The most rapidly rotating He-strong emission line star: HR7355
aa r X i v : . [ a s t r o - ph ] J a n Astronomy & Astrophysics manuscript no. 7355 c (cid:13)
ESO 2018November 4, 2018
The most rapidly rotating He-strong emission line star: HR 7355 ⋆ (Research Note) Th. Rivinius , S. ˇStefl , R.H.D. Townsend , and D. Baade ESO - European Organisation for Astronomical Research in the Southern Hemisphere, Chile Bartol Research Institute, University of Delaware, Newark, DE 19716, USA ESO - European Organisation for Astronomical Research in the Southern Hemisphere, Karl-Schwarzschild-Str. 2,85748 Garching bei M¨unchen, GermanyReceived: < date > ; accepted: 24/01/2008 ABSTRACT
Aims.
We searched for massive stars with Balmer-emission consistent with magnetically confined circumstellar material.
Methods.
Archival spectroscopic and photometric data were investigated.
Results.
HR 7355 is a formerly unknown He-strong star showing Balmer emission. At V = 6 .
02 mag, it is one of thebrightest objects simultaneously showing anomalous helium absorption and hydrogen emission. Among similar objects,only σ Ori E has so far been subjected to any systematic analysis of the circumstellar material responsible for theemission. We argue that the double-wave photometric period of 0.52 d corresponds to the rotation period. In tandemwith the high projected equatorial velocity, v sin i = 320 km s − , this short period suggests that HR 7355 is the mostrapidly rotating He-strong star known to date; a class that was hitherto expected to host stars with slow to moderaterotation only. Key words.
Stars: emission line – Stars: circumstellar matter – Stars: magnetic – Stars: chemically peculiar
1. Introduction
In the early B-type spectral range a subclass of He-strongstars is found, i.e. stars showing Helium lines with abnor-mally large equivalent widths. The chemical surface abun-dances of these stars are influenced by the presence of astrong magnetic field, resulting in a He overabundance thattypically varies in strength over the stellar surface.Because He-strong stars are sufficiently luminous to har-bour radiatively driven winds (as diagnosed by ultravioletabsorption line diagnostics; see Shore & Brown 1990), theyrepresent ideal laboratories for understanding the processof magnetic wind confinement (Babel & Montmerle 1997).Typically, the fields of these stars are too strong for them tobe amenable to the magnetohydrodynamical (MHD) sim-ulations (e,g, ud-Doula & Owocki 2002). However, an al-ternative Rigidly Rotating Magnetosphere (RRM) modelfor the circumstellar distribution of magnetocentrifugallyconfined wind plasma by Townsend & Owocki (2005) hasshown much promise in reproducing the detailed opticalvariability of the archetype emission-line He-strong star σ Ori E.To date, our knowledge on He-strong stars is limited toslow to moderate rotators, as no rapid rotators had beenfound. This goes as far as to the conclusion that slow ro-tation is an intrinsic property of He-strong stars (Walborn1983; Zboril & North 1999), that has to be taken into ac-count by the search for the origin of the magnetic field.
Send offprint requests to : Th. Riviniuse-mail: [email protected] ⋆ Based on observations collected at La Silla, ESO-Chile(Prop. 063.H-0080, 073.D-0234)
This work not only reports the discovery of one morebright massive star to host a magnetosphere for applicationof the above model, but also extends the parameter rangein which He-strong stars are found by a factor of about twoin rotational velocity space.HR 7355 (HD 182 180, HIP 95 408) is a little-observedB2Vn star of 6 th magnitude ( V = 6 . , B = 5 . v sin i =320 km s − , while Glebocki & Stawikowski (2000) report v sin i = 270 ±
30 km s − . Hipparcos photometric data in-dicate that the star is a periodic variable, with P = 0 .
26 d(Koen & Eyer 2002).
2. Observations
During guaranteed-time observations with FEROS at theESO-1.52m telescope in 1999, that included several mag-netic and emission-line stars in the target list, HR 7355 wasobserved once on July 25, 1999 (HJD=2451385.507) with
S/N = 280 and noted as a weak emission star, but thisresult was not published (Fig. 1, upper left panel). FEROSis an echelle instrument covering the spectral range 3600-9200 ˚A with a spectral resolving power of ∆ λ/λ = 48 000(Kaufer et al. 1999). When re-scanning available FEROSspectra of Be stars in a search for candidate hot starshaving magnetospheres, we noticed that a second FEROSspectrum of HR 7355 had been obtained on July 05, 2004
Th. Rivinius et al.: The most rapidly rotating He-strong emission line star: HR 7355 (RN)
Fig. 1.
