On the Mass Accretion Rates of Herbig Ae/Be Stars. Magnetospheric Accretion or Boundary Layer?

Round 1

Reviewer 1 Report

I have read the review "On the mass accretion rates of Herbig Ae/Be
stars. Magnetospheric Accretion or Boundary Layer?" with great
interest. The author displays a great deal of knowledge and provides
it in a form easily accessible to readers without deep prior
knowledge in the field. All in all, I think this is a great review.

I have one major criticism though that needs to be addressed. In
addition, I make a few minor suggestions that I believe would make the
article more readable, but I want to stress that the author should
feel free to implement them or not as he sees fit.

Major point
-----------

I have a major concern with the main conclusion of the text:

How can the accretion rates be so different for MA and BL models? In
both cases, the same gravitational energy is released as mass travels
from infinity (OK, the shape of the gravitational potential means that
only the inner few R_* matter) to the stellar surface. In both
scenarios that energy has to go somewhere. In the MA scenario, it is
all released in the accretion shock, split into different wavebands
from direct X-ray emission to heating up the stellar photosphere
(optical veiling). If BL scenarios give higher mass accretion rates
for the same stars, a large fraction of the energy needs to go
elsewhere. Where does it go?

On a very basic level, the equation in line 422 is not specific to the
BL model. It describes the energy released in any accretion
process. That is all the energy there is and energy conservation
tells us that that energy has to go somewhere in both the BL and he MA
scenario. Note that a very similar equation is the starting point for
the Calvet & Gullbring (1998) model, there "F" is essentially the
L_acc from the equation in line 422.

Looking at the problem from the point of view of the total energy
conversation, one can argue that the differences between MA and BL
models *must* be due to systematics in the way the models are
applied. This starts with simple model assumptions (e.g. MA shock
models typically assume that "half the energy is radiated inwards and
half outwards", but of course the inwards energy is re-radiated
elsewhere or inflates the stellar photoshere, but it certainly not
"lost"; MA models often assume that "half the mass flux is on the far
side of the star"), continues with scaling relations (sometimes
derived from non-simultaneous data or, as the text shows, extrapolated
beyond the mass range where they were originally found) to the missing
modelling of BL (a BL is certainly not a single temperature BB, but
rather a turbulent region with a distribution of temperatures). This
also includes observational questions of extinction and estimating
which fraction of the total L_acc is observed in the waveband in
question or how much other processes (winds) contribute to the
observed flux in a specific tracer such as e.g. Halpha. (The latter
point is discussed in Section 7.)

(While not directly relevant here, note that in CTTS there is also a
discrepancy between X-ray shock modelling (direct light from the shock
[e.g. Gunenther+ 2017, Curran+ 2010]) and the Calvet & Gullbring-type
modelling of re-processed light around the Balmer jump.)

It is clearly beyond the scope of the current review to solve these
issues and I agree that UV observations might be decisive to learn
more about with scenario applies. However, simply comparing two
continuum fluxes will likely tell us more about the shortcomings of
our current models than about the physics of accretion.

Suggestions
-----------

Figure 1 is nice and illustrates the concept well. Just one small
comment: It might be helpful to indicate in the image or the caption
that this is a cut in the plane of the star with the disk only shown
on one side, while in truth the disk surrounds the star on all sides
and accretion might happen on more than one spot (or in the BL case,
all the way around).

Around line 100: Evidence for high-densities in X-ray observations
could also be mentioned which also cannot be explained in BL
(e.g. Kastner+ 2002, Brickhouse+ 2010 or others) and similarly UV line
profiles (e.g. one of the papers by Gomez de Castro and Lamzin between
1994 and 1999 or modeling papers from the Palermo group, e.g. Colombo
2016), since this is a UV focussed review.

Line 117: Should there be any spaces in "NaID"?

Line 118: Add a short (< 1 sentence) explanation what an "UXOr phenomenon" is.

Line 144: "revealed accurate"> Maybe "proved accurate", or "was revealed to be accurate"?

Line 147: Do you mean "the same stellar parameters as" or is there a
comparison missing e.g. "lower ... than"?

(This is a minor point, not just a suggestion) Around Line 180: How
about variability? A relation L_acc <-> L_* can only hold if the
accretion rate is steady or "on average". In TTS there is ample
evidence of time-variable accretion rates, which will lead to a larger
scatter in any \dot M <-> line relation for non-simultaneous
datasets. I suggest to at least mention this point, see e.g. Aarnio+
(2014, https://ui.adsabs.harvard.edu/abs/2014IAUS..302...80A/). This
point comes up in the first paragraph of Section 3, but in a different
context.

