3 Results

Surprisingly, we found not only a large sample of 685 objects showing light curves similar to those observed for Galactic Be stars (we call them type-4 stars, with stochastic or sometimes quasi-periodic variations), but also 139 objects showing outbursts (called type-1 stars), 154 showing high and low states (called type-2 stars), and 78 stars showing periodic or near-periodic long-term oscillations (type-3 stars). 18 type-1 stars also showed sudden luminosity jumps like those observed in type-2 stars. We called them type-1/type-2 stars. In Figs. 1-6, we show examples of light curves for objects of the type-1 to type-4 subclasses. Basic information (OGLE name, distance (in arcsec) to the nearest star detected with Dophot, V magnitude, B-V and V-I colours, scatter of I-band magnitudes, and optional comments) on these stars is given in Tables 1-6. It is striking that type-2 and type-3 light curves have not been observed in any Galactic variable, to the best of our knowledge, and very few cases of Galactic type-1 stars seems to have been detected (see Sect. 3.1). Therefore all these stars clearly deserve further study. In Figs. 7 and 8 we show the V versus B-V, and V-Iversus B-V diagrams for the entire sample of stars. It is observed that the number of Be star candidates declines towards the extremes in both diagrams, indicating that the ranges of magnitude and colour we chose for our search were probably near-optimum to include all the Be stars in the database. Regarding the main sequence, most stars are displaced to the red in the V versus B-V diagram; this is in principle consistent with hydrogen recombination (free-bound and free-free) emission from circumstellar envelopes around Be stars (Dachs et al. 1988). Keeping in mind that our data have not been corrected for interstellar or circumstellar reddening, the sharp blue cut-off in the B-V colour is notable. This observational fact is consistent with the vanishing of the Be phenomenon in spectral types earlier than B0, partly due to the efficiency of radiation pressure to remove circumstellar gas in hotter stars.

3.1 Type-1 stars

 

 

Table 1: Type-1 stars. $\Delta \Phi$ is the distance (in arcsec) to the nearest star detected with Dophot.

Star	$\Delta \Phi$	V	B-V	V-I	rms
003623.36-733922.3	0.601	16.158	-0.233	-0.212	0.012
003832.64-732234.9	0.076	15.264	-0.139	-0.117	0.01
003918.20-733656.6	0.245	14.548	0.484	0.714	0.017
003922.09-732531.6	0.031	16.931	-0.041	-0.146	0.019
004036.15-732921.6	0.04	16.548	0.13	-0.03	0.062
004207.86-734501.9	0.136	16.804	-0.055	0.049	0.02
004215.08-731710.4	0.097	15.966	-0.157	-0.055	0.019
004231.84-732200.9	0.024	15.376	-0.106	-0.019	0.077
004326.72-725910.3	0.439	16.858	-0.128	-0.127	0.018
004502.43-732318.6	0.137	16.777	-0.048	0.039	0.012
004624.69-724657.3	0.609	19.333	0.203	0.463	0.045
004624.74-731941.7	0.102	15.851	0.069	0.169	0.09
004631.41-730335.6	0.085	17.113	-0.118	-0.002	0.04
004646.81-731849.2	0.054	16.701	-0.013	0.207	0.015
004650.39-731017.7	0.092	15.513	-0.105	-0.022	0.014
004653.17-732330.2	0.327	15.252	-0.033	0.012	0.014
004717.43-725227.6	0.119	17.071	0.353	0.403	0.136
004757.42-731050.8	0.122	15.714	-0.036	0.041	0.02
004800.14-730728.6	0.055	16.354	-0.226	0.046	0.101
004803.08-725404.2	0.347	16.704	-0.127	-0.126	0.015

 

 

Table 2: Type-1 stars showing transient quasi-periodic outbursts. ART means "after removing trend", BHS "before high state" and BF "before fading".

