Reply to the Referee’s Report for ApJ/300877/ART
We would like to thank the referee for his/her thorough review of our
manuscript. The paper is much improved after his/her suggested
revisions.
2/25/09 2:14 PM
Referee report - ApJ/300877/ART
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Referee report
Article ID: ApJ/300877/ART
Title: Abundant Circumstellar Silica Dust and SiO Gas Created by a Hypervelocity Collision in the ~12 Myr HD172555
System
Edited Feb 25
"Abundant Circumstellar silica dust and SiO gas created by a hyper-velocity collision in the ~12 Myr HD172555 system"
by C.M. Lisse et al.
This paper reports on the analysis of the Spitzer/IRS spectrum of the young (1.2 10^7 yr) main sequence star HD172555. This star has a
quite peculiar thermal emission spectrum of circumstellar dust in that the shape of the feature at about 10 micron peaks sharply at 9.3
micron and shows additional structure at shorter wavelengths. The 10 micron spectrum is found to be very similar to that of the T Tauri star
FN Tau, and somewhat similar to that of HD23514, a young (10^8 yr) main sequence star.
The authors model the thermal spectrum and find that the peak is best represented by glassy silica (tektite and obsidian) and that the
structure at shorter wavelengths can be fitted by gas-phase SiO2. Amorphous silicates are not seen. Fitting a power-law for the grain-size
distribution and setting the maximum size of the grains to a millimeter they obtain a mass in dust of 10^21 kg. For a larger upper limit of
100m this increases to 10^22 kg. The minimum size of the grains adopted in their model is not mentioned. The temperature of the
grains is warm (~200-335 K). The origin of the silica dust and molecules is explained by a collision between two large bodies, hundreds of
kilometers in size, impacting at a high relative velocity (dv >= 10 km/sec) and that is shattered and partly vaporized in the event.
Subsequently, silica may have condensed from the melt and from gas phase SiO. A high impact velocity is thought to be needed in order
to process silicates into silica.
In comparing this event to solar-system events, they conclude that it not similar to the collision event in which the Earth's moon was formed
(to low dv), but more like an impact that stripped Mercury of its surface layers (the chemical composition of the surface layers of Mercury
seems comparable to what is derived for the dust around HD172555). An alternative hypothesis, that is not excluded by the authors,
is that the dust is produced by a stirring-up of a planetisimal belt, for instance due to planetary migration, causing (multiple?)
collisions of large bodies at high velocities. The event must have happened quite recently (< 0.1 Myr) as otherwise the small silica
particles would have been swept out of the system by radiation pressure or accreted onto the star because of the Poynting-Robertson and
stellar wind drag.
---major comments
1. Regarding the quality of the data. It seems to me that up to 6 micron the spectrum is very noisy. Molecular SiO2 produces a sharp drop
from 5-6,5 micron, and the spectrum appears to show this. However, in view of the noise I doubt whether one can really conclude this?
Please explain.
The error bars on the data, as displayed, are 2-sigma. Looking at the data, the rather noisy part
appears to our eyes to be in the 5-6 um region. Above 6 um the SNR is better than 5 per spectral
element, and with the numerous spectral elements, and a careful chi-squared fitting, the modeling
is robust. The fundamental of SiO2 emission occurs at 8 um, and has a sharp dropoff to the blue
side (in the 7.5-8 um region), and then essentially no emission strength in the 6-7.5 um region. It
is this spectral behavior that is driving the fits. The referee is correct in stating that the 5-6 um
behavior of the data, while consistent with the spectral model, is also noisy and not conclusive.
2. For the assumptions on the modeling of the thermal spectrum the authors refer to other papers of the same first author. I have checked
some of these paper, but could not find a detailed discussion of essential assumptions. Information needs to be added regarding:
Our 2008 HD113766a paper, section 3, goes into some detail about the basic tenets of the
modeling, the adoption of 7 different classes of materials (olivines, pyroxenes, water ice/gas,
amorphous carbon, carbonates, PAHs, metal sulfides, and phyllosilicates, representing the
majority dominant dust species for the most abundant atomic species H, C,N,O, Si, Mg, Fe, S).
We had been asked by the reviewer of that paper to include more detail of the modeling, and we
thought that that would be enough to use in reference in this work. Since many of the referee’s
questions relate to the modeling, we have decided instead to add to the HD172555 manuscript
the detailed discussion of the spectral modeling. Following the results of the Deep Impact and
STARDUST experiments, this section explicitly lists the species in the model, the particle size
range applicable to the fitting (0.1 – 1000 um), how each component is fitted independently to the
spectrum, so that the dust is modeled not as aggregate particles, but as a mixture of optically thin
fine particles (it is always important to remember that we would not be seeing any spectral
features unless there is a large preponderance of micron sized particles in the observed dust –
larger particles are optically thick and show little spectral behavior above blackbody), and how we
have done the minimal amount of modeling consistent with recent sample return and remote
sensing results.
- The best fit presented in Table 2 consists of quite many components (11!). Are all these species really needed? What if one is excluded;
does this result in a significantly poorer fit? (So, please perform an f-test like analysis to show that all these components are really
needed).
