Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://www.eso.org/~mvandena/v351ori_s2p.ps.gz Äàòà èçìåíåíèÿ: Tue Sep 3 20:29:50 2002 Äàòà èíäåêñèðîâàíèÿ: Mon Oct 1 23:46:26 2012 Êîäèðîâêà: Ïîèñêîâûå ñëîâà: dust disk

THE REMARKABLE HERBIG AE STAR V351 ORI
M.E. van den Ancker University of Amsterdam, Amsterdam, The
Netherlands
P.S. Th' e University of Amsterdam, Amsterdam, The Netherlands
D. de Winter Universidad Aut'onoma de Madrid, Madrid, Spain
ABSTRACT. The photometric behaviour of the Herbig Ae star V351 Ori was
investigated combining data from the literature with new photometry. It is shown
that this object changed from a Herbig Ae star with strong photometric variations,
due to extinction by circumstellar dust clouds, to that of an almost non­variable
star. Such a behaviour is not unique; it has been found also in the star BN Ori.
This suggests that such transitions as well as the opposite must occur quite often
during the evolution of these intermediate mass stars towards the main­sequence. A
provisional model to explain V351 Ori's behaviour, in which it is assumed that a
temporarily strong accretion of matter onto the star took place, is proposed.
1. INTRODUCTION
V351 Ori = HD 38238 is an irregular variable Hff­emission line star, located in the
direction of the Lynds 1630 dark cloud region. Although V351 Ori fulfills all primary
criteria for membership of the Herbig Ae/Be stellar group (A or B­type emission­line
star with infrared excess, located in a plausible region of recent star formation), as
well as many secondary ones (large brightness variations, spectral signatures of mass
accretion), it was only first recognized by Koval'chuk (1985) and by Zajtseva (1986)
to belong to this special class of intermediate­mass pre­main sequence objects. As a
consequence of this late discovery, the star was not well studied and a more careful
analysis of all available photometric and spectroscopic data of this object was in place,
the results of which are presented in this paper.
2. PHOTOMETRIC BEHAVIOUR
During the last seven years, V351 Ori was monitored by the ESO ``Long Term Pho­
tometry of Variables''­group, using the Str¨omgren Automatic Telescope at La Silla,
Chile. The new y data are, together with visual photometry of V351 Ori collected
from literature, plotted in Fig. 1. Colour­magnitude diagrams with these data are
shown in Fig. 2. When we examine the lightcurve in Fig. 1, we notice that V351 Ori
has changed its photometric behaviour drastically over the period of 14 years covered
by this diagram. In the period before JD 2446000 the star shows large brightness
variations. When we look at the colour­magnitude diagrams in the Johnson UBV RI
system over this period (Fig. 2b), we notice that for V ! 10: m 0 there seems to be a
linear relation between the V magnitude and the colour­indices, in which the colours
1

Figure 1. Lightcurve of V351 Ori. Squares represent data of Koval'chuk (1985), plus signs those of
Kilkenny et al. (1985), crosses those of Chkhikvadze (1990) and open circles are data of the LTPV group.
Figure 2. (a) Colour­magnitude diagrams of V351 Ori in the Str¨omgren uvby system. (b)
Colour­magnitude diagrams of V351 Ori in the Johnson UBV RI system. Plot symbols have the same
meaning as in Fig. 1. The solid lines indicate the interstellar reddening direction.
become redder as V increases. The slope of this relation is compatible with that of
the normal (i.e. R V = A V =E(B \Gamma V ) = 3:1) interstellar extinction law. This suggests
that these variations are caused by variable amounts of extinction of the starlight.
As the stellar brightness decreases even further, we see the blueing effect; with
increasing V , the colours now become bluer. Such an effect can be explained by
assuming the presence of a dust envelope around V351 Ori. The radii of the particles
in this envelope are small enough to scatter the blue photons of the stellar radiation,
producing a faint blue emission. The complete colour­magnitude diagram in Fig. 2b
can now be explained by assuming the passing by of obscuring dust clouds, revolving
around the star. Due to extinction by the particles in the cloud, the light from the
star becomes fainter and redder. Below some brightness level, the scattered blue light
from the dust envelope dominates, and produces the observed blueing effect.
If we only look at the first part of the light curve in Fig. 1 and the colour­magnitude
diagrams in Fig. 2b, we would come to the conclusion that the photometric behaviour
of V351 Ori is like that of a typical Herbig Ae star with large photometric variations,
such as UX Ori. However, in Fig. 1 we can see that V351 Ori changed its photometric
behaviour from that of a Herbig Ae star showing large photometric variations to that of
a nearly constant one. This process seems to have started around JD 2446000. Before