Changes in several representative lines between the spectra taken in 1999 (solid line) and 2004 (dotted). The 1999profile has H α emission extending out to several times beyond v sin i (the latter indicated by the vertical dotted lines,upper left) Note that for the Balmer lines (left column) a wider range in velocity is shown than in the other panels.(HJD=2453191.879) with S/N = 270 (Evans et al. 2005).The latter spectrum has been retrieved from the ESO-science archive and reduced with the FEROS standard DataReduction System, available from ESO. The two spectrasuffer from a somewhat imperfect continuum normaliza-tion, not untypical for echelle data. This tends to limitthe accuracy of equivalent width measurements for Starkbroadened lines such as the Balmer lines, and for hot starssuch as HR 7355, Helium lines can similarly be affected.
3. Analysis
Photometric data have been obtained from the Hipparcossatellite archive. Two points were removed as outliers, leav-ing 41 remaining photometric measurements spanning theinterval from JD=2447967 to JD=2449061.We repeated the analysis by Koen & Eyer (2002) andwere able to confirm their period for sinusoidal varia-tions: P sin = 0 . ± . Figure 1 shows a selection of the spectral lines observed.Most intriguing are the changes in the He i lines, and thewidth of the Balmer emission.In classical Be stars (Porter & Rivinius 2003), where theemission arises from a Keplerian disk having near-circularparticle orbits, the highest kinematic velocity possible isthe orbital velocity at the stellar surface, typically from afew to about five hundred km s − for late and early type B-stars, respectively. In the present case, however, the Balmeremission extends from − − . For a strongemission line, peaking at several times the flux of the lo-cal continuum, scattering processes can broaden its base;however, the line seen in HR 7355 is certainly too weak forsuch broadening to be occurring. We are thus led to to con-clude that the extent of the Balmer emission in HR 7355 isgoverned by the non-Keplerian kinematics of the emitting material itself. Between 1999 and 2004, the emission de-creased strongly both in strength and kinematic width, butremained present as distinct peak on the blue side slightlyoutside v sin i , yet less obvious as a filling-in of the absorp-tion on the red side.The photospheric absorption lines also differ betweenthe two epochs. The He i lines show the most striking vari-ations – the equivalent widths (EWs) of some change bymore than a factor of 2 (see Table 1). The changes ariseacross the entire line width, affecting the Stark broadeningwings as well as the line cores. Such behaviour cannot beexplained by pulsation; radial pulsation displaces the entireline, while non-radial pulsation distorts the profile withinthe limits of v sin i , but tends to conserve the total EW.Neither behaviour is consistent with that seen in HR 7355.To the contrary, the He i profiles obtained in 2004, in partic-ular the 4388 and 4713 lines (Fig. 1), resemble the signatureof a spot on a rotating star (cf. Sect. 4.2).Variations are also seen in lines other than He i , but theyare much weaker. The wings of the Balmer lines are some-what deeper in the spectrum taken 2004 than in the onefrom 1999, and the EW of Si iii is slightly larger, i.e. byabout 1 . σ , see Table 1. The uncertainty of the latter mea-surement has been estimated from a series of manual andsemi-automated measurements, with integration limits cho-sen to embrace all potential continuum normalization er-rors, so σ = 25 m˚A is probably conservative. There are alsosome lines that do not change: The C ii ii i
4. Discussion
The high projected equatorial velocity of HR 7355 suggeststhat we see the star close to equator-on. If an oblique-dipole magnetic field is responsible for the confinement ofthe Balmer-emitting circumstellar material, the density ofthis material should be highest at the twin intersections be-tween the magnetic and rotational equators (see Sect. 4.4,also for an explanation of the H α variations, which arerather long-term changes rather than being rotational). h. Rivinius et al.: The most rapidly rotating He-strong emission line star: HR 7355 (RN) Fig. 2.
The Hipparcos photometry sorted with the singleand double-wave periods. As epoch the date of the 1999FEROS spectrum has been chosen; the respective phasesof both spectra are indicated by arrows.When viewed from an equator-on aspect, these high-densityregions will transit the stellar disk twice per rotation cy-cle, leading to a double-wave photometric signature. In thisscenario, the rotation period is identified as twice the ob-served sinusoidal value, i.e. P dw = 0 . ± . . ± .
07 cycles apart, i.e. having almost the samephase. If we are to seek a common origin for the spectro-scopic and photometric variations, this small phase differ-ence is not compatible with the strong changes seen in thespectra. However, with the double wave period, the cycleseparation becomes 3464 . ± .
