X-ray light curves and its spectrum suggest a physical change in its stellar wind over the last decade. It has been proposed that the mass-loss rate has decreased by a factor of about 2 over the last 15 years. We complement these recent results by investigating the past evolution and the current state of η Car in the near-infrared (IR). Methods. We present JHKL photometry of η Car obtained at SAAO Sutherland from 2004–2013 with the Mk II photometer at the 0.75 m telescope and JHKs photometry with SIRIUS at the 1.4 m IRSF telescope from 2012–2013. The near-IR light curves since 1972 are analyzed. Results. The long-term brightening trends in η Car’s JHKL light curves were discontinuous around the 1998 periastron passage. After 1998, the star shows excess emission above the extrapolated trend from earlier dates, especially in J and H, and the blueward, cyclical progression in its near-IR colors is accelerated. The near-IR color evolution is strongly correlated with the periastron passages. After correcting for the secular trend we ?nd that the color evolution matches an apparent increase in blackbody temperature of an optically thick near-IR emitting plasma component from about 3500 K to 6000 K over the last 20 years. Conclusions. We suggest that the changing near-IR emission may be caused by variability in optically thick bremsstrahlung emission. Periastron passages play an important role in the observed excess near-IR emission after 1998 and the long-term color evolution. We thus propose the hypothesis that angular momentum transfer (via tidal acceleration) during periastron passages leads to sudden changes in η Car’s atmosphere resulting in a long-term decrease in the mass-loss rate.
关键词： stars: individual: η Carinae,stars: winds, out?ows,stars: mass-loss,stars: variables: S Doradus,stars: massive
更新于2025-09-10 09:29:36
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Wind asymmetry imprint in the UV light curves of the symbiotic binary SY Mus
摘要： Context. Light curves (LCs) of some symbiotic stars show a di?erent slope of the ascending and descending branch of their minimum pro?le. The origin of this asymmetry is not well understood. Aims. We explain this e?ect in the ultraviolet LCs of the symbiotic binary SY Mus. Methods. We model the continuum ?uxes in the spectra obtained by the International Ultraviolet Explorer at ten wavelengths, from 1280 to 3080 ?. We consider that the white dwarf radiation is attenuated by H0 atoms, H? ions, and free electrons in the red giant wind. Variation in the nebular component is approximated by a sine wave along the orbit as suggested by spectral energy distribution models. The model includes asymmetric wind velocity distribution and the corresponding ionization structure of the binary. Results. We determined distribution of the H0 and H+, as well as upper limits of H? and H0 column densities in the neutral and ionized region at the selected wavelengths as functions of the orbital phase. Corresponding models of the LCs match well the observed continuum ?uxes. In this way, we suggested the main UV continuum absorbing (scattering) processes in the circumbinary environment of S-type symbiotic stars. Conclusions. The asymmetric pro?le of the ultraviolet LCs of SY Mus is caused by the asymmetric distribution of the circumstellar matter at the near-orbital-plane area.
关键词： scattering,binaries: symbiotic,stars: individual: SY Mus,stars: winds, out?ows
更新于2025-09-10 09:29:36
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Analysis of colour and polarimetric variability of RW?Aur?A in 2010–2018
摘要： The results of UBVRIJHKLM photometry and VRI polarimetry of a young star RW Aur A observed during unprecedented long and deep (up to (cid:2)V ≈ 5 mag) dimming events in 2010–11 and 2014–18 are presented. The polarization degree p of RW Aur A at this period has reached 30 per cent in the I band. As in the case of UX Ori-type stars (UXORs), the so-called ‘bluing effect’ in the colour–magnitude V versus V ? Rc, V ? Ic diagrams of the star and a strong anticorrelation between p and brightness were observed. However, the duration and the amplitude of the eclipses as well as the value and orientation of the polarization vector in our case differ signi?cantly from those of UXORs. We conclude that the dimmings of RW Aur A occurred due to eclipses of the star and inner regions of its disc by the axisymmetric dust structure located above the disc and created by the disc wind. Taking into account both scattering and absorption of stellar light by the circumstellar dust, we explain some features of the light curve and the polarization degree–magnitude dependence. We found that near the period of minimal brightness the mass-loss rate of the dusty wind was >10?9 M(cid:3) yr?1.
