Winds of Wolf−Rayet stars: phenomenology of mass loss
Population I Wolf–Rayet (WR) stars are the evolved descendants of massive (M ≥ 25M) O-type stars. The dense, fast radiatively-drivenWR winds efficiently hide all vital information about basic characteristics of the stellar cores. Observing binary systems with WR components, we put firm limits on the...
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irk-123456789-794042015-04-01T03:02:45Z Winds of Wolf−Rayet stars: phenomenology of mass loss Marchenko, S.V. Plenary Sessions Population I Wolf–Rayet (WR) stars are the evolved descendants of massive (M ≥ 25M) O-type stars. The dense, fast radiatively-drivenWR winds efficiently hide all vital information about basic characteristics of the stellar cores. Observing binary systems with WR components, we put firm limits on the sizes and luminosities ofWR stars. In attempt to understand dynamics of the outflows, we study the micro-structure of WR winds, finding that they are composed from numerous dense clumps. The growing evidence that WR stars may be linked (collapsars) to the long Gamma Ray Bursters raises a question about rotation rates of WR stars. First results based on the observations of globally-structured winds of some WR stars show that they may be considered as moderately fast rotators. There is one more fascinating feature of the WR mass loss: some carbon-reach WR stars may form dust, thus placing them among the first prodigious dust-producers in the early Universe. Though we are yet to find how dust is formed in the extremely hostile environment of WR winds, we have made substantial progress over the past decade. Recent high spatial resolution near/mid-infrared imaging at the HST, Keck and Gemini, combined with abundant optical/UV spectroscopy and photometry, allows to map rapidly changing environments of the dust-forming regions and derive some basic properties of the freshly formed dust. 2005 Article Winds of Wolf−Rayet stars: phenomenology of mass loss / S.V. Marchenko // Кинематика и физика небесных тел. — 2005. — Т. 21, № 5-додаток. — С. 23-29. — Бібліогр.: 54 назв. — англ. 0233-7665 http://dspace.nbuv.gov.ua/handle/123456789/79404 en Кинематика и физика небесных тел Головна астрономічна обсерваторія НАН України |
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Plenary Sessions Plenary Sessions Marchenko, S.V. Winds of Wolf−Rayet stars: phenomenology of mass loss Кинематика и физика небесных тел |
| description |
Population I Wolf–Rayet (WR) stars are the evolved descendants of massive (M ≥ 25M) O-type stars. The dense, fast radiatively-drivenWR winds efficiently hide all vital information about basic characteristics of the stellar cores. Observing binary systems with WR components, we put firm limits on the sizes and luminosities ofWR stars. In attempt to understand dynamics of the outflows, we study the micro-structure of WR winds, finding that they are composed from numerous dense clumps. The growing evidence that WR stars may be linked (collapsars) to the long Gamma Ray Bursters raises a question about rotation rates of WR stars. First results based on the observations of globally-structured winds of some WR stars show that they may be considered as moderately fast rotators. There is one more fascinating feature of the WR mass loss: some carbon-reach WR stars may form dust, thus placing them among the first prodigious dust-producers in the early Universe. Though we are yet to find how dust is formed in the extremely hostile environment of WR winds, we have made substantial progress over the past decade. Recent high spatial resolution near/mid-infrared imaging at the HST, Keck and Gemini, combined with abundant optical/UV spectroscopy and photometry, allows to map rapidly changing environments of the dust-forming regions and derive some basic properties of the freshly formed dust. |
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Article |
| author |
Marchenko, S.V. |
| author_facet |
Marchenko, S.V. |
| author_sort |
Marchenko, S.V. |
| title |
Winds of Wolf−Rayet stars: phenomenology of mass loss |
| title_short |
Winds of Wolf−Rayet stars: phenomenology of mass loss |
| title_full |
Winds of Wolf−Rayet stars: phenomenology of mass loss |
| title_fullStr |
Winds of Wolf−Rayet stars: phenomenology of mass loss |