Line 310: free -> few

Line 364-366: Is the source of this scatter known? If it is related to
e.g. uncertainties in the stellar parameters that effect all
measurements from the same star in (mostly) the same way, then
relative measurements are still possible, i.e. increasing the Halpha
flux by a factor of 5 is less than the scatter in the relation, but
might still indicate a significant increase in accretion rate even if
we cannot say by what amount exactly.

Line 422: "The reference M_acc values ... with inclinations ....": I
do not see an inclination dependence in the formula on line 422.

Fig 4 and 5: The caption describes what is shown in each panel, but it
would be much easier for the reader if the relevant parameters (T, M,
logg for Fig 4) could also be printed into each panel. There is
sufficient space in the top left of each panel. A legend that explains
color and line style would also help (obviously, that is only required
in one of the panels).

Author Response

My response is attached in a pdf file

Author Response File: Author Response.pdf

Reviewer 2 Report

The review is well written and provides a good overview on the accretion properties of HAe/Be stars. I have only minor comments detailed below.

General:
--------
Section 2 is relatively long and may benefit from introduction subsections.

The argument of similar photospheric and hot spot temperatures appears quite late in the review (p. 6) while it may be one of the fundamental challenges for testing model predictions and explains some of the challenges in deriving accretion rates for HAeBes. Therefore, it may be beneficial to introduce this argument earlier in the text.

When discussing the magnetic field measurements, it may be useful to state the type of measurement, i.e., if large scale ordered magnetic fields are probed or if the field may be in the form of rather local (higher order multipole) components. Only sufficiently large scale fields may be able to disrupt/truncate the disk leading to MA.

Specific
---------

Abstract:
"... there is not yet a consensus on the validity of this scenario versus the BL one."

Line 104:
"Nowadays accretion rates in CTTs are commonly derived assuming MA..."
This is certainly true, the basic assumption is essentially an energy argument: the kinetic energy of the accretion flow is lost as radiation. The shape of the (post-) shock emission depends on the details physical conditions. While the author certainly knows this, the sentence may be improved to make this line of reasoning more clear.

Line 138:
"...establishing a calibration relating the observed “Balmer excess” (∆D_B) in UX Ori with Macc."
A few more explanations on the Balmer excess may be appropriate here (I noticed that it is better described in the text surrounding Eq 8, but pulling this discussion to earlier in the text may benefit the reader). Perhaps also a figure for the Balmer excess?

Line 161:
"... that an alternative accretion mechanism may work for these objects ..."
work -> be in operation

Line 162:
"...strong support to initial claims from spectropolarimetry..."
to _the_ initial claims

Line 164:
"...a fraction of the HBe stars..."
Can you give a rough quantitative number?

Line 209:
"...they lack from a careful ..."
remove "from"

Line 210:
"... which still constitutes a major limitation for the applicability of MA line modelling in these objects []."
Because?

Line 216:
"... analogous to the WTTs are not defined []."
Well, the lack of a _definition_ is not the real obstacle but the challenge to reliably identify them against the field population. Probably worth rephrasing.

Line 220:
"... line morphologies and variability are inconsistent with a scaled-up T Tauri MA scenario []."
Because?

Line 224:
"... the primary requirement for MA to proceed is the presence ..."
proceed -> apply

Line 304:
"... or from indirect methods comprising empirical correlations with the previous accretion rates."
I guess "previous" means "model-based rates"? Aren't all empirical relations ultimately based on samples where model-based values are available?

Line 323:
"... assuming a simple dipole geometry for the magnetic field."
Maybe add that this is motivated by the fact that the dipole reaches the furthest out from the star?

Line 375:
The Dupree cite: Add that this is for a CTTS?

Line 400:
"... can be extended to the HAeBes with similar accuracies."
add "provided that MA also applies to these stars"

Sentence before Eq. 3:
"...can be described as an optically thick region characterized by a single blackbody..."
Isn't this the zeroth order approximation? Maybe add a cautious word here? Or make clear that this is "just" meant in energy terms not spectrally.

Line 473:
"...(using a Kurucz template with a given stellar temperature and surface gravity)..."
Add "e.g." before "a Kurucz" or so.

Line 536:
"... changing it by 1 dex results in significant variations in the expected excesses ..."
Be specific, i.e., what means significant in actual numbers here?

Line 555:
"... 100 hours with the future World Space Observatory 6 will in principle be devoted to observe ..."
Remove "in principle"?

Author Response

my response is attached in a pdf file

Author Response File: Author Response.pdf