04 – a half-phase difference,which is much more plausible.The two IUE spectra of HR 7355, SWP39549 and 39556,do not show any significant differences. The spectra are sep-arated by 8.879, d; with the above periods this correspondseither to 34.06 or to 17.03 cycles, both values having a smallphase difference consistent with the absence of variations. σ Ori E
The changes in the He i lines seen in Fig. 1 are a clear indi-cator of abundance variations across the surface. In partic-ular, the extent of the variability across the entire width ofthe lines, including the Stark-broadened wings, can hardlybe attributed to any other mechanism. The same kind ofabundance variations can be seen in σ Ori E; there, the He i equivalent width changes in anti-phase compared to lines ofCarbon, Oxygen, Silicon, and Magnesium. The Hydrogenlines of σ Ori E are also modulated, however in a more com-plicated fashion due to a combination of photospheric andcircumstellar effects. In general, the variations of HR 7355,inasmuch that they can be estimated from only two spec-tra, are quite similar to those seen in σ Ori E, as reportede.g. by Reiners et al. (2000).
Table 1.
Equivalent widths of selected lines, compared toequivalent widths measured from model spectra by Zboril(2000, see Sect. 4.3 for details). Measurements are given inm˚A, and the typical error in the observed values are about10 % for the strong lines of H and He, and about 25 m˚A forthe weak lines of C ii and Si iii . Line 1999 2004 He/H=0.4 He/H=0.1H γ i i i ii iii Due to the significant effects of rotational broadening, onlythe strongest photospheric lines can easily be measuredand used to constrain the star’s fundamental parameters.Unfortunately, second only to the (emission-contaminated)Balmer lines in this respect are the He i lines, and these ofcourse are strongly variable. Nevertheless, at least roughestimates can be attempted.The luminosity classification as a dwarf star and thebroadening wings of the Hydrogen lines are consistent witha surface gravity log g ≈ .
0. Using this as a reference point,the spectral features, including the equivalent widths of theBalmer lines, point toward an effective temperature on theorder of 20 000 K. This value is bracketed by the publishedspectral classifications, B2V (Hiltner et al. 1969) and B5IV(Jaschek et al. 1964), but is closer to B2V (see Table 10 ofTrundle et al. 2007, for instance).The very high projected equatorial velocity of v sin i =320 km s − makes HR 7355 the most rapidly rotating He-strong star known to date. Statistically, the class of He-strong stars is deficient in rapid rotators (Walborn 1983;Zboril & North 1999), with σ Ori E ( v sin i = 165 km s − and i ≈ ◦ ) being one of the record holders so far.Assuming sin i = 1 and identifying the double wave pe-riod as rotational, the stellar radius of HR 7355 would beabout 3.3 R ⊙ , too small for a B2V star even at the ZAMS(Balona 1995). However, HR 7355 is rotating sufficientlyclose to the critical limit that gravity darkening could biasthe measurement of v sin i toward lower apparent values(Townsend et al. 2004).To derive a preliminary estimate of the range in sur-face Helium abundances, we compared the FEROS spectraagainst models published by Zboril (2000). These modelsuse He/H abundance ratios ranging from 0.1 to 1.0 for asequence of effective temperatures and surface gravities, in-cluding T eff = 20 000 K, log g = 4 .
0. The results from equiv-alent width measurements for a few lines in the observedspectra are compared in Table 1 against values measuredfrom the models for the above parameters and He/H=0.1and 0.4. The typical EW-error of 10 % was derived by aconservative estimate from repeated manual measurements.We conclude that the disk-averaged stellar hemisphere ob-served in 1999 was enriched in Helium by a factor of about4, while the one observed in 2004 was about normal oreven slightly depleted in Helium. A more detailed study isneeded to refine these numbers, however.
Th. Rivinius et al.: The most rapidly rotating He-strong emission line star: HR 7355 (RN)
The absorption-line changes of HR 7355 are a clear indica-tion of spatial structure in the surface abundances at thevery least of Helium, Silicon, and possibly Hydrogen. ForB-type stars, this structure is the typical signature of astrong magnetic field, on the order of several kiloGauss.The presence of emission – with an extension out to almost ± − that is more likely to be kinematic ratherthan due to scattering – lends independent support to thepresence of a strong field that is able to confine circumstel-lar plasma and torque it into co-rotation.The RRM model (Townsend & Owocki 2005) assumesa magnetic field sufficiently strong that the circumstellarenvironment is completely dominated by the field, i.e. windplasma upflowing from the star is forced to follow the fieldlines, but does not influence them. The model predicts thesteady accumulation of plasma at points along field lineswhere the effective (gravitational plus centrifugal) potentialis at a local minimum. For an oblique dipole field – thesort most commonly detected in chemically peculiar stars –the locus formed by such minima resembles a warped disk;moreover, the distribution of accumulated plasma in thisdisk is concentrated into two elongated cloud-like regions,centered along the twin intersections between magnetic androtational equators.The RRM model predicts a distinctive observational sig-nature for the warped magnetospheric disk. Because it co-rotates with the star, it exhibits double-peaked emissionwith a strength that varies due to optical depth and oc-cultation effects. Depending on rotational and magnetic in-clination, the disk may transit the stellar disk either onceor twice per rotation cycle, in both cases absorbing pho-tospheric flux. In HR 7355, the equator-on aspect meansthat two such eclipses should occur per rotation cycle (seeTownsend 2007), in accordance with our assumption thatthe double-wave period corresponds to the rotation period.The changes in the total emission strength between1999 and 2004 cannot be constrained to a short or longtimescale from the two observations alone. However, theyare not easily ascribed to any short-periodic type of vari-ation, and are more likely to have occured on longertimescales, for instance during a breakout of accumulatedmaterial from its magnetic confinement (see Appendix ofTownsend & Owocki 2005; see also ud-Doula et al. 2006). In the handful of other He-strong stars showing convincingcases of H α emission – δ Ori C, V 1046 Ori and HD 64 740 –there are only brief reports noting the emission and its vari-ability, but no in-depth investigations have been publishedso far (Walborn 1974; Pedersen 1979; Bohlender et al. 1991;Bolton et al. 1998). Further spectra are required for thoseto obtain a sufficient database, both in amount and qual-ity, for a detailed study with current models. A few morecandidates mentioned by Zboril et al. (1997) are suspectedcandidates on the basis of photometry alone or might shownebular H β emission instead of circumstellar one.