Figure 3. Observed (squares) and extinction­corrected (circles) SEDs for V351 Ori. Also shown are the
Kurucz model (left dashed line), the disk model (middle dashed line) and the Planckian (right dashed
line) fitted to the infrared excess and the total model (solid line), fitted to the extinction­free SED.
this date, the star showed large (? 2 m ) variations in y with a maximum brightness of
ú 8: m 8 in this passband. Between JD 2446000 and JD 2448200, V351 Ori only varied
by several tenths of a magnitude in y, and the star's maximum visual brightness dims
by 0: m 16 to y ú 9: m 0. Between JD 2448200 and JD 2448250, the maximum brightness
of V351 Ori rapidly increases to y ú 8: m 84, equal to its level before JD 2446000. After
JD 2448250, the variations in brightness nearly vanished.
3. STELLAR PARAMETERS
Using newly obtained low­resolution spectra, we determined V351 Ori to be an A6 or
A7 weak emission­line star, well in agreement with the spectral type of A7 III given
by Zajtseva (1986). From new high­resolution (R = 55; 000) spectra of V351 Ori in
the Hff and Na i D lines, we notice that the Hff emission has a very strong reversed
P Cygni profile, indicating accretion of matter towards the star. Indications for the
presence of a disk­like structure around V351 Ori come from the presence of a weak
and broad component in the sodium lines.
The extinction­free spectral energy distribution (SED) of V351 Ori, employing
only data taken near maximum brightness, was analyzed by comparing it with a
Kurucz (1991) model with T eff = 7650 K and log g = 3.6, corresponding to the
spectral type of A7 III. The photometry used in the construction of this SED were
taken from literature and were all obtained in the period when the star still showed
strong photometric variations. The resulting SED of V351 Ori is shown in Fig. 3.
As can be seen from this plot, the quality of the fit to the Kurucz model is very
good in the ultraviolet and the optical wavelength ranges, with a large amount of
excess radiation in the infrared. The fact that the SED in the near infrared can be
approximated by a straight line indicates the presence of an extended disk or dust shell
around the star, whereas the strong bump in the far­infrared suggests the presence of
an additional cooler component. We modelled this SED using a simple parametrized
disk model, with T / r \Gamma0:9 , similar to the ones by Beckwith et al. (1990). An
additional Planckian component with a temperature of 78 \Sigma 4 K represents the dust
shell. These two models are also shown in Fig. 3.

4. DISCUSSION AND CONCLUSIONS
We have shown that V351 Ori changed from a Herbig Ae star with strong photometric
variations to a almost non­variable one. Such a change had previously only been
observed in the Herbig Ae star BN Ori around 1947 (Shevchenko et al. 1996). A
first conclusion that can be drawn from the fact that we have actually been able to
observe such transitions in the relatively short period these stars have been observed
systematically, is that such transitions from Herbig Ae stars with strong photometric
variations to almost non­variable ones must occur quite often during the evolution of
such stars towards the main­sequence. This also implies that the reverse process must
occur frequently.
Since before JD 2446000 the photometric variations of V351 Ori were caused by
dust clouds orbiting the star, it seems that around this date some event started which
either destroyed the dust in these clouds, or moved the dust clouds from our line of
sight. This event continued until JD 2448200, after which the star rapidly became
nearly constant in brightness. So far, we can only speculate about the cause of this
event. It could be that it is correlated with a temporarily strong accretion of dust
onto the star. Such an event will be accompanied by shock­fronts which create an
extremely hot gaseous disk near the equatorial plane close to the star. This hot
optically thick gaseous disk blocked part of the visual and blue light of the star,
causing a visual brightness decrease of about 0: m 2. At the same time, this hot object
radiates ultraviolet energy superposed on that of the central star. This is causing the
increase of the ultraviolet brightness of the star. When the accretion of dust stops, the
visual and blue stellar light regain its maximum brightness, whereas the ultraviolet
contribution of the hot disk disappears, and the ultraviolet brightness drops.
Since all infrared observations of V351 Ori were made during the period in which
the star still showed photometric variations, it is very important to obtain new in­
frared photometry of this object. If the dust clouds close to the star have indeed
accreted onto the central star, we should expect to see a clear decrease of the infrared
excess of the star. Further optical monitoring of V351 Ori remains necessary to see
whether the dust clouds start to re­occur, in which case the star will become variable
once more.
References
Beckwith, S.V.W., Sargent, A.I., Chini, R.S., G¨usten, R. 1990, A. J. 99, 924
Chkhikvadze, J.N. 1990, Bull. Abastumani Astroph. Obs. 68, 43
Kilkenny, D., Whittet, D.C.B., Davies, J.K., Evans, A., Bode, M.F., Robson, E.I.,
Banfield, R.M. 1985, SAAO Circ. 9, 55
Koval'chuk, G.U. 1985, Kinem. Fizika Nebesn. Tel. 1, 30
Kurucz, R.L. 1991, in Precision photometry: Astrophysics of the galaxy, ed. A.G.
Davis Philip, A.R. Upgren, K.A. Janes, L. Davis press, Schenectady, New York
Shevchenko, V.S., Ezhkova, O., Tjin A Djie, H.R.E., van den Ancker, M.E., Blondel,
P.F.C., de Winter, D. 1996, Astr. Ap., in press
Zajtseva, G.V. 1986, Astrofizika 25, 471