| title_full_unstemmed |
Winds of Wolf−Rayet stars: phenomenology of mass loss |
| title_sort |
winds of wolf−rayet stars: phenomenology of mass loss |
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Головна астрономічна обсерваторія НАН України |
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2005 |
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Plenary Sessions |
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http://dspace.nbuv.gov.ua/handle/123456789/79404 |
| citation_txt |
Winds of Wolf−Rayet stars: phenomenology of mass loss / S.V. Marchenko // Кинематика и физика небесных тел. — 2005. — Т. 21, № 5-додаток. — С. 23-29. — Бібліогр.: 54 назв. — англ. |
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Кинематика и физика небесных тел |
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AT marchenkosv windsofwolfrayetstarsphenomenologyofmassloss |
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2025-07-06T03:27:53Z |
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2025-07-06T03:27:53Z |
| _version_ |
1836866573737918464 |
| fulltext |
WINDS OF WOLF–RAYET STARS: PHENOMENOLOGY OF MASS LOSS
S. V. Marchenko
Department of Physics and Astronomy, Western Kentucky University
1 Big Red Way, Bowling Green, KY 42101–3576, USA
e-mail: sergey.marchenko@wku.edu
Population I Wolf–Rayet (WR) stars are the evolved descendants of massive (M � 25M�) O-type
stars. The dense, fast radiatively-driven WR winds efficiently hide all vital information about basic
characteristics of the stellar cores. Observing binary systems with WR components, we put firm
limits on the sizes and luminosities of WR stars. In attempt to understand dynamics of the outflows,
we study the micro-structure of WR winds, finding that they are composed from numerous dense
clumps. The growing evidence that WR stars may be linked (collapsars) to the long Gamma Ray
Bursters raises a question about rotation rates of WR stars. First results based on the observations
of globally-structured winds of some WR stars show that they may be considered as moderately
fast rotators. There is one more fascinating feature of the WR mass loss: some carbon-reach
WR stars may form dust, thus placing them among the first prodigious dust-producers in the early
Universe. Though we are yet to find how dust is formed in the extremely hostile environment of
WR winds, we have made substantial progress over the past decade. Recent high spatial resolution
near/mid-infrared imaging at the HST, Keck and Gemini, combined with abundant optical/UV
spectroscopy and photometry, allows to map rapidly changing environments of the dust-forming
regions and derive some basic properties of the freshly formed dust.
INTRODUCTION
Discovered in 1867 by C. J. E. Wolf and G. Rayet, the Population I Wolf–Rayet stars form a rather peculiar
class of massive stars whose spectra are completely dominated by broad, intensive emission lines. A gradual
improvement of models of stellar evolution helped to realize that the Population I WR stars represent a final
evolutionary stage of the massive (M ≥ 25M� for a nearly solar metallicity) O-type stars (see [28] and references
therein). WR stars consist of the hot H- (CNO cycle) or He-burning cores that drive very strong winds, making it
generally impossible to see the stellar surface (average mass-loss rates Ṁ ∼ 10−5M�yr−1 and terminal velocities
v∞ ∼ 1000 – 4000 km s−1). Due to the extremely high mass loss rates coupled with high wind velocities, WR stars
represent a substantial, if not leading, component affecting the “ecology” and structure of the interstellar
medium, as well as star formation in galaxies (including what one now calls “Wolf–Rayet galaxies” [7]). WR stars
are considered as unequivocal “fiducial marks” of the starburst regions. The combination of high luminosity and
sensitivity to Z (relative heavy metal content) makes WR stars excellent tracers of the metallicity gradients in
distant galaxies. Rapid evolution of WR stars places them on a verge of exploding as Supernovae (hypernovae),
providing unique information about the very final moments of massive star evolution. The importance of
this pre-SN stage is highlighted by the recently established link between WR stars and Gamma-Ray Bursts
(e.g., [38]).