关键词： binaries: general,stars: winds, out?ows,stars: individual: RW Aur,accretion, accretion discs,stars: variables: T Tauri, Herbig Ae/Be
更新于2025-09-09 09:28:46
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Wind inhibition by X-ray irradiation in HMXBs: the influence of clumping and the final X-ray luminosity
摘要： Context. In wind-powered X-ray binaries, the radiatively driven stellar wind from the primary may be inhibited by the X-ray irradiation. This creates the feedback that limits the X-ray luminosity of the compact secondary. Wind inhibition might be weakened by the effect of small-scale wind inhomogeneities (clumping) possibly affecting the limiting X-ray luminosity. Aims. We study the influence of X-ray irradiation on the stellar wind for different radial distributions of clumping. Methods. We calculate hot star wind models with external irradiation and clumping using our global wind code. The models are calculated for different parameters of the binary. We determine the parameters for which the X-ray wind ionization is so strong that it leads to a decrease of the radiative force. This causes a decrease of the wind velocity and even of the mass-loss rate in the case of extreme X-ray irradiation. Results. Clumping weakens the effect of X-ray irradiation because it favours recombination and leads to an increase of the wind mass-loss rate. The best match between the models and observed properties of high-mass X-ray binaries (HMXBs) is derived with radially variable clumping. We describe the influence of X-ray irradiation on the terminal velocity and on the mass-loss rate in a parametric way. The X-ray luminosities predicted within the Bondi-Hoyle-Lyttleton theory agree nicely with observations when accounting for X-ray irradiation. Conclusions. The ionizing feedback regulates the accretion onto the compact companion resulting in a relatively stable X-ray source. The wind-powered accretion model can account for large luminosities in HMXBs only when introducing the ionizing feedback. There are two possible states following from the dependence of X-ray luminosity on the wind terminal velocity and mass-loss rate. One state has low X-ray luminosity and a nearly undisturbed wind, and the second state has high X-ray luminosity and exhibits a strong influence of X-rays on the flow.
关键词： hydrodynamics,stars: early-type,outflows,X-rays: binaries,stars: mass-loss,stars: winds
更新于2025-09-09 09:28:46
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Eta Carinae’s 2014.6 spectroscopic event: Clues to the long-term recovery from its Great Eruption
摘要： Aims. Every 5.5 years, η Car’s light curve and spectrum change remarkably across all observed wavelength bands. These so-called spectroscopic events are most likely caused by the close approach of a companion. We compare the recent spectroscopic event in mid-2014 to the events in 2003 and 2009 and investigate long-term trends. Methods. Eta Car was observed with HST STIS, VLT UVES, and CTIO 1.5 m CHIRON for a period of more than two years in 2012?2015. Archival observations with these instruments cover three orbital cycles and the events of 2003.5, 2009.1, and 2014.6. The STIS spectra provide high spatial resolution and include epochs during the 2014 event when observations from most ground-based observatories were not feasible. The strategy for UVES observations allows for a multidimensional analysis, because each location in the re?ection nebula is correlated with a di?erent stellar latitude. Results. Important spectroscopic diagnostics during η Car’s events show signi?cant changes in 2014 compared to previous events. While the timing of the ?rst He ii λ4686 ?ash was remarkably similar to previous events, the He ii equivalent widths were slightly larger, and the line ?ux increased by a factor of ～7 compared to 2003. The second He ii peak occurred at about the same phase as in 2009, but was stronger. The He i line ?ux grew by a factor of ～8 in 2009?2014 compared to 1998?2003. The N ii emission lines also increased in strength. On the other hand, Hα and Fe ii lines show the smallest emission strengths ever observed in η Car. The optical continuum brightened by a factor of ～4 in the past 10?15 years. The polar spectrum shows fewer changes in the broad wind emission lines: the Fe ii emission strength decreased by a factor of ～2 (compared to a factor of ～4 in our direct line of sight). The He ii equivalent widths at FOS4 were larger in 2009 and 2014 than during the 2003 event. Conclusions. The basic character of η Car’s spectroscopic events has changed in the past two to three cycles. The ionizing UV radiation dramatically weakened during each pre-2014 event but not in 2014. The strengthening of He i and N ii emission and the weakening of the lower-excitation Hα and Fe ii wind features in our direct line of sight implies a substantial change in the physical parameters of the emitting regions. The polar spectrum at FOS4 shows fewer changes in the broad wind emission lines, which may be explained by the latitude-dependent wind structure of η Car. The quick and strong recovery of the He ii emission in 2014 supports a scenario, in which the wind-wind shock may not have completely collapsed as was proposed for previous events. As a result, the companion did not accrete as much material as in previous events. All this may be the consequence of just one elementary change, namely a strong decrease in the primary’s mass-loss rate. This would mark the beginning of a new phase, in which the spectroscopic events can be described as an occultation by the primary’s wind.
关键词： stars: mass-loss,stars: massive,stars: winds, out?ows,stars: individual: eta Carinae,stars: variables: S Doradus
更新于2025-09-04 15:30:14