We present first results of our study of a sample of Galactic LBV, aimed to
contribute to a better understanding of the LBV phenomenon, by recovering the
mass-loss history of the central object from the analysis of its associated
nebula. Mass-loss properties have been derived by a synergistic use of
different techniques, at different wavelengths, to obtain high-resolution,
multi-wavelength maps, tracing the different emitting components coexisting in
the stellar ejecta: the ionized/neutral gas and the dust. Evidence for
asymmetric mass-loss and observational evidence of possible mutual interaction
between gas and dust components have been observed by the comparison of mid-IR
(Spitzer/IRAC, VLT/VISIR) and radio (VLA) images of the nebulae, while
important information on the gas and dust composition have been derived from
Spitzer/IRS spectra.Comment: 5 pages, 4 figures. To appear in proceedings of 39th Liege
  International Astrophysical Colloquium: The multi-wavelength view of Hot,
  Massive Star

Buemi, Carla S.

Fazio, Giovanni

Hora, Joseph L.

Leto, Paolo

Trigilio, Corrado

Umana, Grazia

English

arXiv

We present first results of our study of a sample of Galactic LBV, aimed to contribute to a better understanding of the LBV phenomenon, by recovering  the mass-loss history of the central object from the analysis of its associated nebula. Mass-loss properties have been derived  by a synergistic use of different techniques, at different wavelengths, to obtain high-resolution, multi-wavelength maps, tracing the different emitting components coexisting in the stellar ejecta: the ionized/neutral gas and the dust. Evidence for asymmetric mass-loss and observational evidence of possible mutual interaction between gas and dust components have been observed by the comparison of  mid-IR (Spitzer/IRAC, VLT/VISIR) and radio (VLA) images of the nebulae, while important information on the gas and dust composition have been derived from Spitzer/IRS spectra