Star	$\Delta \Phi$	V	B-V	V-I	rms	Time-scale (days)
004957.09-730204.5	1.776	17.129	-0.133	0.122	0.049	332, ART
005039.05-725751.4	0.028	17.211	0.15	0.301	0.078	136
005118.72-732846.3	0.158	15.765	0.039	0.248	0.013	216, ART
005149.34-724134.3	0.069	16.026	-0.013	0.163	0.02	38, BF
005157.18-730811.8	0.183	16.544	-0.058	0.092	0.025	22, ART
005235.60-723751.7	0.03	14.573	-0.145	-0.114	0.045	94
005312.58-725533.6	0.039	16.024	-0.121	-0.029	0.037	58
005321.08-724548.3	0.095	15.526	-0.201	-0.067	0.049	231
005355.66-724359.2	0.151	15.56	-0.092	-0.057	0.114	85 or 42, ART, BHS
005623.53-723926.3	0.071	16.225	0.065	0.309	0.022	215, ART
005758.52-722228.7	0.084	15.794	-0.088	0.002	0.03	40, ART, BF
005802.33-724137.5	0.031	17.092	-0.167	-0.154	0.057	16, ART
005950.21-722817.4	0.123	17.342	-0.112	0.03	0.017	173, ART
010001.47-724046.9	0.062	15.876	-0.187	-0.039	0.154	133 or 101, ART
010023.52-723302.7	0.025	15.6	-0.194	-0.079	0.022	168, ART
010120.64-721118.7	0.097	15.489	-0.065	0.078	0.019	75, ART
010332.59-720326.5	0.086	14.91	-0.207	-0.129	0.146	127, ART
010409.79-723835.6	0.086	15.325	-0.097	0.046	0.026	112, BHS
010838.56-723633.5	0.036	16.115	-0.075	0.092	0.035	136

Figure 1: Examples of type-1 Be star candidates showing sharp outbursts.

Figure 2: Examples of type-1 Be star candidates showing hump-like outbursts.

The outbursts shown by type-1 stars were basically of two types: those characterized by a sudden rise of luminosity followed by a gradual decline, generally lasting tens of days with amplitudes $\mathrel{\copy\laxbox}$0.2 mag, and those with usually slower declines, lasting hundreds of days with amplitudes >0.2 mag, and characterized by more symmetric rising and fading timescales. In general, in both kinds of outbursts, the stars were redder when brighter and we could not find any evidence of a correlation of outburst rising delay with wavelength. Some stars showed both kinds of outbursts, and sometimes it was difficult to classify them when the outburst was faint and not well resolved, or in the cases of noisy light curves. For this reason we do not provide a sub-classification for type-1 stars. Not always these stars are showing a constant quiescence brightness level. For 16% of type-1 stars (22 objects) it was possible to identify an outburst time-scale, i.e.  a typical time for outburst recurrence, although this recursion sometimes was observed only as a transient phenomenon and not during the whole observing period. Basic information for type-1 stars is given in Tables 1 and 2. Exemplary light curves of this type of variable stars are shown in Figs. 1 and 2. The outburst time-scales, given in Table 2, run from 16 to 332 days, with a mean of 126 days. The time-scale histogram shows a decrease of cases toward longer periods; 68% of the objects show time-scales shorter than 150 days. There is no clear correlation between outburst time-scale, and stellar colour or apparent magnitude.

Our type-1 objects probably correspond to the bumpers found in the Large Magellanic Cloud by Cook et al. (1995) after analysing the MACHO collaboration database. These authors report on the discovery of a group of bright main sequence stars with $V \sim 15{-}17$ which have a constant luminosity for extended periods of time, but show occasional outbursts at 10-30% amplitude level. They found that the episodes in more than 50% of the cases are asymmetric and shorter than about 50 days with a more rapid brightening than dimming. The fact that H$\alpha$ and H$\beta$ emission was observed in the spectra of 7 of these stars led Cook et al. (1995) to suggest that these stars could be related to, or maybe identical to, Galactic Be stars.

