From: Chris Lidman (clidman@eso.org)
Date: Sat Nov 13 2004 - 09:51:02 PST
Hi Gabriele,
I've read through the first part of your evolution paper and
I've made some minor corrections to the LaTex source. I've
also made some comments, see the attached files called Notes.txt, on
how the text could be made clearer.
I'll go through the rest of the paper this week.
Cheers, Chris.
PS Have you had any news from Gaston?
-- European Southern Observatory Alonso de Córdova 3107, Vitacura Casilla 19001, Santiago 19 CHILEPh. +56 2 463 3106 FAX +56 2 463 3001
General Comments
================
I'm now mostly satisfied with the scientific content; however, the
text needs work. If you wish, we can iterate. I am in Santiago all
next week, so we can work on this together.
I've edited the text until the beginning of section 3.2. I will wait
your reply to my suggestions before continuing further.
All my changes are in bold. For the moment, I've mostly limited myself
to grammatical corrections. In some case, I've rearranged the words
in, but I've tried to keep the meaning unchanged. You should read my
modifications and these notes carefully.
There are some parts that need to be re-written. In these cases, I
have not modified the text, but I provide suggestions on how it
could be improved and in some cases I provide examples.
If I disagree with something in the text, I leave it untouched, but I
add a comment to these notes.
Other General Comments applicable to the whole text
===================================================
An alternative to using "equivalent width-like measurements" is to use the
shorter term "pseudo-equivalent widths"
There are a few "motherhood" statements. These sentences are sweeping
statements that are generally true, but do not add much to the paper.
I note them here, but I have not deleted them in the text. You should
consider deleteing them.
In some cases, you use 1991T/1999aa-like SNe and 1991T-like SNe. It would
be better to stick to one of these descriptions, otherwise the reader
might get confused. The same goes for 1991bg/1986G-like versus 1991bg-like
descriptions.
Introduction
============
I think you need to work on the introduction. The logical flow from
one sentence to another and one paragraph to another is not as good as
it could be.
The term "their brightness homogeneity" in the first sentence is bit
heavy. I think that you could delete it and repharse the sentence to.
"Precision measurements of cosmological parameters using high-redshift
SNe~Ia as distance indicators assumes that the shape-brightness
corrections {\bf that are} derived from local samples are applicable
to high-redshift SNe~Ia"
Then, before launching into the next paragraph, you might want to
expand this point in the first paragraph with the arguments that
are currently used to support this assumption. Some of these
points are addressed in paragraph 3 and could be moved here.
- Greg's rise time paper
- Gerson's comparison of the light curves
- The range of environments in which low redshift SNe are discovered is
very broad. One can safely asumme that high redshift SNe occur within a
similar range of environments.
Then, one could slightly rephrase the 2nd paragraph to
"The spectral energy distribution provides an alternative avenue for
testing this assumption. The average metallicity of the Universe
increases with cosmic time, so it is not unreasonable to expect that
high redshift SNe~Ia are in environments that have lower
metallicities. The effect on the spectral energy distribution of a
lower metallicty progenitor has been modelled by
\citet{1998ApJ...495..617H} and \citet{2000ApJ...530..966L}. These
studies find that such SNe~Ia are expected to show enhanced flux in
the UV, weaker absorption features in the optical and a shift in
the minima of optical features to redder wavelengths."
Then paragraph 3 can be shortened since part of it has been
to paragraph 1.
Motherhood statement: The physical origin of these differences and
their possible drift as function of redshift have to be investigated.
The phrase "without conclusive results" is too strong. In all the work
that has been done to date (and here you might want to mention Gerson's
work on comparing low and high z lightcurves), the conclusion is are
not inconclusive. There does not seem to be a detectable difference between the
lightcurves of high and low z SNe. This does not mean that we will not
find differences in the future and it does not mean that the width-magnitude
relations that we use are valid.
"Possible systematic differences ..." Is it fair to say that the differences
are possible? I though that this fact is now well established.
Instead of using the term "sub-group", it may be better to use the
term "sub-type". The type is Ia and the sub-type is one of normal /
91T/99aa-like / 86G/91bg-like. I note that you do use the term sub-type
in a couple of other paragraphs.
Be caeful of the phrase
"and they are sometimes called SN~1999aa-like supernovae"
How commonly used is the term SN1999aa-like? If this is only used by the
SCP, then we cannot use the adjective "commonly".
Should we mention the real wierdos which do not fall into one of the
three sub-types?
The following sentence is too sweeping.
"The identification of SN~1991T/SN~1999aa-like supernovae at
high-redshift is therefore an important step in securing the use of
SNe~Ia for cosmology."
Try
"The identification of SN~1991T/SN~1999aa-like supernovae at
high-redshift is an important step in demonstrating that the range
of SNe~Ia that are observed at low redshifts is also observed at
high redshifts."
You might want to remove information from the second last paragraph
which is either in the abstract or later in the paper. Here
is a suggestion on how to shorten and merge the last and 2nd last
paragraphs.
"During 2000, 2001 and 2002, the Supernova Cosmology Project (SCP)
obtained the spectra of 20 high redshift SNe~Ia with FORS2 on the ESO Very
Large Telescope \citep{Lidman}. In this paper, we analyze the 14
spectra with the best signal-to-noise ratios and we perform a
quantitative comparison between these spectra and the spectra of
low-redshift SNe. We also develop approach to identify SN~Ia sub-types
at $z$ $\sim$ 0.5. The paper is organized as follows. The dataset and
identification scheme are presented in section \ref{data}. The
comparison of high- and low-redshift SNe are presented in section
\ref{measurements} together with a statistical analysis of the
results."
