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Equivalent width of emission lines

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  • hughallen10
    I am monitoring the strength of the Balmer emission lines in the VV Cep spectrum as the Be star goes into eclipse behind the red supergiant. The Equivalent
    Message 1 of 13 , Nov 4, 2017
      I am monitoring the strength of the Balmer emission lines in the VV Cep spectrum as the Be star goes into eclipse behind the red supergiant. The Equivalent Width calculator in BASS works very nicely but I'm conscious that the result should really be corrected for changes in the Vmag of VV Cep. Is there a formula for doing this? I have a light curve for VV Cep from which I can read the approximate Vmag on a given date
      Cheers
      Hugh


    • andyjwilson_uk
      Hi Hugh, You should be safe using equivalent width without flux calibration, which is what I think you are asking. It is also a good measure as it is
      Message 2 of 13 , Nov 4, 2017
        Hi Hugh,

        You should be safe using equivalent width without flux calibration, which is what I think you are asking. It is also a good measure as it is independent of resolution, so results from different spectrographs may be compared.

        It is the width in Angstroms of a rectangular line of equal area to the actual absorption line whose height is that of the continuum. As though the line were a perfect rectangle with no flux in the line.

        Typically it is easier to imagine for an absorption line that for an emission line, but the maths is the same. The below link gives insight into the maths and meaning.


        Cheers,
        Andy


        ---In astrobodger@..., <hughallen10@...> wrote :

        I am monitoring the strength of the Balmer emission lines in the VV Cep spectrum as the Be star goes into eclipse behind the red supergiant. The Equivalent Width calculator in BASS works very nicely but I'm conscious that the result should really be corrected for changes in the Vmag of VV Cep. Is there a formula for doing this? I have a light curve for VV Cep from which I can read the approximate Vmag on a given date
        Cheers
        Hugh



         
      • hughallen10
        Thanks Andy. I was concerned that since the continuum flux and emission flux are actually indepenedent in VV Cep, a change in the continuum flux would lead to
        Message 3 of 13 , Nov 4, 2017
          Thanks Andy. I was concerned that since the continuum flux and emission flux are actually indepenedent in VV Cep, a change in the continuum flux would lead to an apparent change in the equivalent width of the emission line even though the emission flux was actually constant. Have I misunderstood?
          Hugh
        • andyjwilson_uk
          Yes, Hugh. You are right that a change in the continuum flux will change equivalent width even if the line itself remains at the same underlying strength. Flux
          Message 4 of 13 , Nov 4, 2017
            Yes, Hugh. You are right that a change in the continuum flux will change equivalent width even if the line itself remains at the same underlying strength. Flux calibrating won't help you out with the equivalent width as you would get an identical result whether or not it is flux calibrated. What you want is a different measure.

            I think your ideal measurement is the line area of a flux calibrated line. I notice BASS has a line area measurement. Though that still leaves you with the problem of flux calibrating the spectrum.

            Cheers,
            Andy

            ---In astrobodger@..., <hughallen10@...> wrote :

            Thanks Andy. I was concerned that since the continuum flux and emission flux are actually indepenedent in VV Cep, a change in the continuum flux would lead to an apparent change in the equivalent width of the emission line even though the emission flux was actually constant. Have I misunderstood?
            Hugh
          • John Paraskeva
            Hi Hugh, Andy, The units of Equivalent Width are wavelength, so as Andy says, are not affected by the flux scaling. EW is affected by a change in continuum
            Message 5 of 13 , Nov 4, 2017
              Hi Hugh, Andy,

              The units of Equivalent Width are wavelength, so as Andy says, are not affected by the flux scaling. EW is affected by a change in continuum since EW is the with of a rectangle having the same are as the measured peak/dip. Setting Y axis scaling to use 0 as the minimum will show this graphically as a hatched region.

              It may be useful to use the VV_CEP Visual Magnitudes to re-scale each profile to use Absolute flux (erg/cm2/s/A) units. Although not perfect, this would make measurements of areas more meaningful.  See the "Absolute Flux" tab of the "Normalise Flux Scale" screen (under Image menu).

              cheers
              John



              From: "barnards.star12@... [astrobodger]" <astrobodger@...>
              To: astrobodger@...
              Sent: Saturday, 4 November 2017, 19:45
              Subject: [astrobodger] Re:: Equivalent width of emission lines

               
              Yes, Hugh. You are right that a change in the continuum flux will change equivalent width even if the line itself remains at the same underlying strength. Flux calibrating won't help you out with the equivalent width as you would get an identical result whether or not it is flux calibrated. What you want is a different measure.

              I think your ideal measurement is the line area of a flux calibrated line. I notice BASS has a line area measurement. Though that still leaves you with the problem of flux calibrating the spectrum.

              Cheers,
              Andy

              ---In astrobodger@..., <hughallen10@...> wrote :

              Thanks Andy. I was concerned that since the continuum flux and emission flux are actually indepenedent in VV Cep, a change in the continuum flux would lead to an apparent change in the equivalent width of the emission line even though the emission flux was actually constant. Have I misunderstood?
              Hugh


            • andyjwilson_uk
              Excellent John! I missed that screen yesterday when I had a quick look for flux calibration. Out of interest does this do an integration across the wavelength
              Message 6 of 13 , Nov 5, 2017
                Excellent John! I missed that screen yesterday when I had a quick look for flux calibration.

                Out of interest does this do an integration across the wavelength range for a V filter, or is it just an approximation at a single wavelength?