The Galactic counterparts of type-1 objects could be the outbursting Be stars found by Hubert & Floquet (1998) analyzing Hipparcos photometry. They find short-lived outbursts in $\omega$ CMa and other 13 Be stars, and also a long-lived outburst in $\upsilon$ Cyg and another 7 Be stars. The outbursts were preferentially detected in early Be stars with rather low to moderate projected rotational velocities. The average time scale for the recurrence of short-lived outburst is, however, slightly larger than that observed for our SMC sample. Hubert & Floquet (1998) argue that both kinds of outbursts probably have the same origin and can be explained by discrete high density emitting plasma events seen under low inclination angles with respect to the rotational axis. The number of cases presented by these authors is too small and the photometry too scarce to make a detailed comparison with our type-1 stars, however.

Although at present we cannot exclude the Be star nature of type-1 objects, we will present, in the next section, two possible alternative explanations for these objects.

3.2 Type-2 stars and type-1/type-2 stars

Type-2 stars show sudden brightness jumps of an amplitude of a few tenths of magnitude, with the star remaining at a rather constant brightness level for hundreds of days. Type-2 stars usually don't show additional variability, like the irregular variations observed in type-4 stars, except for rather smooth long-term changes. Type-2 stars can be found in basically the same colour range as type-1 stars, but type-2 stars are statistically bluer than type-1 stars. Eighteen type-1 stars in our database also showed high-low states like those observed in type-2 stars. These type-1/type-2 stars were only found in the range of 15.5<V<17 and in a narrow colour box characterized by  -0.2<B-V<0.2 and -0.2<V-I<0.25. The existence of these "transition" stars supports the conjecture that type-1 and type-2 stars could correspond to the same kind of object. Basic information for type-2 and type-1/type-2 stars is given in Tables 3 and 4.

 

 

Table 3: Type-2 stars.

Star	$\Delta \Phi$	V	B-V	V-I	rms
004234.39-733058.2	0.043	15.989	-0.148	0.002	0.021
004258.05-730230.7	0.036	16.059	0.028	0.176	0.019
004319.46-731705.7	0.099	16.403	-0.056	0.132	0.032
004330.69-732034.2	0.028	16.428	-0.194	-0.177	0.009
004414.25-731215.5	0.086	17.297	-0.107	-0.028	0.023
004443.05-731316.5	0.068	16.217	-0.072	0.028	0.017
004504.35-724449.9	0.076	17.906	0.336	0.549	0.051
004510.40-731648.1	0.104	15.855	-0.105	-0.022	0.009
004539.31-731040.1	0.433	17.849	-0.104	0.702	0.088
004546.74-733233.1	0.008	16.284	-0.032	0.158	0.032
004556.84-730919.8	0.068	17.128	-0.057	0.031	0.013
004601.22-731821.9	0.11	16.374	0.033	0.255	0.018
004610.97-732535.2	0.037	15.439	0.009	0.032	0.146
004633.48-730352.1	0.026	15.489	-0.066	0.063	0.04
004639.77-725241.0	0.047	15.614	-0.179	-0.173	0.011
004651.07-730215.3	0.048	15.801	-0.026	0.176	0.026
004655.32-731207.6	0.094	16.164	-0.113	0.103	0.03
004714.29-731044.0	0.09	17.635	-0.022	0.043	0.022
004714.53-731349.7	0.131	15.068	-0.196	-0.172	0.009
004719.31-732533.6	0.11	16.449	-0.074	0.145	0.073

 

 

Table 4: Type-1/type-2 stars. The nomenclature is as in Table 2.