Section 2.1
===========
Which SNe in table 1 use spectrally determined dates and which ones use
lightcurve dates. I guess that the ones with an error of 1 day have been
determined from the lightcurve and those with an error of 2 days are
from the spectrum?
If space is an issue, Table 1 can be simplified. The instrument, the setup, the
telescope and the exposure time are already reported in Lidman et al.
Figure 1 is now much better. It looks a bit crowded. Have you though
about putting all on the same plot, i.e. one set of axes. The names
should have the standard IAU nomenclature, i.e. SN 2001go instead of
sn01go.
I do not think it is necessary to mention IRAF here, so I deleted
the sentence. However, some intermediate sentence is required,
so I added
"A full description of the observations and the data reduction
are given in Lidman et al."
Section 2.2
===========
It is probably too early to descibe the technique that we use as the
standard technique.
The following sub-sentence
"the identification often relies on a visual inspection of the spectra."
is true, but it is not something that is addition to the "standard
technique", it is part of it. Hence I've rephased this sentence and the
previous one.
You use the term 'photospheric SN~Ia". What does this mean. Wouldn't SN~Ia
be enough?
Table 2. You descibe the SII/W feature in Type Ib/c as absent/narrow. I guess
this should be absent, since an absent feature cannot be narrow.
Table 2. Footnote c is missing. Footnotes h,e,f,g do not need a separate
column and can be attached to the wavelength region. Instead of a,b, use
a+b, or create a new footnote (before and and around maximum light.
I did not understand the text to footnote f. In footnote i, can you sepecify
the Hydrogen line - I presume it is H-beta.
max. can be expanded to maximum light.
Section 2.3
===========
Figure 2. I've send you a spectrum of 2002fd without the telluric features.
Figure 2. If you mark telluric absorption, you should also mark the
one that starts at 9300 Angstroms.
The sentence "The increasing sample of supernova discovered in the
local universe tend to show that normal and peculiar slow-decliner SNe
may not be two different classes of objects but could form a
`continuum'." seems out of place in this paragraph.
If 91T/99aa SNe are hard to classify, is it not more likely that the
true fraction as been understimated rather than overestimated?
The following sentence tries to address a couple of issues.
"The expected number could be biased by different SNe search
techniques and by the strengthening evidence that spectroscopically
SN~1991T/SN~1999aa-like SNe do not always come with broad light curves
and vice-versa, see for example SN~1999ee, SN~2002cx or SN~1999aw"
It would be better to split this sentence into several.
"The precise number depends on the details of the methods that are
used in the search. Many SN~1991T/SN~1999aa-like SNe may have already
been observed at high redshift but not clearly identified because the
signal-to-noise ratio in the spectrum was too low or the lightcurve
was not exceptionally broad. It is becoming clearer that
SN~1991T/SN~1999aa-like SNe do not always have broad light curves
and vice-versa, see for example SN~1999ee, SN~2002cx or SN~1999aw"
You may want to add a couple of references at this point.
Section 2.4
===========
This section is very short and we don't seem to say much. Is it it
worth including in the paper? The first and last spectra are very
noisy.
Figure 3. Use the full IAU name in the figure, i.e. SN 2001go and not
sn01go.
Is the host spectrum fixed between epochs 5.6 and 29.5?
Section 3.1
===========
The horizontal bars in Fig. 4 correspond to errors of +/- 2 days. Some
of these should be +/- 1 day as recorded in table 1.
I've reordered the paragraphs. First paragraph is on normals, second
is on perculiars and later paragraphs is on high z.
You might want to replace the following sentence
"In Fig.~\ref{ca_vel} the solid line indicates the average trend for
Ca~{\sc ii}~H\&K time evolution computed using a large data set of
local normal type Ia supernovae. The gray band shows the {\it rms} of the
data set around the empirical model."
with
"The mean trend for normal SNe~Ia, from 10 days before maximum light
to 40 days after maximum light, is shown in Fig.~\ref{ca_vel}. The
shaded area represents the dispersion (one standard deviation about
the mean). The trend and the dispersion have been computed from a large
sample of nearby supernova." ... and you might want to say a bit more
about this large nearby sample.
The following sentence is a bit vague.
"Assuming that intrinsically low-luminosity high-z SNe are similar to
those in the local universe, measured expansion velocities values on
the lower edge of the distribution shown in Fig.~\ref{ca_vel}, would
suggest a possible under-luminous SN."
You discuss the velocity of the CaII feature in low luminosity
objects, but you have not mentioned the velocity of 91T like objects. Is
the velocity of the ejecta in these objects above the mean line of
figure 4? This might be an interesting point to mention. I note that
2002fd lies below the band.
Were the velocities for the low and high z data measuredin the same way?
If this is true, you might want to point this out in the text.
I've added the following sentence. I am not sure if it accurately
descibes what you have done.
"The line profile is modelled with a Gaussian and the contiuum over the
width of the line is modelled as a straight line."
Section 4.2
===========
In figures 5,6 and 7, you have not added horizontal bars.
References
==========
There is no need to list all the names.
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