BASIC PARAMETERS OF WR STARS
Population I WR stars are divided into three broad and, probably, evolutionary-successive spectral classes: WN,
WC, and WO which are based on appearance of particular emission lines in their spectra (see [19] and references
therein). WN stars exhibit the predominance of emission lines of helium and nitrogen, with some presence of
carbon, silicon and hydrogen. Spectra of WC stars are dominated by carbon and helium, with an absolute
absence of hydrogen. The rare WO class is similar to WC, except that oxygen is seen much more clearly. Each
broad class is further sub-divided into subclasses, based on the overall ionization of WR wind. Hence, WN2 and
WN11 would define the WN stars of the highest and lowest ionization (effective temperature of the stellar core),
respectively. The dense, fast radiatively-driven WR winds efficiently hide all vital information about the basic
characteristics (size, temperature, luminosity, chemical composition) of the stellar (hydrostatic) cores. Hence,
the zone which may have some resemblance of a main-sequence star photosphere, must be placed in a rather
c© S. V. Marchenko, 2004
23
arbitrarily chosen region of the outmoving envelope of WR star. This makes direct estimation of the basic
parameters difficult, if possible at all.
Membership of some WR stars in open clusters and associations allows to put rather loose limits on the ab-
solute magnitudes [19]: Mv = −2.4 to –7.2 for the WN class, with the lowest Mv corresponding to WN2;
Mv = −3.3 to –4.6 for the WC class; Mv = −2.8 for WO. The terminal velocities of the WR winds are, probably,
the best established values, as they are measured directly from the UV, optical and IR spectra [43] with a typical
accuracy of 10%–20%. They span a broad range: from v∞ ∼ 500km s−1 for WN11 stars to v∞ ∼ 4500km s−1 for
the WO class. The mass-loss rates come from a wide variety of approaches: from the model-dependent estimates
based on radio fluxes (e.g., [23] and the “standard model” calculations (see below)), to the most direct values
provided by binary systems with WR components [50]. All the observed and theoretically estimated mass-loss
rates are rather extreme, sometimes in excess of Ṁ ∼ 10−5 M� yr−1, which is quite sufficient to alter evolu-
tion of a massive star. The scarce information about the masses of WR stars comes from binary systems [19]:
MWR = 2–55M�.
As we turn to the temperatures of WR winds and their wind-driving cores, we must call for a help of
the so-called “standard model” of WR stars (e.g., [14, 17]). The model solves the transfer equation in the co-
moving frame subject to a statistical and radiative equilibrium, assuming an expanding, spherically-symmetric,
homogeneous WR atmosphere. The stellar radius (R∗) is defined as the inner boundary of the model atmosphere
and is located at the Rosseland optical depth of about 20 with the stellar temperature (T∗) defined by the usual
Stefan–Boltzmann relation. The “standard model” does not solve the momentum equation, so that a density
or velocity structure is required. For the supersonic part, the velocity is parameterized with a classical β-type
law of the form [22]:
v = v0 + (v∞ − v0)(1 − R∗
r
)β ,
with R∗ = 1 and v∞ = 1, β = 1–3 and v0 ≤ 0.5v∞ [31] providing reasonably good fits to the observed WR
emission lines in the optical and UV. This [supersonic] velocity field defines the density profile of the expan-
ding wind which is ultimately connected to a hydrostatic density structure at a depth, such that the velocity
and velocity gradient match at the interface. The subsonic velocity structure is usually set by corresponding
hydrostatic models. This pre-specified wind velocity profile could be the largest deficiency of the “standard
model”. Indeed, while being able to provide an excellent fit to the UV–IR spectral energy distribution and
reasonably well match the shapes of the emission parts of the profiles (Fig. 1, adopted from [34]), the model
fails the most critical test, being unable to fit the absorption components of the P Cygni profiles. Can we put
any observational restrictions on the otherwise unknown wind velocity law? It is a difficult, if not impossible,
task for a single WR star. However, in a favourably oriented binary one could be able to perform an analysis
of atmospheric eclipses, combining the resonance lines in the UV (large distances from the core) and the
subordinate optical transitions (compact and intermediate-size line formation zones – see some examples in [2]).
For now, only rough constraints can be placed on the velocity law in the case of the best studied binary V444
Cyg (Fig. 2, adopted from [29]). The traditional β-velocity law provides good fits to some optical lines in V444
Cyg (mainly formed at r � a/2, i.e., at a half-orbital separation). However, there is a systematic and increasing
with distance deviation from the β law at r � 3/4a, indicating that a significant and extended wind acceleration
may occur at large, r > 10RWR, distances from the WR core.
