From: Chris Lidman (clidman@eso.org)
Date: Wed Oct 08 2003 - 07:40:15 PDT
Hi Gaston,
I have read through the paper and here are comments. We can discuss
these during Fridays phone meeting.
Cheers, Chris.
Comments on Gaston's Paper - version 02/10/2003
General Comments
================
The definition of the spectral features is now clear and unambiguous.
The motivations are better explained. However, they are spread out
through the introduction. This could still be improved.
The figures are in black and white. There are 3 SNe groups
(over-luminous, under-luminous and normals), yet I see 4 shading schemes
- black, dark grey, light grey and open. This makes it much more
difficult to interpret the graphs. I suggest that you use a clearer
colouring and shading scheme, as had been used in earlier versions of
this paper.
The following combination may be more suitable.
- 91bg-like SNe - blue, solid and lightly shaded
- Normal - green and solid
- 91T-like SNe - red and open
In this way, it will be much easier for the reader to see how the
families of different SNe are grouped with respect to the eachother.
In the caption to each figure, it may be useful to explicitly state
the differences that are noted in the text. Ie. Under-luminous SNe lie
above and 91T like SNe lie above. People usually look at the figures
first and then read the text later.
The paper presents three correlations between absolute luminosity and
spectral indicator: R(Si II)_EW, t_br and alpha(2+3). I assume that
you looked for many others. In most of the EW plots, the three type Ia
classes: under-luminous over-luminous and normal, clearly separate. It
should be explicitly mentioned (in a table) which correlations you
tried and the measured dispersion about some linear or quadratic
fit. In this sort of paper, a negative result is equally important.
I have a serious concern about distances.
You have used H_0 = 65, Freedman et al derive 72. Many of the
distances (and absolute magnitudes) come from Saha et al. (The Sandage
camp) who deduce significantly lower values for the Hubble
constant. Freedman et al. also derive distances to the host of several
of these
SNe via Cephieds. How does the correlation change when the distances
of Freedman and a Hubble constant of 72 are used instead. Although
the final word as to who is right about the Hubble constant and cephied
distances has not been said, most astronomers lean towards the Freedman
et
al results. We should probably do the same. At the very least we should
check which camp the distance in table 7 come from and make sure they
are coming from the one camp and not both.
The number of supernovae used in the B-V vs EW plot is far too small. I
do
not think that this section should be in the paper.
More specific suggestions
=========================
Title
-----
The title is awkward. Also don't use acronyms in the title.
How about a main title with "A new set of spectral indices for type Ia
supernovae."
with the subtitle "Correlations with absolute luminosity"
Abstract
--------
"easily reproducible" sounds bad. Perhaps "empirical" is better
and more accurate.
You use the term secondary calibrator. This is true for the alpha(2+3)
parameter (see figure 12.), but it seems to me that this particular
parameter can also be used as a primary calibrator. Surely, the latter
is more interesting. Then, later in the abstract, you mention that
there is no need to obtain photometric points beyond maximum
light. This is certainly not true for secondary calibrators. For
spectroscopic primary calibrators, like alpha(2+3), one will need at
least one and probably more photometric points. as one still needs to
measure or infer the apparent magnitude at maximum light.
Introduction
============
There are some peculiar type Ia SNe that are not classified as 91bg,
91T or normal. Some examples are 2000cx, 2002cx and 2002ic. I don't
know
if we should mention these in the introduction or not.
Section 3.
=========
I think that you should drop the sentence starting with "Since spectra
..."
since it contradicts slightly what is said in the previous sentence.
Section 4.
==========
Free expansion is always true. It is not restricted to the hot
photospheric
phases. In the way this is written, one would get the impression that
free
expansion does not occur during the nebular phase.
Section 4.1
===========
Table 4
=======
I feel that the ratio of the intrinsic dispersion to the mean
equivalent width (it is called the relative dispersion in the text) is
not a meaningful parameter. Since it can be calculated from the
parameters that are already in the table and since it is made from
parameters that may change in different ways, I do not think that
there is a strong argument for including it.
Section 4.2
===========
In the 3rd paragraph.
"... where underluminous SNe are again excluded." I could not find
where they had been excluded before.
Section 4.3
===========
In the second paragraph, you use the phrase "... seem to be on the rise
..." The word "rise" might confuse people - i.e. rise in the
lightcurve.
Try to use an other word or rephrase the sentence.
Equation 3
==========
What are the reasons for using a functional form rather than the cubic
spline as is done in other cases. Why were you motivated to use it.
Why did you not use a spline? From later sections this becomes clear.
The
reason why you use the functional form is that it parameterizes the
time the "discontinuity" occurs. This should be stated in a short
paragraph
and not left to the reader to guess.
Figure 5.
=========
In the caption to figure 5, you mention that the light grey symbols
are used for underluminous SNe. There are no underluminous SNe in
this plot. See my earlier comment about the colouring scheme for the
plots.
Top of the second column of page 11
===================================
You mention that SNe in group b) have greater SiII abundances.
Be careful here! Do you really mean that the "abundance" of SiII is
higher in these SNe?
Section 5
=========
1st paragraph: This is the first time that stretch and Delta m15 are
mentioned. Please add references that define these quantities.
2nd paragraph: Whose template B-band template did you use?
To get a more recent idea of the accuracy of distance indicators,
please read the work published by the HST key project on distance
indicators. Freedman, W. et al. 2001, ApJ, 553, 47.
H_0=65 is no longer standard. People use numbers that vary between 70
and
72. Use the Freedman et al result H_0=72 +/-3.
The relation between "Fe II 4800" and intrinsic colours. Where does this
come from. There is a section on this at the end of the paper, but
readers
will not be aware of this relation until the end. Did you actually use
this
relation to correct for reddening? From the text, it would seem that you
did.
Table 7.
=======
What is the major source of uncertainty in estimating the peak
magnitude.
Is it the distance estimate?
There are two footnotes with the label "d".
5.1.3
=====
The fact that you are able to find correlations which reduce the scatter
to below the measurement error on M_B is a concern. It can only mean
that
the errors on M_B are over-estimated.
A paragraph describing how one might alpha(2+3) this parameter
practically for
high redshift supernovae is needed. The most fundamental question here
is "can we measure this parameter with sufficient accuracy for it
to be useful"
The last paragraph is very shaky. Alpha(2+3) is inferred for three SNe
by
inverting equation 6. It should not be and the inferred values should
be deleted from table 7. The EW(2+3) values are also inferred, however,
we are probably on more secure ground here as the spectra from which
EW(2+3) are inferred are near maximum light.
5.2
===
This is very speculative. There are too few SNe and two of them show
disagreement. Additionally, the error bars are large. It does not
compete with the other measures.
It is something you should pursue with future SNe, but I do not think
that it should appear in this paper. I think it weakens the paper
considerably.
Conclusion
==========
Again, I think that the relative dispersion is not a meaningful
parameter. It
is mentioned in the second paragraph.
In general, the conclusion is far too long and there is far too much
detail. The conclusion is not a discussion. Try to limit this section
to just a few paragraphs. You should emphasise alpha(2+3). It is the
most
important part of the paper.
This archive was generated by hypermail 2.1.4 : Wed Oct 08 2003 - 07:43:44 PDT