5. Conclusions
HR 7355 is a previously unknown spectroscopically variablestar, and as such it should no longer be used as a spectral standard star. In its capacity as the newest member of theHe-strong class, it is not only one of the brighter stars inthis class, but is also the most rapidly rotating.In addition to its spectral variability, HR 7355 is peri-odically variable in photometry, with either a single-wavesinusoidal lightcurve of P sin = 0 . ± . P dw = 0 . ± . σ Ori E (Townsend et al. 2005), and we are optimisticthat it can explain the behaviour of HR 7355 too.
Acknowledgements.
RHDT is supported by NASA grantLTSA/NNG05GC36G. We thank the FEROS consortium forobserving HR 7355 in 1999, during the time guaranteed to theconsortium for building the instrument. This study made use of theSIMBAD and ADS databases, as well of the ViZiR catalog services.We thank the second referee, C. J. Evans, for his suggestions how toimprove the presentation of this work.
References
Abt, H. A., Levato, H., & Grosso, M. 2002, ApJ, 573, 359Babel, J. & Montmerle, T. 1997, A&A, 323, 121Balona, L. A. 1995, MNRAS, 277, 1547Bohlender, D. A., Walker, G. A. H., & Bolton, C. T. 1991, JRASC,85, 202Bolton, C. T., Harmanec, P., Lyons, R. W., Odell, A. P., & Pyper,D. M. 1998, A&A, 337, 183Evans, C. J., Smartt, S. J., Lee, J.-K., et al. 2005, A&A, 437, 467Glebocki, R. & Stawikowski, A. 2000, Acta Astronomica, 50, 509Hiltner, W. A., Garrison, R. F., & Schild, R. E. 1969, ApJ, 157, 313Jaschek, C., Conde, H., & de Sierra, A. C. 1964, ObservatoryAstronomical La Plata Series Astronomies, 28, 1Kaufer, A., Stahl, O., Tubbesing, S., et al. 1999, The Messenger, 95,8Koen, C. & Eyer, L. 2002, MNRAS, 331, 45Pedersen, H. 1979, A&AS, 35, 313Porter, J. M. & Rivinius, T. 2003, PASP, 115, 1153Reiners, A., Stahl, O., Wolf, B., Kaufer, A., & Rivinius, T. 2000,A&A, 363, 585Shore, S. N. & Brown, D. N. 1990, ApJ, 365, 665Townsend, R. H. D. 2007, MNRAS, in preparationTownsend, R. H. D. & Owocki, S. P. 2005, MNRAS, 357, 251Townsend, R. H. D., Owocki, S. P., & Groote, D. 2005, ApJ, 630, L81Townsend, R. H. D., Owocki, S. P., & Howarth, I. D. 2004, MNRAS,350, 189Trundle, C., Dufton, P. L., Hunter, I., et al. 2007, A&A, 471, 625ud-Doula, A. & Owocki, S. P. 2002, ApJ, 576, 413ud-Doula, A., Townsend, R. H. D., & Owocki, S. P. 2006, ApJ, 640,L191Walborn, N. R. 1974, ApJ, 191, L95Walborn, N. R. 1983, ApJ, 268, 195Zboril, M. 2000, A&A, 363, 1051Zboril, M. & North, P. 1999, A&A, 345, 244Zboril, M., North, P., Glagolevskij, Y. V., & Betrix, F. 1997, A&A,324, 949 h. Rivinius et al.: The most rapidly rotating He-strong emission line star: HR 7355 (RN) List of Objects ‘HR 7355’ on page 1‘ σ Ori E’ on page 1‘HR 7355’ on page 1‘HD 182 180’ on page 1‘HIP 95 408’ on page 1‘ σ Ori E’ on page 3‘ δδ