What is about the WR radii? Estimates of the core radius R∗ and temperature T∗ require an extrapolation
using a v(r) wind expansion law, as well as knowledge of the physics of the optically thick inner WR wind.
Both domains remain practically unexplored. However, it is obvious that stars in binaries cannot be larger than
the space available for them. Applying this simple principle to Wolf–Rayet binaries in which the constraints
are most severe, i.e., to systems with the shortest periods and smallest separations, one may obtain rather
strict upper limits for the WR radii, in general RWR � 10R� [39], in line with the independent estimates
obtained by modelling the light curves of eclipsing binary systems with WR components [3]. These alternative
values were substantially smaller than the core radii predicted by earlier versions of the “standard model”
(e.g., [16, 21]). Incorporation of heavier chemical elements and higher ionization stages in the line lists used in
radiation transfer algorithms, as well as a self-consistent treatment of line blanketing in a stellar wind has led
to a gradual convergence of the theoretical and observational results [9].
SMALL-SCALE STRUCTURING OF WR WINDS
All adequately (good time and spectral coverage, high S/N and spectral resolution) observed WR stars show
outwardly-moving, numerous emission “spikes” on the tops of much broader emission profiles. They were in-
terpreted as arising from overdense, small-scale structures embedded into a rapidly accelerating and outmoving
24
Figure 1. Synthetic spectrum of the resumably single star WR3. Top panel: Spectroscopic comparison between IUE
spectrophotometry, ubvr photometry and 2MASS JHK photometry for WR3 and synthetic spectrum (dotted) reddened
by E(B − V ) = 0.4 mag. Inset is the rectified IUE spectrum, together with the synthetic spectrum (dotted), degraded
to the resolution of SWP/LORES and includes the correction for atomic Lyα with log N(H i)=21.5 cm−2. Other panels
compare the rectified optical and near-IR observations with our synthetic model (dotted)
25
Figure 2. V444 Cyg: thick line − observationally imposed limits on the WR wind velocity law (spectroscopy); dotted
line − β = 1.2 law ([31]; profile fitting); dashed line − [1], light curve solution; dotted-dashed lines − [48], theory
WR wind. So far, the best theoretical explanation relates the rapid rise of these small-scale structures to a spe-
cific line-driven instability inherent to any line-driven wind [45]. Numerous direct observations and attempts
at their interpretation [24, 46] allow one to draw the following “collective portrait” of the small-scale inho-
mogeneities in a WR wind [30]. Within a given wind volume, R∗ <r < 10R∗, there might be ∼ 102 relatively
large, rcl � R�, probably optically thick clumps of a total mass of � 5% of the ambient wind with a density
contrast of > 100. Such density fluctuations are common in numerical simulations [13]. One may assume
that roughly the same proportion of large clumps survives to reach much larger distances from the star [47].
The large, massive clumps may accelerate at a much slower pace, thus seemingly defying the wind velocity laws
traditionally accepted in modelling. The probable hierarchy of clump sizes results in much more numerous (at
least 104 [25]), small and optically thin clumps, presumably forming the bulk of the wind. One may only guess
about the kinematics of this populace.
Once included in the framework of the “standard WR model”, the micro-structuring (clumping) of the WR
winds allowed to: (i) produce much better fits of the emission line profiles (especially in the red-shifted electron-
scattering wings [15]), (ii) bring theoretical spectral energy distributions closer to the observed infrared and radio
fluxes [44]. Probably, the most important issue is the resulting factor 2–5 downward revision of the mass-loss
rates in the structured WR winds [15, 18]. This profoundly influences the evolution of massive stars, as the mass
loss, along with the initial mass/chemical composition and rotational rate, completely controls evolutionary
pathways of massive stars [28]. There is another important consequence of the micro-structured WR wind.