The fits are redone without each component and the chi-squared re-checked to see if the
goodness of fit has changed significantly (column “Model chi-squared if not included” in Table 2).
We do not list species for which the model chi-squared does not change significantly as having
been detected, although they may not be ruled out, either.)
- It is not clear to me that for all of their components the authors use measurements (of powders) to obtain extinction characteristics,
or whether they derive these from measurements of optical constants for some of the species. Please explain. In case of the former, what
l
do they assume outside of the wavelength measured? In case of the atter, what do the authors assume for the shape of the grains?
We use laboratory thermal emission spectra. As noted by the referee, Mie theory and its
assumption of spherical grain structure is not very applicable to the case of randomly oriented
astrophysical grains produced by a number of different kinetic and thermodynamic processes,
and then evolved by collisions, shocks, etc. We do not assume any spectral behavior outside the
wavelength measured.
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Homogeneous spheres? Or some other (shape) distribution. This may be relevant as homogeneous spheres produce a peak-location
that is always bluer than less symmetric particles. To illustrate a possible implication, Chen et al. 2006 use solid cristobalite spheres.
If Chen et al. would use less symmetric particles the peak of their cristobalite grains would shift to the red, perhaps producing a good
fit without the need for silica.
Agreed, and this is why we do not use Mie theory!
- The authors do not assume the grains to be part of aggregates (and representing the monomers in the aggregate), in which case it would
be very likely that the small and large monomers are in thermal contact. If the origin of the bulk of the grains released in the presumed
collision is formed by shattering it seems likely that the dust is in aggregates, and should be represented by the same temperature
(distribution). In this case, can the spectrum be fitted?
Our model assumes a run of temperature with particle size. The largest particles are optically
thick, and behave like blackbodies, and thus are set to LTE. We allow the temperature of the
smallest (0.1um) particles of each component to vary independently, and interpolate between the
2 extremes for intermediate particle sizes.
In doing this, we found that the best fit model had the temperature of all components roughly
equivalent – except for the very absorbing small amorphous carbon particles, which always “run”
hot, and the gigantic blackbody particles, which are always at LTE.
If not, it seems (again within the collision hypothesis) that the origin of the bulk of the grains is through condensation of the melt or from the
gas phase as in this case, I can imagine that individual components are more likely to be formed. So, add a short discussion of
chemically homogeneous particles vs. chemically in-homogeneous aggregates.
- A remark about nomenclature (though I'm not an expert in this!). The fact that your 'silica' is mixed with significant amounts of Al2O3
(essentially causing the pure silica peak at about 8.5 micron to shift to ~9.7 micron) to me seems to imply this is actually a glassy silicate
(i.e. not a glassy silica).
We disagree – only one of the two ‘silica’ components has any appreciable alumina component
(the tektite), and this at the 14% level. We have amorphous silicates of both olivine and pyroxene
composition in the model, and they look spectrally nothing like the amorphous silicate species.
The main peak for crystalline quartz is at ~9 um, and the main emission peak we are seeing in
our data is at 9.3 um. Alumina has emission features in the 11 – 20 um region, not the 8 – 10 um
region, that are much more muted than the silica/silicate features, and which would not shift the
emission peak to ~9.7 um. An appropriate terminology for the material may be the catch-all
“amorphous silicate of silica composition”, except that silicas and silicates are normally clearly
distinguished in the geological literature. Since the stoichiometry of the tektite and obsidian
material is close to the SiO2, and not the SiO3 or SiO4 of the silicates, and the spectral signature
of the material is clearly highly unusual for any silicate, but nor for the silicas, we find the
amorphous silica terminology to be the best.
3. The discussion takes up almost half of the paper. The authors must make this more concise, in any case by removing repetitive
statements. In my view section 4.4 and 4.5 can be joined and shortened considerably.
---Minor Comments
The English can often be improved. I will give some examples, but these are only meant as an illustration. A thorough 'clean up'
of the paper is needed.
Introduction
- If HD172555 and beta Pic have the same spectral type and class, and are both at a small distance, then indeed their B-V must
be the same.
We prefer to turn the argument around, and the fact that the B-V is the same is a confirmation of
the assignment of HD172555 to the Beta Pic moving group.
- ..HD172555 should also have a very rich circumstellar disk. Rich in what sense?
We have changed the text from “By implication, HD172555 should also have a rich circumstellar
disk. “ to read “By implication, HD172555 should also have an extensive circumstellar disk.”
- Provide a reference for '...the estimated age for the initiation of planet formation"
We have added in the references Wetherill 1990; Yin et al. 2002; Chambers 2004 to bolster this
point.
Section 2
- In finding the best Kurucz model its irrelevant to assume an age (12 Myr).
The sentence containing the Kurucz discussion has been recast as “The stellar photospheric flux
was then estimated by minimum χ2 fitting of a Kurucz stellar atmosphere models to optical and
2MASS (0.3 – 3 um) archival photometry1. See URL http:// http://nsted.ipac.caltech.edu/”
- Don't use phrases such as "we were amazed"
Agreed. We have changed this sentence to read “On the other hand, a literature search of
possible gas species produced a possible match with the fundamental ro-vibrational linear stretch
complex of the SiO molecule – it has P, Q, and R branches at 7.5 - 10 um, centered at ~8 um.”