UMANA, Grazia Maria Gloria

BUEMI, CARLA SIMONA

TRIGILIO, CORRADO

LETO, PAOLO

OA@INAF - Istituto Nazionale di Astrofisica

2011Publication Year2023-01-20T15:59:02ZAcceptance in OA@INAFThe nebulae around LBVs: a multiwavelength approachTitleUMANA, Grazia Maria Gloria; BUEMI, CARLA SIMONA; TRIGILIO, CORRADO; LETO, PAOLO; Hora, Joseph L.; et al.Authorshttp://hdl.handle.net/20.500.12386/32966HandleBULLETIN DE LA SOCIÉTÉ ROYALE DES SCIENCES DE LIÈGEJournal80NumberThe nebulae around LBVs:a multiwavelength approachGrazia Umana1, Carla S. Buemi1, Corrado Trigilio1, Paolo Leto1Joseph L. Hora2 and Giovanni Fazio21 INAF-Osservatorio Astrofisico di Catania, Via S. Sofia 78, 95123 Catania, ITALY2 Harvard-Smithsonian Center for Astrophysics, 60 Garden St. MS-65,Cambridge, MA 02138-1516, USAAbstract: We present first results of our study of a sample of Galactic LBV, aimed to contribute to a bet-ter understanding of the LBV phenomenon, by recovering the mass-loss history of the central object fromthe analysis of its associated nebula. Mass-loss properties have been derived by a synergistic use of differenttechniques, at different wavelengths, to obtain high-resolution, multi-wavelength maps, tracing the differentemitting components coexisting in the stellar ejecta: the ionized/neutral gas and the dust. Evidence for asym-metric mass-loss and observational evidence of possible mutual interaction between gas and dust componentshave been observed by the comparison of mid-IR (Spitzer/IRAC, VLT/VISIR) and radio (VLA) images of thenebulae, while important information on the gas and dust composition have been derived from Spitzer/IRSspectra.1 IntroductionLuminous Blue Variables are luminous (intrinsically bright, L ∼ 104L⊙) stars, which show differentkinds of photometric and spectroscopic variabilities. They are massive (M ∼ 22−120M⊙, Meynet &Maeder 2005), characterized by intense mass-loss rates (10−6 − 10−4M⊙yr−1), which can occur alsoin the form of eruptive events. LBVs are quite rare objects in our Galaxy. This is probably connectedto their very short lifetime (some 104 yrs). The most recent census of Galactic LBVs counts 12effective members and 23 candidates (Clark et al. 2005) and a few LBV (and candidates) have beenalso reported in some nearby galaxies.LBVs represent a crucial phase in massive star evolution during which a star loses enough mass tobecome a ∼ 20M⊙ WR star. To test evolutionary models, it is extremely important to quantify a keyparameter: the total mass lost during the LBV phase, i.e. the gas (ionized, neutral, and molecular, if itexists) and the dust. Another important aspect of the study of circumstellar envelopes is to determinethe mass-loss archeology of the central star and in particular how the mass-loss behavior (multipleevents, bursts) is related to the physical parameters of the central object.“The multi-wavelength view of hot, massive stars”; 39th Liège Int. Astroph. Coll., 12-16 July 2010Bulletin de la Société Royale des Sciences de Liège, Vol. 80, 2011, p. 335 - 340Figure 1: IRAC composite image of Wray 17-96 with north up and east to the left, FOV = 5′′.Emission from the central star is evident at 3.6µm (blue), while the warm dust is well traced by the8.5µm (red)2 The projectA good understanding of the physical conditions in LBV ejecta requires multi-wavelength observa-tions, tracing the different emitting components coexisting in the stellar ejecta: the ionized/neutral gasand the dust. The study of both components provides two kinds of information: current mass-loss,via direct observations of stellar winds (the gas component), and mass loss history of the central star,by analysis of the dust component/s. The detailed knowledge of the gas and dust distribution allowsus to evaluate the total (gas+dust) mass of the nebula, the presence of different shells related to dif-ferent mass-loss episodes, and thus the total mass lost by the central object during this critical phaseof its evolution. Moreover, it could provide evidence for gas and dust mutual interactions which are apossible cause of the quite complex morphologies often observed in the LBV nebulae (LBVNs).In the last few years we have started a systematic study of a sample of Galactic LBVs and LBVcandidates aimed at deriving their mass-loss properties for a better understanding of the LBV phe-nomenon in the wider context of massive star evolution. Our approach is based on a synergistic use ofdifferent techniques, at different wavelengths, that allows us to analyze the several emitting compo-nents coexisting in the nebula. In particular, we performed a detailed comparison of mid-IR and radiomaps, with comparable spatial resolution, to sort out the spatial differences in the maps in order todetect particular features which can be associated with mass-loss during the LBV phase: asymmetricwinds versus symmetric winds in asymmetric environments; single events versus multiple events. Inthe framework of our LBV project, we obtained Spitzer/IRAC observations aimed at detecting andresolving the faint dust shells ejected from the central stars, and Spitzer/IRS observations to char-acterize the dust content of the nebula via mid-IR spectra. The ionised fraction of the nebulae hasbeen mapped via high-angular resolution radio (VLA) observations. For the more compact nebulae,images in the mid-IR have been obtained