Some of WR stars are known to be prodigious dust formers (see below). All the channels of dust formation
are deemed to be inoperative unless WR winds are highly structured, thus providing vitally important density
enhancements and shielding from the abundant UV photons [4]. Shielding implies a substantial optical depth
of the inhomogeneities. Assuming that dust is produced in the optically thick part of the wind (encompassing
� 0.05Ṁ), one finds a good correspondence with the overall efficiency of the WR dust formation [54].
MACRO-STRUCTURES IN WR WINDS AND ROTATION OF WR STARS
It has recently been demonstrated that, although the mass and mass-loss rate are still the determining factors
for evolution in the upper H–R diagram, rotation is an equally important parameter [27]. Although the rotation
periods of O stars, predecessors of the WR phase, are relatively well-known, very few measurements have been
made for their WR descendants. Based on the width of absorption lines (which are rarely present in WR spectra),
a value of v sin i ∼ 500 km s−1 was claimed for WR138 by [36] and of v sin i ∼ 150–200 km s−1 for WR3 by [37].
However, for stars with strong winds, one can never be sure that the widths of absorption lines are not dominated
by other mechanisms such as turbulence or wind expansion. Indeed, our recent investigation shows that WR3
is likely a single WR star with the turbulence-broadened absorption features arising in a WR wind [34]. But
there is an indirect evidence that at least some WR stars may be fast rotators. Currently, favoured model for
26
the long Gamma Ray Bursters is the collapse of a rapidly rotating massive star [26]. On the other hand, model
predictions of [27] are that WR stars should be extremely slow rotators (� 50 km s−1), since most of the angular
momentum is carried away by the high mass-loss rate before and during the WR evolutionary phase.
Is there any possibility to gain important information about the rotational velocities of WR stars? The winds
of WR stars have been demonstrated to be highly variable. In particular, one type of structure in the wind that
may account for some of these variations, Co-rotating Interaction Regions (CIRs), are thought to be closely
linked to the rotation of the star. The widely applicable model explains drifting density enhancements as arising
from co-rotating interacting regions, i.e., regions of interaction of a slow, “overloaded” wind with a relatively
faster “normal” outflow, gradually being brought into the line of sight by stellar rotation [8] in a form of
rotating spiral-like structure. These global, large-scale density fluctuations are thought, by consensus, to be
driven from a photosphere in the case of OB stars, or are caused by [similar: magnetic field and/or pulsations?]
perturbations at the base of WR wind. The initial perturbation rapidly propagates through the wind, being
gradually carried by rotation, thus generating a spiral-like structure in the density distribution which leads to
a very characteristic, large-scale periodic variability pattern in the emission lines of a WR star. Hence, CIRs
may be the only way to gain access to rotational periods of WR stars.
In the past decade, two clear cases of this phenomenon have been identified through repeated spectroscopic
observations. WR6 ([49]: P =3.76 days) and WR134 ([42]: P = 2.25 days) show periodic variations without any
indication of a companion. It is generally accepted that the detected periods are the rotation periods of the stars.
The periods were found to be independent of distance in the wind, indicating that the CIRs do not suffer from
differential rotation but rather enjoy “solid wind” rotation and, therefore, provide a direct measurement of
the rotation period of the underlying star. One more example comes from the recently concluded observational
campaign on the WN8 star WR123 where we find a dominant (both in the photometric and spectral data)
P ∼ 10h. For typical radii of WR stars, we find equatorial rotation speeds of ∼ 40–90 km s−1. Hence, WR stars
may be considered as moderately fast rotators.
THE WOLF–RAYET “DUSTARS”
There is one more fascinating feature of the WR mass loss: some carbon-reach WR stars (the ones which
belong to the WC spectral class) may form dust. Even though present-epoch dust production output for all
Galactic WC stars is � 1% of the total Galactic rate [5, 11], the dust-generating WC stars are regarded as
outstanding for three main reasons: (i) The absolute rate of dust production is extraordinarily high, reaching
Ṁ ∼ 10−6M� yr−1 [20, 52]. (ii) The dust is formed in a hot, extremely hostile environment, posing a formidable
theoretical problem. (iii) In the early (age ∼1 Byr) Universe, WR stars could be very common, but unique
sources of dust, along with subsequent dust-generating SN events, since WR stars evolve much more rapidly
than any lower-mass stars, commonly associated with dust production in the “modern” Universe. It is not clear
how much (and what kind of) dust can be produced in a SN explosion [12]. However, it is quite clear that
the copious amounts of the carbon-rich dust produced in the WR winds may survive for at least ∼ 102 years [35],
thus effectively reaching (and enriching) the ISM.