Star	$\Delta \Phi$	V	B-V	V-I	rms	Time-scale (days)
004402.00-733129.4	0.221	15.456	-0.003	0.232	0.016	284
004406.65-732938.2	0.034	16.285	-0.029	0.114	0.02
004650.21-732807.3	0.042	15.442	-0.192	-0.191	0.017
004738.11-731126.1	0.12	15.484	-0.114	0.022	0.008
004800.73-732253.0	0.137	15.842	-0.048	0.144	0.018
004817.58-725028.8	0.133	16.329	-0.109	-0.048	0.016
004848.27-732611.3	0.062	15.462	-0.098	0.016	0.067	39, BF
004858.25-724119.4	0.054	16.157	-0.074	0.076	0.015
005045.50-730112.9	0.071	16.398	-0.057	0.038	0.019
005053.20-731030.8	0.046	15.925	-0.054	0.157	0.096
005105.65-731311.5	0.044	15.911	-0.032	0.208	0.047
005112.13-725656.2	0.08	16.196	-0.082	0.09	0.034	52, ART
005141.43-731129.9	0.051	16	-0.123	0.024	0.091
005147.58-730924.7	0.066	15.629	-0.073	0.077	0.045
005456.92-731200.4	0.056	16.344	-0.156	-0.143	0.01
005651.99-724027.1	0.059	15.735	-0.065	-0.015	0.067
005854.06-722841.9	0.114	17.014	-0.04	0.134	0.042
010701.71-724754.9	0.036	15.601	-0.152	-0.053	0.021

3.3 Type-3 stars

In these periodic objects, we searched for periods longer than 0.2 days using a variety of period searching algorithms implemented in graphic C-routines written by us, and also in IRAF. We found periodicities in the range of 17 to 720 days. The accuracy for periods longer than 200 days is low, and these variations can easily be quasi-periodic rather than strictly periodic. 51% of the periods were found in the range of 17-100 days. The mean period was 135 days. Many of the stars showed significant aliases in the periodogram in addition to the main period, and 2 showed evidence of a variable period. Five stars turned out to be eclipsing binaries, with evidence of additional variability not related to changing aspects of the binary system. In general, type-3 stars were redder when brighter. All the relevant information is given in Table 5. The 9 type-3 stars in our sample with periods longer than 200 days have V-I colours in the range of 0.0-0.2 mag, and B-V colours in the range of -0.1-0.0 mag. They correspond to stars with visual magnitudes between 15.5 and 18.0 mag. In general, type-3 stars show similar colours than the other types of stars we are discussing, but they appear more homogeneously distributed in Figs. 7 and 8, with a tendency to concentrate in the red part of the colour-colour diagram.

 

 

Table 5: Type-3 stars. ART means "after-removing-trend", ChP changing period, LA low amplitude, MA multiple aliases, and VA variable amplitude.