                Cheers,
                Andy


                ---In astrobodger@..., <johnparaskeva@...> wrote :

                Hi Hugh, Andy,

                The units of Equivalent Width are wavelength, so as Andy says, are not affected by the flux scaling. EW is affected by a change in continuum since EW is the with of a rectangle having the same are as the measured peak/dip. Setting Y axis scaling to use 0 as the minimum will show this graphically as a hatched region.

                It may be useful to use the VV_CEP Visual Magnitudes to re-scale each profile to use Absolute flux (erg/cm2/s/A) units. Although not perfect, this would make measurements of areas more meaningful.  See the "Absolute Flux" tab of the "Normalise Flux Scale" screen (under Image menu).

                cheers
                John


              • hughallen10
                Just what I was after, thanks John. I intend to use the attached AAVSO light curve to read off an approximate Vmag for all my VV Cep spectra, and then repeat
                Message 7 of 13 , Nov 5, 2017
                  Just what I was after, thanks John. I intend to use the attached AAVSO light curve to read off an approximate Vmag for all my VV Cep spectra, and then repeat the equivalent width calculations. Attached is the first flux calibrated example, for the last spectrum I captured on 29th October
                  Cheers
                  Hugh
                • John Paraskeva
                  Hi Andy, The Absolute Flux function essentially re-scales the data using the visual magnitude. The user guide shows the calculation. regardsJohn From:
                  Message 8 of 13 , Nov 5, 2017
                    Hi Andy,

                    The Absolute Flux function essentially re-scales the data using the visual magnitude. The user guide shows the calculation.

                    regards
                    John



                    From: "barnards.star12@... [astrobodger]" <astrobodger@...>
                    To: astrobodger@...
                    Sent: Sunday, 5 November 2017, 8:43
                    Subject: [astrobodger] Absolute Flux Calibration

                     
                    Excellent John! I missed that screen yesterday when I had a quick look for flux calibration.

                    Out of interest does this do an integration across the wavelength range for a V filter, or is it just an approximation at a single wavelength?

                    Cheers,
                    Andy


                    ---In astrobodger@..., <johnparaskeva@...> wrote :

                    Hi Hugh, Andy,

                    The units of Equivalent Width are wavelength, so as Andy says, are not affected by the flux scaling. EW is affected by a change in continuum since EW is the with of a rectangle having the same are as the measured peak/dip. Setting Y axis scaling to use 0 as the minimum will show this graphically as a hatched region.

                    It may be useful to use the VV_CEP Visual Magnitudes to re-scale each profile to use Absolute flux (erg/cm2/s/A) units. Although not perfect, this would make measurements of areas more meaningful.  See the "Absolute Flux" tab of the "Normalise Flux Scale" screen (under Image menu).

                    cheers
                    John




                  • John Paraskeva
                    Hi Hugh, You profile looks good. Don t forget, you can add a Y-axis text option for Absolute Flux (erg/cm2/s/A) Be interested to see your results across
                    Message 9 of 13 , Nov 5, 2017
                      Hi Hugh,

                      You profile looks good.

                      Don't forget, you can add a Y-axis text option for "Absolute Flux (erg/cm2/s/A)"

                      Be interested to see your results across multiple sessions. Might work well as an animation or phase plot?

                      regards
                      John




                      From: "hughallen10@... [astrobodger]" <astrobodger@...>
                      To: astrobodger@...
                      Sent: Sunday, 5 November 2017, 14:39
                      Subject: Re:: Re: [astrobodger] Re:: Equivalent width of emission lines

                       
                      Just what I was after, thanks John. I intend to use the attached AAVSO light curve to read off an approximate Vmag for all my VV Cep spectra, and then repeat the equivalent width calculations. Attached is the first flux calibrated example, for the last spectrum I captured on 29th October
                      Cheers
                      Hugh


                    • andyjwilson_uk
                      Thanks John, I missed that bit though it was obvious! Cheers, Andy
                      Message 10 of 13 , Nov 6, 2017
                        Thanks John, I missed that bit though it was obvious!

                        Cheers,
                        Andy
                      • hughallen10
                        Hi again John, Here s a graph of Equivalent Width (EW) measurements, not just on my own Alpy spctra but also some from Jim Ferreira in California and an Alpy
                        Message 11 of 13 , Nov 19, 2017
                          Hi again John,
                          Here's a graph of Equivalent Width (EW) measurements, not just on my own Alpy spctra but also some from Jim Ferreira in California and an Alpy spectrum from Steve Cuthbert on the BAA spectroscopy database. Lots of interesting details in there. It's nice that there is broad agreement with the higher resolution data that Ernst Pollmann is posting on the ARAS forum including the brightening cycles around mid-September and late October. Fascinating stuff!
                          Cheers
                          Hugh
                        • andyjwilson_uk
                          That is fantastic agreement between different observers and setups Hugh! Cheers, Andy
                          Message 12 of 13 , Nov 20, 2017
                            That is fantastic agreement between different observers and setups Hugh!
                            Cheers,
                            Andy
                          • John Paraskeva
                            I agree. Thanks for sharing this. regardsJohn From: barnards.star12@gmail.com [astrobodger] To: astrobodger@yahoogroups.co.uk
                            Message 13 of 13 , Nov 21, 2017
                              I agree. Thanks for sharing this.

                              regards
                              John


                              From: "barnards.star12@... [astrobodger]" <astrobodger@...>
                              To: astrobodger@...
                              Sent: Monday, 20 November 2017, 17:13
                              Subject: [astrobodger] Re:: Equivalent width of emission lines

                               
                              That is fantastic agreement between different observers and setups Hugh!
                              Cheers,
                              Andy


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