by using VLT/VISIR in the N and Q mid-IR bands.336Bulletin de la Société Royale des Sciences de Liège, Vol. 80, 2011, p. 335 - 3403 ResultsMany interesting results have been obtained from our imaging and spectroscopic program. In particu-lar, extended dusty shells have been detected around some of our targets (see Fig. 1) and evidence forasymmetric mass-loss and of possible mutual interaction between gas and dust components is sug-gested by the comparison of VLT/VISIR and VLA images of the more compact nebulae. The analysisof the mid-IR spectra has provided information on the gas and dust composition, allowing identifica-tion of the mineral composition of LBV ejecta and to discriminate between crystalline or amorphousdust components. Moreover, the presence of low-excitation atomic fine structure lines points outthe existence of a photodissociation region (PDR), an extra component of neutral/molecular gas thatshould be taken into account when one determines the total budget of mass lost by the star during itsLBV phase. We present examples of our results in the following sections. More details can be foundin a series of papers devoted to the project.Figure 2: Center: multi-configuration 6-cm VLA map (red) superimposed on the 11.26-µm VISIRmap (blue) of IRAS 18576+0341. Both maps have north up and east to the left. The center image is20′′ across. A zoomed (FOV 8′′ ) of the VLA map (left) and of the VISIR map (right) are also shown(adapted from Buemi et al. 2010).3.1 IRAS 18576+034This is the object which shows the most extreme difference between the ionized gas component(traced by free-free emission ) and the dust component (Fig. 2). High spatial resolution and highsensitivity images of IRAS 18576+0341 were obtained using the mid infrared imager VISIR at theVery Large Telescope and the Very Large Array interferometer (see Buemi et al. 2010 for details) Theapproximately circularly-symmetric, mid-IR nebula strongly contrasts with the asymmetry that char-acterizes the ionized component of the envelope, as seen in the radio and [Ne II] line images. Amongpossible scenarios for the cause of the observed asymmetry in the ionized gas morphology are anunseen external ionizing source (either a companion or shocked gas) or holes in the dusty material.However, at the moment, it is not possible to discriminate amongst them.The detailed mid-IR maps allowed us to determine the size of the dusty nebula (Ddust=7′′), thedust temperature distribution, and the total dust mass. From the total dust mass (Buemi et al. 2010),assuming a gas to dust ratio of 100, a total nebular mass of ∼ 0.5M⊙ is derived. LBVNs are believedto form from strong, eruptive episodes (Smith & Owocki 2006) even if the origin from more or lesssteady outflows cannot be ruled out (Nota et al. 1995; Voors et al. 2000). From the observed size of thedusty nebula, in the hypothesis that this material is expanding at constant velocity (70 km s−1, Clark337Bulletin de la Société Royale des Sciences de Liège, Vol. 80, 2011, p. 335 - 340et al. 2009), we derive a dynamical age of ∼ 3500 yrs and a mass-loss rate, averaged over the nebulaformation, of ∼ 1 × 10−4M⊙yr−1. Moreover, the dust distribution in the nebula is consistent with astrong mass-loss episode that occurred ∼ 2000 years ago (Buemi et al. 2010), indicating the mass-lossis not constant, with different quantity of mass released during episodes of different duration. Thisresult is corroborated by the current-day mass loss rate of 3.7 × 10−5M⊙yr−1 from the central objectas measured in the radio (Umana et al. 2005), which is smaller than the average value necessary to fillup the circumstellar nebula.3.2 HR CarHR Car is surrounded by a faint, low-excitation nebula which is difficult to observe because of thehigh luminosity of the central object. One of the most striking properties of the nebula is the completedisagreement between the large scale optical structure, showing a SE-NW bipolar morphology (Weiset al. 1997) and the inner, strongly asymmetric, radio nebula (see White 2000). Our spectroscopicSpitzer/IRS observations of the inner nebula reveal a rich mid-IR spectrum showing both solid stateand atomic gas signatures (Umana et al. 2009). The characteristic broad feature at 10µm indicates thepresence of amorphous silicates, suggesting that dust formation occurred during the LBV outburst.This is in contrast to the detection of crystalline dust in other Galactic LBVs that are probably moreevolved. The crystalline dust is similar to the dust observed in red supergiants that has been consideredto be evidence of dust production during evolutionary phases prior to the outburst (Waters et al. 1998).Strong low-excitation atomic fine structure lines such as 26.0µm [Fe II] and 34.8µm [Si II] indicate,for the first time, the presence of a PDR around this object class. While the physics and chemistry ofthe low-excitation gas appears to be dominated by photodissociation, a possible contribution due toshocks can be inferred from the evidence of gas phase Fe abundance enhancement.3.3 HD 168625Our mid-IR spectroscopic observations (IRS) of this LBV candidate detected spectral features at-tributable to polycyclic aromatic hydrocarbons (PAHs), indicating the presence of a PDR around theionized nebula. This result enlarges the number of LBV and LBV candidates where the presence ofa PDR has