Star	$\Delta \Phi$	V	B-V	V-I	rms	Period (days)
003813.22-734144.7	0.291	16.607	-0.045	0.022	0.058	$187\pm19$
003832.82-734404.2	0.385	17.447	0.448	0.609	0.106	$79\pm3$, MA
003833.35-731510.1	0.177	16.467	0.019	0.084	0.064	$164\pm3$
003927.34-733309.6	0.322	15.855	-0.158	0.057	0.055	$\sim$27, ART, MA
003952.15-730057.7	0.53	18.815	-0.042	0.022	0.077	$107\pm6$, double eclipse?
004019.36-730402.9	2.172	16.207	-0.033	0.146	0.059	$200\pm22$, VA
004024.94-734416.1	1.58	16.531	-0.018	0.04	0.01	$\sim$448
004211.38-732437.7	0.345	17.916	-0.089	0.01	0.058	$74\pm3$
004212.21-734120.0	0.017	15.921	-0.197	-0.138	0.039	$46\pm1$, alias 1.01932
004323.84-733930.1	0.636	16.341	-0.19	-0.145	0.016	$62\pm2$, alias 0.98134
004336.91-732637.7	0.097	14.178	0.181	0.331	0.031	$118\pm7$, MA
004357.18-732019.9	0.139	16.51	0.386	0.668	0.065	$29\pm1$
004454.66-732802.9	0.183	14.817	0.286	0.576	0.03	$17.37\pm0.15$, VA
004517.55-732343.6	0.044	17.513	0.306	0.222	0.094	$171\pm15$
004541.10-731219.2	0.082	17.149	0.089	0.24	0.032	$182\pm18$
004541.80-724932.3	0.046	17.252	0.308	0.48	0.206	$85\pm3$
004554.14-731404.3	0.781	15.483	0.517	0.709	0.01	$99\pm4$, LA
004633.06-731919.1	0.129	17.15	-0.012	0.181	0.021	$38\pm1$, ART
004633.76-731204.3	0.092	14.058	0.206	0.385	0.057	$184\pm18$, eclipsing
004652.03-731423.7	0.084	15.995	-0.069	0.087	0.021	$\sim$567, other 67?
004653.24-724300.0	0.495	19.469	0.03	0.12	0.166	$66\pm3$
004702.75-730618.3	0.126	16.57	0.122	0.243	0.016	$17.3\pm0.15$, ART, eclipsing
004723.53-730347.0	0.11	16.328	-0.043	0.162	0.016	$101\pm2$, ART, eclipsing
004748.20-731906.1	0.073	18.355	0.194	0.392	0.079	$177\pm10$
004750.14-731316.4	0.222	15.475	0.269	0.471	0.009	$30.0\pm0.3$, LA
004816.01-730635.7	0.212	15.917	0.123	0.338	0.04	$26.45\pm0.20$, strange phase curve, also type-1?
004833.67-732955.6	0.221	19.403	0.129	0.417	0.14	$161\pm12$
004843.23-731415.8	0.111	16.736	-0.081	0.066	0.062	$\sim$357, VA
004849.06-724309.2	0.088	17.588	0.446	0.639	0.108	$150\pm15$, rather time-scale

 

 

Table 6: Type-4 stars.

Star	$\Delta \Phi$	V	B-V	V-I	rms
003618.14-734624.6	0.086	14.929	0.353	0.779	0.279
003618.15-734247.4	1.419	16.177	-0.131	-1E-3	0.036
003619.44-732553.1	0.132	19.519	0.258	0.516	0.182
003621.53-732610.6	0.05	14.741	0.067	0.214	0.124
003623.02-734651.0	0.016	19.551	0.484	0.531	0.304
003645.28-733259.7	1.863	15.59	-0.024	0.2	0.015
003652.74-732857.9	3.222	14.397	0.081	0.177	0.016
003715.60-734546.0	1.804	16.09	0.453	0.647	0.013
003726.27-731420.0	0.511	19.63	0.318	0.559	0.25
003730.99-731112.4	1.965	15.846	0.109	0.209	0.01
003744.41-730447.6	0.109	16.996	-0.087	-0.051	0.028
003804.71-735150.5	0.329	19.434	0.009	0.092	0.065
003810.16-730138.4	1.22	17.296	0.59	0.767	0.02
003813.77-731340.0	0.208	17.335	-0.081	0.116	0.016
003817.01-734852.6	0.025	15.199	0.462	0.709	0.28
003827.69-733314.5	1.499	16.126	-0.156	0.124	0.021
003833.29-732629.4	1.317	16.725	0.414	0.619	0.011
003836.25-734813.4	0.303	15.383	-0.14	-0.082	0.022
003837.66-731158.3	0.093	14.894	-0.085	0.057	0.028

3.4 Type-4 stars

Figure 3: Examples of type-1/type-2 Be star candidates showing outbursts and high-low states.

Figure 4: Examples of type-2 Be star candidates showing high and low states.

Figure 5: Examples of type-3 Be star candidates showing periodic oscillations.

Figure 6: Examples of type-4 Be star candidates showing random variability.

They are the most numerous in our sample, sharing colour ranges with the other types of variables. They usually show stochastic variability in time scales running from days to years (Fig. 6). In some occasions we observed quasi-periodic oscillations on time scales of weeks, and in others short-duration, eclipse-like events. Information on type-4 stars is given in Table 6.