Two basic processes of dust formation prevail among the WC stars:
(i) “single” channel: constant, sustained formation in single WC stars, only of the coolest (WC9, 10 and some
WC8) subtypes. The IR emission excess, arising from re-radiation of stellar UV photons by the hot dust and
superposed on an underlying hotter stellar emission component, is in the form of a nearly black-body radiation
at Td ∼ 1000–1600 K from a shell with inner critical diameter 0.5÷1.5 · 103R� (0.5÷1.5 · 104R�). Presumably,
the winds of hotter single WC stars are too rarefied to form dust at a distance where the UV radiation has
dropped sufficiently to allow dust formation to occur. In any case, even in cool WC stars, a smooth wind flow
will not form dust; clumping is required for an efficient grain growth [4].
(ii) “binary” channel: episodic formation in binary WC + O systems with eccentric orbits. The key factor here is
the compression by wind-wind collision involving H-rich material from the O-star and the C-rich WC wind. This
allows dust formation to occur, which is dramatically enhanced at each periastron passage. Among the some
seven systems in which episodic dust formation has been detected so far [52], all have (confirmed or suspected)
long periods of several years, with no preference for hot- or cool-type WC stars. Presumably, the dust is
formed relatively far downstream along the shock interface, where the temperature has fallen sufficiently from
the initially extremely high values of 106−7 K. The shock cone wraps around the weaker-wind O-star, so that
IR dust emission should arise in a preferred direction far beyond the O-star, as seen from the WR star.
Hot (T ≤ 1500 K) circumstellar WR dust was only recently spatially resolved around some WC +O binaries
[33, 40, 41, 51]. All the above-cited near-IR observations have targeted the hottest dust only. The apparent sub-
arcsecond sizes of the barely resolved hot dust regions made next to impossible any direct application of quan-
titative models. The first mid-IR (λλ 8–18μm) images of the spatially-resolved dust cloud around WR112 [35]
27
provided data on: (a) the temperature profile in the envelope, proving that, as anticipated, the temperature
follows a thermal equilibrium profile; (b) the characteristic size and chemical composition of the dust grains,
finding amorphous carbon as a main constituent of dust particles of a ∼ 0.5μm characteristic size; (c) the ab-
solute rates of dust formation, up to Ṁdust ∼ 6 · 10−7 M� yr−1. Finishing our mid-IR survey in 2004, we find
two more spectacular dust envelopes around Wolf–Rayet binaries WR48a and WR140. The latter example is
the most instructive. Indeed, this particular long-period (P = 7.93 yr), highly eccentric (e= 0.881: [32]) binary
serves as a prototype for studies of wind-wind collision phenomena in massive binaries. As an indicator of its
importance, the last periastron passage in 2001 was followed by dozens of astronomical facilities, from X-ray
to near-IR. Our recent high-quality λ 12.3μm images show concentric dust arcs around WR140 which can be
unequivocally related to the 1993 and 2001 dust formation episodes, thus helping to understand dynamics of
dust formation.
The obvious next step is to obtain high-quality mid-IR spatially-resolved spectra of the carbon-based dust
clouds. This may answer a broad range of important and fundamental questions: Are there any discernible
spectral features which may point to a specific grain material? – different modifications of amorphous carbon-
based dust produce distinctly different mid-IR spectra [6, 53]. Are there any traces of polycyclic aromatic
hydrocarbons (e.g., [10])? Are there any signs of gradual chemical evolution of dust? How fast is “aging”
(annealing? chemical evolution?) of the dust in the presence of intense radiation field? May the wind-embedded
shocks change dust properties?
Acknowledgements. The work was supported by the Projects of Distinction Fund of the Western Kentucky
University.
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