been confirmed, implying the importance of such a component in the budget of total masslost by the central object during this elusive phase of massive star evolution.We have analyzed and compared the mid-IR and radio maps, and derive several results concerningthe associated nebula (Umana et al. 2010). While the overall torus-like shape of the dust morphologyis confirmed, the higher resolution and sensitivity of our images allow us to discern finer details ofthe dust distribution, most notably the highly structured texture of the nebula, and provide a betterlocalization of the dust ring, with its north-west and south-east condensations (Fig. 3). There is alsoevidence for grain distribution variations across the nebula, with a predominant contribution fromlarger grains in the northern part of the nebula while PAH and smaller grains are more segregated inthe southern part.Besides via optical emission, the ionized part of the nebula can be traced by radio observations,without suffering of intrinsic extinction. We have obtained a 3.6-cm VLA map by using the interfer-ometer in two configurations to determine the structure down to sub-arcsec scale without resolvingout the more extended emission (Fig 4). The overall ionized nebula is reminiscent of the dust dis-tribution, with one main difference: the brightest radio emission is located where there is a lack ofthermal dust grains, corroborating the hypothesis of the presence of a shock in the southern portionof the nebula as consequence of the interaction of a fast outflow with the slower, expanding dustynebula. Such a shock would be a viable means for PAH production as well as for changes in the338Bulletin de la Société Royale des Sciences de Liège, Vol. 80, 2011, p. 335 - 340grain size distribution. Finally, from the detection of a central radio component, very probably as-sociated with the wind from the central massive supergiant, we derive a current mass-loss rate ofṀ = (1.46 ± 0.15) × 10−6M⊙yr−1.3.4 Future prospectsThe study of the LBV phenomenon has been hampered by the lack of a significant sample of objectswith associated nebulae. The presence of an extended, dusty circumstellar nebula can be identifiedby its IR/mid-IR fingerprints. Therefore, we can search among proposed candidates by assessing thepresence of observational characteristics that define an LBV. A good possibility is offered by the morethan 400 bubbles identified at 24µm by Mizuno et al. (2010) in the Galactic Plane survey conductedwith MIPS on the Spitzer Space telescope (MIPSGAL). These small (≤ 1′ ) rings, bubbles, disksor shells are pervasive through the entire Galactic plane in the mid-infrared. Whatever the nature ofthese 24µm sources, the implications of such a large number in the Galactic plane is remarkable andthey provide a powerful ”game reserve” for evolved massive stars as already pointed out by Wachteret al (2011).Figure 3: VISIR map of HD 168625 in the Q1continuum filter. The brightness levels rangefrom 0 to 8.06 Jy arcsec−2.Figure 4: The multi-configuration 3.6-cm VLAmap of HD 168625. The point-like centralsource, whose coordinates coincide with thoseof the central object, is probably related to thecurrent-day stellar wind of the LBV.AcknowledgementsThis research is supported in part by ASI contract I/038/08/0 “HI-GAL” and by PRIN-INAF 2007.ReferencesBuemi, C. S., Umana, G.., Trigilio, C., Leto, P., Hora, J. L., 2010, ApJ, 721, 1404Clark, J. S., Crowther, P. A., Larionov, V. M., Steele, I. A., Ritchie, B. W., Arkharov, A. A. 2009, A&A 507, 1555Clark, J. S., Larionov, V. M., Arkharov, A., 2005, A&A, 435, 239Meynet, G., Maeder, 2005, A&A, 581, 598Mizuno, D. R., Kraemer, K. E., Flagey, et al. 2010, AJ 139, 1542Nota A., Livio M., Clampin M., Schulte-Ladbeck R., 1995, ApJ, 448, 788Smith N., Owocki S. P., 2006, ApJ, 645, L45339Bulletin de la Société Royale des Sciences de Liège, Vol. 80, 2011, p. 335 - 340Umana, G.., Buemi, C. S., Trigilio, C., Leto, P., 2005, A&A , 437, L1Umana, G.., Buemi, C. S., Trigilio, C., Hora, J. L., Fazio, G. G., Leto, P. 2009, ApJ, 694, 697Umana, G.., Buemi, C. S., Trigilio, C., Leto, P., Hora, J. L., 2010, ApJ, 718, 1036Voors R. H. M. et al. 2000, A&A, 356, 501Weis, K., Duschl,W.J., Bomans, D.J., Chu, Y.H., Joner, M.D. 1997, A&A, 320, 568Wachter, S., Mauerhan, J., Van Dyk, S., Hoard, D.W., & Morris, P. 2011, in Proceedings of the 39th LiègeAstrophysical Colloquium, eds. G. Rauw, M. De Becker, Y. Nazé, J.-M. Vreux & P.M. Williams, BSRSL80, 291Waters, L. B. F. M., Morris, P. W., Voors, R. H. M., Lamers H. J. G. L. M., & Trams, N.R. 1998, AP&SS, 255, 179White, S. M., 2000, ApJ, 539, 851DiscussionF.J. Perez: Did you study the change in the structure of the dust grains, especially in the regions witha higher density?G. Umana: Not yet. But we are going to do a more detailed modeling.L. Oskinova: I would like to point out a paper by Barniske et al. (2008, A&A 486,971). The SpitzerIRS spectrum was modeled using the DUSTY code. The continuum of the IRS spectrum can be wellmodeled assuming a specific cut-off for the size of the grains.340Bulletin de la Société Royale des Sciences de Liège, Vol. 80, 2011, p. 335 - 340

The nebulae around LBVs: a multiwavelength approach

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The nebulae around LBVs: a multiwavelength approach

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