- Alpha Tau, an O-star on the asymptotic giant branch. Alpha Tau is a K5III.
We meant to say that it is an oxygen star, not an O star as in OBAFGKM.  This has been
clarified by re-writing the sentence as follows :” Examination of the transition moment calculations
of Drira et al. (1997) and the IRAS/LRS and ISO-SWS measurements of Alpha Tau, a oxygenrich K-star on the asymptotic giant branch with a known SiO absorption feature (Cohen and
Davies 1995) demonstrated a possible fit to the residuals.”
Section 3
- "The minerals disproportionate into SiO and metal" I think you mean decompose or break-up
While disproportionate is a correct chemical term, denoting non-stoichiometric evaporation, we
like decompose better for the target audience, and have replaced ‘disproportionate’ with
‘decompose’. Thanks!
- Again: "SiO gas if disproportionating olivines..."
We have re-written this sentence to read “Despite detailed searches, there are no obvious
features of FeO or MgO mineral species (expected in addition to SiO gas if olivine or pyroxene
pyrolitic decomposition has occurred).”
- Mention minimum adopted grain size in 3.2. How sensitive is the derived mass for a_min?
The minimum grain size, particles of 0.1 µm in radius, is now explicitly mentioned: “The
best-fit
model size distribution for the HD172555 circumstellar dust producing the sharp
silica feature is dn/da = a-3.95±0.10, 0.1 < a < 1000 µm, more small particle dominated
than a purely collisional equilibrium distribution of dn/da = a-3.50 (Dohnanyi 1969). “
The derived total mass is not sensitive to the minimum grain size at all, the vast majority of the
mass (but not the surface area) resides in the largest particles.
- "... the mass in these particles is not trivial." do you mean insignificant?
We have changed this sentence to read “We note that given that the relative surface area in this
population is significant, the relative amount of mass is likely to be important as well.”
- Explain P-R
The sentence where P-R drag is first mentioned now reads “(Here recent is defined as
within the condensation lifetime of SiO gas at 5.8 AU from a L* = 9.5 A5 star, and within
the Poynting-Robertson (P-R) drag lifetime and radiation pressure blowout time for dust
a few microns in radius (Burns et al. 1979, Chen et al. 2006), or within a fraction of 1
Myr.)”
- Remove "In terms of the thermal temperature ... is ~206 K" as it doesn't really add anything (Don't you mean radiative equilibrium is
LTE?).
We have substantially re-written this section to read “This technique has worked well in
comparison to imaging measures of the dust location. E.g., we placed in the location of the
dominant emitting dust from the inner cavity wall of the HD100546 disk at 13 AU (Lisse et al.
2007a), and STIS 1-D spectroscopic measurements find a cavity for this disk of radius 13.2 AU
across (Grady et al. 2001, 2005). Our best-fit model for the HD69830 circumstellar dust ring puts
the dust at 1 AU from the K0V primary (Lisse et al. 2007b), while recent Gemini AO
measurements have shown that any warm dust in the system resides at < 2 AU from the primary
(Beichman et al. private communication 2007).
For the HD172555 circumstellar material, scaling from the T T1ejecta = 340K for the hottest Deep
Impact dust at 1.51 AU and L* = 1.0, we find for the smallest grains with Tmax = 335 K that rdust =
5.8 ± 0.6 AU. LTE for grains larger than 100 µm in radius at this distance is ~206 K. This agrees
well with Wyatt et al. 2007b's Spitzer 24/70 µm based estimate of 4-5 AU for the location of the
dust with respect to the HD172555 primary, and with our finding in the best fit of an associated
reservoir of ~200 K blackbody-like dust with no spectral signature - there are large pieces of dust
associated with the very fine grained material producing the sharp spectral features. A location of
5.8 AU for the circumstellar material is also consistent with the location of the innermost dust belt
surrounding b Pic (6.4 AU from the star), as determined by Okamato et al. (2004). The equivalent
location in our solar system is at ~1.9 AU from the Sun, or between the orbit of Mars and the
inner edge of the asteroid belt.”
Section 4
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- "an asteroid of radius >= 440 km radius asteroid" remove 'radius asteroid'
Done.
- 'oligarch' sized planetesimal. This is to my knowledge not a common phrase. If I'm right, please remove it from the text
(also in the abstract) as it took me until page 18 before I understood its meaning.
The term “oligarch” is becoming more and more common in the terrestrial planet formation
community. It is denotes the few planetisimals that win the accretion game enough to rise to
Ceres to Mars size, and come to dominate the behavior of all planetisimals in the latter part of the
planet formation process. We prefer to keep this terminology.
"undense circumstellar torus" undense is not an English word.
we estimate the time scale for SiO
recondensation to occur via 2-body collisions in an circumstellar torus in the
molecular flow regime to be ≤ 0.1 Myr.”
We have removed the term “undense” and changed the text to read “…